Background
∗ At Japan Proton Accelerator Research Complex (J-PARC) in Tokai Village, Ibaraki Prefecture, we accelerate high-intensity proton beams to near light speed, conducting experiments to observe and reveal unknown phenomena in elementary particles and atomic nuclei. We have continuously endeavored to increase 'beam power,' a key indicator determined by the number of accelerated protons supplied to experimental facilities, crucial for the success of our experiments.
Achievements
∗ At the accelerator, 'Main Ring,' we have incrementally increased beam power since the start of operations in 2008. On December 25, 2023, we surpassed our initial target, achieving a beam power of 760 kW through significant enhancements. Efficiently recovering and reusing energy stored in the electromagnets not only allowed us to supply beam power 1.5 times greater with the same electricity consumption but also resulted in significant energy savings.
Meaning
∗ At J-PARC, the 'T2K experiment' delves into neutrino fundamentals. Neutrino research in Japan is playing the leadingrole in the world, with Nobel Prizes being awarded. It is expected that the beam power increase will enhance the T2K experiment and the following Hyper-Kamiokande project will produce new results first in the world.
Electromagnets at J-PARC's Main Ring
We surpassed our initial target, achieving a beam power of 760 kW, a crucial indicator at the Japan Proton Accelerator Research Complex (J-PARC), while concurrently achieving significant energy savings. This success paves the way for the Hyper Kamiokande project, with expectations of a drastic enhancement of the T2K experiment.
The Japan Proton Accelerator Research Complex (J-PARC) in Tokai Village, Ibaraki Prefecture, is a high-intensity proton accelerator facility jointly constructed and operated by the High Energy Accelerator Research Organization (KEK) and the Japan Atomic Energy Agency (JAEA). J-PARC conducts cutting-edge research across a wide range of research, including elementary particle physics, atomic nuclei physics, condensed matter physics, chemistry, material science, and biology. The administrative entity overseeing its operations is called the J-PARC Center.
The proton beam undergoes staged acceleration through three accelerators: a linear accelerator (LINAC), a circular accelerator called the Rapid-Cycling Synchrotron (RCS), capable of accelerating particles up to 3 GeV (gigaelectron volts, a unit of kinetic energy), and another circular accelerator called the Main Ring. Through these accelerators, proton beams are supplied to experimental facilities, including the Neutrino Experimental Facility.
Accelerated protons collide with targets, generating particles (neutrons, K mesons, and neutrinos) used in experiments. Experiment sensitivity depends on the total produced particles, directly proportional to incident protons. Higher sensitivity requires more protons. Achieving this relies on 'beam power' (*1), the number of accelerated protons per unit time, measured in kilowatts (kW).
The Main Ring is a synchrotron circular accelerator initially designed with a target performance of 750 kW. Through numerous beam adjustments and improvements, the beam power reached 500 kW in 2019. Substantial upgrades to the accelerator were implemented, and since the fiscal year 2022, it has been in acceleration tuning operation. On December 25, 2023, it successfully achieved a record-breaking beam power of 760 kW, as shown in Graph 1.
Graph 1: Annual Maximum Beam Power of the Main Ring
We have implemented a technique that employs large-capacity capacitors for the power supplies for the electromagnets, utilizing magnetic force to guide the beam into a circular orbit. This innovative approach efficiently repurposes the energy stored in the magnets, enabling us to achieve a beam power of 760 kW with the same power consumption as before the Main Ring enhancement. This translates to an approximately 1.5 times increase in beam power without an increase in power consumption.
At the Neutrino Experimental Facility, which utilizes protons accelerated in the Main Ring, the apparatus for generating neutrinos has been strengthened to endure high beam power, with improvements in radiation shielding. On December 25, 2023, stable and continuous production of neutrinos was achieved at a record-breaking beam power of 760 kW.
※1. Beam power
Beam power is the product of the kinetic energy of protons and the number of protons extracted per unit of time. It serves as a performance indicator for the accelerator and is the determining factor in the production of secondary and tertiary particles.
Professor IGARASHI Susumu from the Accelerator Division at J-PARC Center: Through the efforts of each team member and excellent teamwork in equipment upgrades and beam tunings, we have successfully reached a long-awaited goal that involved the dedication of many individuals. I am deeply moved and relieved by this accomplishment. Looking ahead, I am committed to continuing our research and striving for even higher achievements.
One of the key experiments utilizing the J-PARC Main Ring beam is the T2K experiment (Tokai to Kamioka Long-baseline Neutrino Oscillation Experiment, *2). This experiment aims to investigate the fundamental properties of neutrinos, elementary particles. Protons accelerated up to 30 GeV in the Main Ring, are extracted and directed to the neutrino experimental facility, where they irradiate a target.
The neutrinos, created by converging the secondary particles generated there, are observed by the Super-Kamiokande detector located 295 km away in Kamioka-cho, Hida City, Gifu Prefecture. To precisely measure the changes caused by the long-distance flight of neutrinos, which have minimal interactions with matter, a substantial number of neutrinos are needed. This, in turn, requires a high beam power of protons, the particles generate neutrinos.
※2. T2K Experiment
To solve the mystery of neutrinos, J-PARC's Main Ring and Experimental Facility generate a high-intensity neutrino beam directed 295 km to the Super-Kamiokande detector, a 50,000-ton water Cherenkov detector operated by Institute for Cosmic Ray Research, the University of Tokyo, located 1,000 meters underground in Kamioka, Hida City, Gifu Prefecture.
Overview of T2K Experiment
The Main Ring, operating as a synchrotron-type accelerator, receives a 3 GeV beam from the preceding accelerator, accelerates it to 30 GeV, and emits a single pulse toward the experimental facility. The electromagnets' magnetic field is then adjusted back to 3 GeV for the next beam injection, and this cycle is repeated. To enhance the beam power in the Main Ring, the following modifications are necessary.
To increase accelerated protons, minimizing beam losses is crucial. Despite repulsive forces, protons undergo over 100,000 turns in the Main Ring. Precision adjustments to magnetic fields and RF accelerator voltages are necessary to minimize beam loss. Even minor errors in magnetic fields can lead to beam loss, so the magnets' fields are finely tuned. Beam tuning in front-end accelerators (Linac and RCS) collaborates to minimize Main Ring beam loss. Exploring optimal conditions, addressing instabilities, and adding equipment contributed to the Main Ring achieving a 500kW beam power in 2019, with 265 trillion protons per pulse, a world record for synchrotron-based proton accelerators.
The operation was on an extended pause starting in the summer of 2021, and a significant modification was implemented to reduce the cycle repetition period from 2.48 seconds to 1.36 seconds.
The modifications encompass various equipment, including devices for injecting the proton beam into the Main Ring, a radio frequency accelerator giving proton energy, a power supply for the main electromagnet to control the proton beam's orbit, and equipment for extracting the accelerated proton beam to the experimental facilities. Following the commissioning of the upgraded equipment and successful confirmation of its stable operation, beam tunings with a repetition rate of 1.36 seconds and beam supply operations to the neutrino experimental facility commenced in 2023. On December 25, the beam power reached 760 kW, as depicted in Graph 2.
In the Neutrino Experimental Facility, electromagnets (electromagnetic horns) that collect secondary particles have been reinforced so that they can be excited with the Main Ring's 1.36-second cycle. Cooling systems for devices, such as the target, exposed to thermal shock from high-power pulsed proton beam irradiation, have been strengthened. Furthermore, facilities like radiation shielding have been reinforced to protect the surrounding environment. These improvements enable to utilize the Main Ring's performance, and to supply more neutrinos stablely for experiments than ever before.
The goal of accelerating a globally unprecedented number of protons, coupled with the challenging task of minimizing beam loss, took 15 years to achieve. This involved incrementally increasing beam power and making substantial improvements to the Main Ring to navigate unforeseen challenges. The efforts include repeated beam tunings and modifications of the Linac and RCS for the performance of the Main Ring. This accomplishment marks a significant milestone in our journey.
We will continue advancing beam tunings to further reduce beam loss and enhance overall operational stability.
The main aim of the T2K experiment is to search the differences in properties between neutrinos and antineutrinos, a phenomenon referred to as 'CP violation.' Understanding this distinction is crucial for unraveling why antimatter vanished, leaving only matter to shape the present cosmos, despite the same amount of matter and antimatter produced during the universe's birth. Achieving this objective necessitates thorough data acquisition.
In the future, toward the Hyper-Kamiokande Project (*3), the next-generation neutrino research following the T2K experiment, we plan to further reduce the repetition rate to 1.16 seconds and expand the RF accelerator to increase the number of extracted protons to 330 trillion, aiming to raise the beam power to 1.3 MW by 2028.
※3. Hyper-Kamiokande Project
This experiment aims to complete the ununified theory of elementary particles and to explore the evolution of the universe by precisely observing proton decay and neutrinos. A new detector is under construction next to the Super-Kamiokande facility in Kamioka Town, Hida City, Gifu Prefecture. The detector, a cylindrical tank filled with ultrapure water, will have a diameter of 68 meters and a depth of 71 meters, with a sensitivity about 10 times higher than the Super-Kamiokande. The experiment is set to begin in 2027.
The T2K Collaboration has started data taking using the enhanced neutrino beam and new neutrino near-detectors from December 2023. The KEK/J-PARC center has upgraded the main ring accelerator and the neutrino beamline to increase the beam power. T2K has also upgraded its neutrino production instruments. The stable operation of neutrino beam has been successfully achieved at a record high beam intensity (about 710 kW), an increase of about 40% compared to before the upgrade. Furthermore, on December 25th, the continuous operation of neutrino beam has been successfully achieved at 760kW, which is greater than the initial design beam power.
The pulsed electromagnet (electromagnetic horn) system, the heart of the neutrino generator, was also upgraded. The current applied to the electromagnetic horn has been increased from 250 kA to 320 kA. This allowed us to increase the neutrino intensity by about 10%. In addition, T2K installed new neutrino detectors that can measure neutrino interactions with even higher precision than before. The newly installed detectors consist of SuperFGD, which detects tracks around a neutrino interaction point inside the detector, High-Angle TPC, which measures momentum of particles emitted over a wide range of angles, and Time-of-Flight, which can detect incoming or outgoing particles and identify particles. Neutrino event candidates were successfully observed during a technical run of the new detectors after the start of beam operation. In 2020, the T2K gave the first hints that the symmetry between matter and antimatter could be violated in neutrino oscillations. With these enhancements, T2K will continue to lead the world in advancing the understanding of neutrino properties and unraveling the mystery of the absence of antimatter in the universe.
Fig. 1 : Illustration of the upgraded neutrino production devices (left) and the new neutrino detectors (right).
T2K is an experiment to study neutrino oscillations (*1) by sending neutrinos produced at the Japan Proton Accelerator Research Complex (J-PARC) in Tokai, Ibaraki, to the neutrino near-detectors and the Super-Kamiokande detector in Kamioka, Hida, Gifu, about 300 km away. The T2K experiment started taking data in 2010 and directly observed electron neutrino appearance for the first time in the world in 2013. In 2014, we started measurements using an anti-neutrino beam to verify CP violation (*2) , and in 2020 we strongly restricted possible values of the neutrino CP phase (*3) for the first time. To obtain evidence of the CP violation, a more precise measurement is required to eliminate 0 and ±180 degrees from the possible range of CP phase with a high degree of confidence. However, in order to realize this, it is necessary to produce more neutrinos and to obtain a deeper understanding of the interactions between neutrinos and nucleus.
(*1) Neutrino oscillation
It is a phenomenon in which a neutrino changes into another type of neutrino as it travels through space. The discovery of this phenomenon showed that neutrinos have mass and earned Prof. Takaaki Kajita the Nobel Prize in Physics in 2015 (shared with Prof. Arthur McDonald).
(*2) CP violation
The "C" in CP symmetry represents the "C transformation," which swaps the particle and the anti-particle (for example, swaps the electron and the positron), and the "P" represents the "P transformation," which swaps the directions of up, down, left, right, front and back with respect to space, as if they were mirror images. When the same physical phenomenon occurs with the same probability when the "C transformation" and the "P transformation" are performed, it is called "CP symmetry". When a phenomenon does not obey CP symmetry, it is called "CP violation". The CP violation is one of the conditions that explain the fact that the current universe is dominated by matter. However, the quark CP violation observed so far is so small that it cannot explain the amount of matter in the universe today. Therefore, the neutrino CP violation is expected to provide a major hint for the mystery.
(*3) CP phase
The CP phase is a fundamental property of the "weak interaction" between elementary particles, introduced by Prof. Makoto Kobayashi and Prof. Toshihide Maskawa to explain CP violation in quarks. The CP phase can take values between -180 and 180 degrees, but for leptons such as electron and neutrino, the values have been completely unknown until recently. T2K experiment excluded nearly half of the range of possible values of the CP phase in 2020 with a confidence level of 99.7% (3 sigma).
The T2K Collaboration, an international collaborative experiment involving approximately 570 researchers from 78 research institutions in 14 countries, has launched a new phase of the experiment using an enhanced neutrino beam and new near-detectors. Neutrinos are produced from decayed pions or other particles produced in interactions between proton beams and a graphite target. The KEK/J-PARC Center upgraded the J-PARC main ring accelerator, including the power supply for the main magnet, to increase the repetition rate of the proton beam from 2.48 seconds to 1.36 seconds, which supplies more protons to the neutrino production target. The T2K experimental group upgraded, modified, and exchanged instruments in the neutrino beam facility such as targets, electromagnetic horns, and beam monitors. The beam commissioning was started in November 2023. The stable production of neutrino beam has been successfully achieved at a record high beam power (about 710 kW), an increase of about 40% compared to before the upgrade. Furthermore, on December 25th, the continuous operation of neutrino beam has been successfully achieved at 760kW, which is greater than the initial design beam power. The heart of the neutrino generator is the electromagnetic horns (Fig.2). The current applied to the three electromagnetic horns was increased from 250kA to 320 kA by upgrading the power supply and other components, thereby improving the focusing efficiency of parent particles such as pions produced at the target. This improves the quality of the neutrino beam delivered to the Super-Kamiokande detector while increasing the number of neutrinos observed by about another 10%.
Fig. 2: Upgraded 2nd electromagnetic horn with improved cooling capacity to enable neutrino production by high-intensity proton beams.
In addition, the T2K Collaboration has started observation using new neutrino near-detectors(Fig.3) at the Neutrino Monitor Building located 280 m downstream of the neutrino production target. By October 2023, three new types of detectors have been installed. A new detector, SuperFGD with a mass of approximately 2 tons of sensitive volume, is located at the center of the upgraded detectors. It has an innovative structure consisting of approximately 2 millions 1cm3 cubes each with 3 holes, made of plastic scintillator. Approximately 56,000 optical fibers penetrate the cubes from three directions and photodetectors are at the ends of the fibers.
Charged particles can be observed in high resolution from three projections and 3D tracks can be reconstructed. Below the SuperFGD, the first High Angle Time Projection Chambers (HATPC) has been installed. The HATPC is a gaseous detectors composed by two field cages that produce a uniform electric field and that uses, as readout system, resistive Micromegas modules. HATPC is an innovative detector that allow for an excellent reconstruction of the track trajectory and hence of the momentum of particles emitted by neutrino interactions in the SuperFGD. Finally, the detectors surrounding the SuperFGD and HATPC are Time-of-Flight detectors. It is used to determine the direction of particles and particle identification. T2K started measurements with the upgraded neutrino beam in December 2023 and succeeded in observing neutrino event candidates from the newly acquired data (Fig. 4, 5).
Fig. 3: A photo of the new detectors.
Fig. 4: An event display of a neutrino interaction candidate in the SuperFGD with a track entering the bottom HATPC while another track is entering the original detectors.
Fig. Fig. 5: The beam timing structure observed with one of the new detectors, Time-of-Flight.
With these improvements, the T2K experiment enters a new phase with an enhanced neutrino beam and novel new detectors. The J-PARC accelerator and neutrino experimental facility are undergoing an upgrade plan to further increase the output power to 1.3 MW (= 1300 kW) while supplying beams for the T2K experiment. Together with the upgraded neutrino production instruments such as the electromagnetic horns with improved focusing efficiency, it will be possible to observe about three times as many neutrino interactions (per unit time) as before, and to reduce the error originating from statistical variations (statistical error) in the observed data. In addition, the new detector can detect large angle scattering in the neutrino interactions, which was not possible with our original detector. This will enable a better understanding of the neutrino-nucleus interactions and therefore reduces the systematic errors. Furthermore, the Super-Kamiokande detector has also improved its detector performance with a much higher neutron detection efficiency by loading gadolinium in the water. The T2K experiment will significantly improve the sensitivity of the measurements through these improvements and proceed to verify the differences in neutrino and antineutrino behavior. The J-PARC high-intensity proton accelerator and neutrino experimental facility are expected to play a key role in the next generation of neutrino research. The new phase of the T2K experiment is an important step toward the next generation of experiments and T2K is expected to continue to lead the world in neutrino research unraveling the mystery of the missing antimatter from our universe.
Question
∗ Muonic helium is composed of an ordinary helium atom with one of its two electrons replaced by a negative muon. Precision spectroscopy measurements of its energy structure can be used to determine the mass of the negative muon and verify the current theory of particle physics. However, this will require a hundredfold improvement over the accuracy of previous measurements.
Findings
∗ Muonic helium atom spectroscopy measurements at J-PARC MLF MUSE D-line were performed directly at zero magnetic field with a precision 3 times better than the previous measurement done in the 1980s. The result obtained is also more precise than the previous indirect measurement at high magnetic field improving the current world record precision by a factor of 1.5, establishing a highly precise spectroscopic method at J-PARC.
Meaning
∗ This is an important milestone for the coming measurements at the H-line planned at high magnetic field that will allow us to measure the energy structure of muonic helium atoms a hundred times more precisely and determine the negative muon mass with greater precision to test CPT invariance by comparing the masses of positive and negative muons (second-generation leptons).
Figure 1: Experimental apparatus for muonic helium atom spectroscopy measurements in the D2 area of J-PARC MLF MUSE (left), and view of the experimental apparatus being assembled from above (right).
Precision spectroscopy measurements of the muonic helium atom energy structure were performed at J-PARC MLF MUSE D-line more precisely than previous measurements done 40 years ago, improving the current world record by a factor of 1.5. This is an important milestone for the coming measurements planned at the H-line with higher muon beam intensity and under a high magnetic field that will permit one to improve further the precision a hundred times and determine more precisely the negative muon mass.
A muonic helium atom, which is composed of an ordinary helium atom with one of its two electrons replaced by a negative muon※1, is a special atom that is not found in nature. It has a small energy level structure, called hyperfine structure※2, that results from the interaction between the remaining electron and the negative muon (due to the intrinsic properties of their respective spin).
※1 Muon
Precision spectroscopy measurements of the muonic helium atom hyperfine structure are the only available experimental results for three-body muonic atoms (i.e., helium nucleus, electron, and negative muon) that can be used to verify and improve the current theory of particle physics for three-body atomic system and quantum electrodynamics (QED), which accurately describes how matter and light interact at the quantum level. It can also be used to determine the mass of the negative muon to test CPT invariance by comparing the masses of positive and negative muons (second-generation leptons).
The hyperfine structure of muonic helium atoms has only been measured twice in the 1980s at the Paul Scherrer Institute in Switzerland (directly at zero magnetic field) and Los Alamos National Laboratory in the United States (indirectly at high magnetic field).
In this study, we succeeded in measuring the hyperfine structure of muonic helium atoms directly at zero magnetic field using the Muon Science Facility (MUSE) D-line at the Materials and Life Science Experimental Facility (MLF) of the Japan Proton Accelerator Research Complex (J-PARC※3) with a precision 3 times better than the previous direct measurement.
The result obtained is also 1.5 times more precise than the previous indirect measurement at high field improving the current world record and establishing a highly precise spectroscopic method. It was also performed for the first time with methane admixture used as an electron donor to form neutral muonic helium atoms efficiently, the prerequisite to measuring the hyperfine structure.
This technique will soon be used at the H-line, which offers about ten times higher muon beam intensity than at the D-line and the possibility of longer measurement time. This will allow us to measure the hyperfine structure of muonic helium atoms at high magnetic field a hundred times more precisely and determine the negative muon mass with greater precision.
A muon is an elementary particle with properties similar to the electron but with a mass about 200 times greater. Both positively and negatively charged muons exist. They can be found naturally in our surroundings, raining down to Earth from space as cosmic rays. However, muons decay in a very short time (about 2 microseconds lifetime). Negative muons can orbit nuclei the same way electrons do to produce a "muonic atom" with different characteristics from ordinary atoms because muons are much heavier.
※2 Hyperfine structure (HFS)
A small energy structure of an atom caused by the interaction between electrons and the nuclei. It is currently used in the cesium atomic clock to define the length of a second.
※3 J-PARC
J-PARC is a large-scale research facility jointly operated by the High Energy Accelerator Research Organization (KEK) and the Japan Atomic Energy Agency (JAEA) in Tokai-mura, Ibaraki Prefecture, Japan. J-PARC is a multi-purpose and multidisciplinary facility that is unique in the variety of secondary-particle beams produced and put to use in cutting-edge research across a wide range of scientific fields, from academic research in particle physics, nuclear physics, condensed matter physics, chemistry, materials science, and biology, to applied research in industrial fields.
Institute for Materials Structure Science (IMSS), High Energy Accelerator Research Organization (KEK) |
P. Strasser (lecturer), R. Iwai (research fellow), S. Kanda (assistant professor), S. Nishimura (special assistant professor), and K. Shimomura (professor). |
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Department of Physics, Nagoya University | S. Fukumura (graduate student), S. Kawamura (graduate student), M. Kitaguchi (associate professor), H. M. Shimizu (professor), and H. Tada (graduate student). |
Graduate School of Science, The University of Tokyo | H. A. Torii (associate professor). |
Graduate School of Arts and Sciences, The University of Tokyo | S. Seo (graduate student). |
There is nothing like good teamwork to achieve new scientific breakthroughs and reach exciting new frontiers.
The hyperfine structure of muonic helium atoms can be precisely measured to determine the mass of negatively charged muons, one of the fundamental physical constants※4, and by comparing it with theoretical predictions, we can also verify the current theory of particle physics. However, there has been no precise measurement of the hyperfine structure of muonic helium atoms since their first measurements back in the 1980s.
In recent years, however, the MuSEUM※5 collaboration at J-PARC has developed a technique for measuring the hyperfine structure of muonium. Muonium and muonic helium are very similar (Fig. 2), both hydrogen-like atoms. They have almost equal hyperfine structures, but these are inverted because of the opposite charge of the muon. The same microwave magnetic resonance technique has the potential to measure the hyperfine structure of muonic helium atoms with a precision up to nearly 100 times greater than the current level. That is how this research was initiated.
※4 Fundamental physical constants
Physical quantities that express the fundamental properties of the natural world and are universal, and immutable throughout the Universe.
※5 MuSEUM Collaboration
Abbreviation for "Muonium Spectroscopy Experiment Using Microwave". An experiment to investigate the energy structure of a special type of atom called muonium, in which an electron orbits around a positively charged muon. Muonium does not exist in nature.
Figure 2: Comparison between a muonium atom (left) and a muonic helium atom (right).
We noted that precisely determining the mass of negatively charged muons using muonic helium atoms allows for testing the CPT theorem※6 by combining it with other experimental results on the mass of positively charged muons (antiparticles).
Additionally, the gas used in the experiment has distinctive characteristics. To form neutral muonic helium atoms, it is necessary to mix helium gas with a small amount of foreign gas that acts as an electron donor. Indeed, when the muon is captured by a helium atom, it quickly ejects both electrons and forms a singly charged muonic helium ion. Subsequently, it cannot capture an electron from neighboring helium atoms because its electron binding energy is much smaller, like hydrogen. In previous experiments performed in the 1980s, the noble gas xenon was used as an electron donor because it is easily ionizable. This time, we used methane (CH4) because it is more efficient, absorbs fewer muons than xenon, which has a higher atomic number, and is less expensive.
※6 CPT theorem
The CPT theorem says that the fundamental symmetry of physical laws under any combination of charge conjugation (C), parity inversion (P), and time reversal (T) holds for all physical phenomena. This means that any particle and its antiparticle must have the same mass, the same magnetic moment (with opposite sign), and the same lifetime.
Figure 3: Diagram showing the relationship between this research and other ongoing experiments at J-PARC and other research facilities.
The precision of measuring the hyperfine structure of muonic helium atoms is mainly determined by the number of muonic helium atoms we can produce. On the other hand, muons captured by a helium atom decay in about 2 microseconds, so the precision of the measurement is essentially determined by the muon beam intensity and the measurement time. To improve the measurement precision as much as possible over a total beamtime of 15 days (carried out in three different cycles), we focused on optimizing the measurement conditions (Fig. 4) and studying different and more advanced analysis methods. We could measure the muonic helium hyperfine structure resonance curve with He + CH4 (2%) at three different pressures (Fig. 5) and extrapolate to zero pressure to determine the muonic helium atom hyperfine structure frequency (Fig. 6).
Figure 4: Schematic view of the experimental setup to measure the hyperfine structure of muonic helium atoms at zero magnetic field. (© 2023 American Physical Society)
Figure 5: Muonic helium hyperfine structure resonance curve measured at zero field with He + CH4 (2%) at (a) 3.0, (b) 4.0, and (c) 10.4 atm, respectively. The red solid lines represent the fitting results. (© 2023 American Physical Society)
Figure 6: Muonic helium atom hyperfine structure frequency as a function of the He + CH4 (2%) gas pressure (red circle). The red solid line shows the linear extrapolation to determine the hyperfine frequency at zero pressure. Previous results from Orth et al. (green diamond) and Gardner et al. (blue square) with the linear extrapolation (blue dashed line) measured with He + Xe(1.5%) are also shown for comparison.
We succeeded in measuring the hyperfine structure of muon helium atoms with the world's highest precision and established the measurement technique using pulsed muons at J-PARC. The measurement results also show that the measurement precision can be improved by a factor of 100 using this technique and the high-intensity muon beams from MLF MUSE H-line at J-PARC. We are currently working on further technological developments to improve the measurement precision.
Compared to positively charged muons, negatively charged muons are measured less precisely in terms of mass, and the difference in properties due to the opposite charge has not yet been well studied. Suppose that more precise measurements of the hyperfine structure of muonic helium atoms are made based on this experiment. In this case, the difference in properties due to the charge, i.e., the difference between particles and antiparticles, could be clarified.
In addition, the theory of complex atoms composed of three or more particles is not well developed at present. Precise measurements of muonic helium atoms, which are composed of three particles (helium nucleus, electron, and muon), can be expected to provide a significant boost to verify and improve the current theory of particle physics.
The muon experiment at the Materials and Life Science Experimental Facility (MLF) of J-PARC was performed under a user program (Proposals No. 2020B0333, No. 2021B0169, No. 2022A0159). This work was supported by the JSPS KAKENHI Grant No. 21H04481.
"Improved Measurements of Muonic Helium Ground-State Hyperfine Structure at a Near-Zero Magnetic Field" , Physical Review Letters 131, 253003 (2023).
Figure 1. The top figure shows the snapshot for the oxide-ion migration. The red and green oxide ions move by breaking and reforming of M2O9 dimers, which enables fast oxide-ion diffusion where the M cation is Nb5+ or Mo6+. The neutron scattering length density distribution from neutron diffraction data at 800 ℃ in the bottom left figure agrees with the time- and space-averaged probability density distribution of oxide ions from ab initio molecular dynamics simulations in the bottom right figure. The interstitial O5 atom in the bottom left figure corresponds to the corner-sharing oxygen atom (Osh in the bottom right figure and squares in the top figure).
Clean energy technologies are the cornerstone of sustainable societies, and solid-oxide fuel cells (SOFCs) and proton ceramic fuel cells (PCFCs) are among the most promising types of electrochemical devices for green power generation. These devices, however, still face challenges that hinder their development and adoption.
Ideally, SOFCs should be operated at low temperatures to prevent unwanted chemical reactions from degrading their constituent materials. Unfortunately, most known oxide-ion conductors, a key component of SOFCs, only exhibit decent ionic conductivity at elevated temperatures. As for PCFCs, not only are they chemically unstable under carbon dioxide atmospheres, but they also require energy-intensive, high-temperature processing steps during manufacture.
Fortunately, there is a type of material that can solve these problems by combining the benefits of both SOFCs and PCFCs: dual-ion conductors. By supporting the diffusion of both protons and oxide ions, dual-ion conductors can realize high total conductivity at lower temperatures and improve the performance of electrochemical devices. Although some perovskite-related dual-ion conducting materials such as Ba7Nb4MoO20 have been reported, their conductivities are not high enough for practical applications, and their underlying conducting mechanisms are not well understood.
Against this backdrop, a research team led by Professor Masatomo Yashima from Tokyo Institute of Technology, Japan, decided to investigate the conductivity of materials similar to Ba7Nb4MoO20 but with a higher Mo fraction (that is, Ba7Nb4-xMo1+xO20+x/2). Their latest study, which was conducted in collaboration with the Australian Nuclear Science and Technology Organisation (ANSTO), the High Energy Accelerator Research Organization (KEK), and Tohoku University, was published in Chemistry of Materials.
After screening various Ba7Nb4-xMo1+xO20+x/2 compositions, the team found that Ba7Nb3.8Mo1.2O20.1 had remarkable proton and oxide-ion conductivities. "Ba7Nb3.8Mo1.2O20.1 exhibited bulk conductivities of 11 mS/cm at 537 ℃ under wet air and 10 mS/cm at 593 ℃ under dry air. Total direct current conductivity at 400 ℃ in wet air of Ba7Nb3.8Mo1.2O20.1 was 13 times higher than that of Ba7Nb4MoO20, and the bulk conductivity in dry air at 306 ℃ is 175 times higher than that of the conventional yttria-stabilized zirconia (YSZ)," highlights Prof. Yashima.
Next, the researchers sought to shed light on the underlying mechanisms behind these high conductivity values. To this end, they conducted ab initio molecular dynamics (AIMD) simulations, neutron diffraction experiments, and neutron scattering length density analyses. These techniques enabled them to study the structure of Ba7Nb3.8Mo1.2O20.1 in greater detail and determine what makes it special as a dual-ion conductor.
Interestingly, the team found that the high oxide-ion conductivity of Ba7Nb3.8Mo1.2O20.1 originates from a unique phenomenon (Figure). It turns out that adjacent (Nb/Mo)O5 monomers in Ba7Nb3.8Mo1.2O20.1 can form M2O9 dimers by sharing an oxygen atom on one of their corners (M = Nb/Mo cation). The breaking and reforming of these dimers gives rise to ultrafast oxide-ion movement in a manner analogous to a long line of people relaying buckets of water (oxide ions) from one person to the next. Furthermore, the AIMD simulations revealed that the observed high proton conduction was due to efficient proton migration in the hexagonal close-packed BaO3 layers in the material.
Taken together, the results of this study highlight the potential of perovskite-related dual-ion conductors and could serve as guidelines for the rational design of these materials. "The present findings of high conductivities and unique ion migration mechanisms in Ba7Nb3.8Mo1.2O20.1 will help the development of science and engineering of oxide-ion, proton, and dual-ion conductors," concludes a hopeful Prof. Yashima.
We hope further research leads us to even better conducting materials for next-generation energy technologies.
Authors | Yuichi Sakuda1, Taito Murakami1, Maxim Avdeev1,2,3, Kotaro Fujii1, Yuta Yasui1, James R. Hester2, Masato Hagihala4, Yoichi Ikeda5, Yusuke Nambu5,6,7, and Masatomo Yashima1,* |
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Title | Dimer-Mediated Cooperative Mechanism of Ultrafast-Ion Conduction in Hexagonal Perovskite-Related Oxides |
Journal | Chemistry of Materials |
DOI | 10.1021/acs.chemmater.3c02378 |
Affiliations | 1 Department of Chemistry, School of Science, Tokyo Institute of Technology 2 Australian Centre for Neutron Scattering, Australian Nuclear Science and Technology Organisation (ANSTO) 3 School of Chemistry, The University of Sydney 4 Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK) 5 Institute for Materials Research, Tohoku University 6 Organization for Advanced Studies, Tohoku University 7 FOREST, Japan Science and Technology Agency |
As the world transitions towards a greener and more sustainable energy economy, reliance on lithium (Li)-ion batteries is expected to rise. Scientists from across the globe are working towards designing smaller yet efficient batteries that can keep up with the ever-increasing demand for energy storage. In recent years, all-solid-state lithium batteries (ASSLBs) have captured research interest due to their unique use of solid electrolytes instead of conventional liquid ones. Solid electrolytes not only make the battery safer from leakage and fire-related hazards, but also provide superior energy and power characteristics. However, their stiffness results in poor wetting of the cathode surface and a lack of homogenous supply of Li ions to the cathode. This, in turn, leads to a loss of capacity in the solid-state battery. The issue becomes more pronounced in thick battery cathode electrode such as millimeter-thick one, which is a more advantageous electrode configuration for realizing inexpensive and high-energy-density battery package, compared to conventional electrode with typical thickness of < 0.1 mm.
Fortunately, a recent study published in Science found a way to overcome this problem. The paper--authored by a team of researchers led by Prof. Ryoji Kanno from Tokyo Institute of Technology (Tokyo Tech)--describes a new strategy to produce solid electrolytes with enhanced Li-ion conductivity. Their work establishes a design rule for synthesizing high-entropy crystals of lithium superionic conductors via the multi-substitution approach.
"Many studies have shown that inorganic ionic conductors tend to show better ion conductivity after multi-element substitution probably because of the flattened potential barrier of Li-ion migration, which is essential for better ion conductivity," points out Prof. Kanno. This was where they started their research. For the design of their new material, the team took inspiration from the chemical compositions of two well-known Li-based solid electrolytes: argyrodite-type (Li6PS5Cl) and LGPS-type (Li10GeP2S12) superionic crystals. They modified the LGPS-type Li9.54Si1.74P1.44S11.7Cl0.3 via multi-substitution and synthesized a series of crystals with composition Li9.54[Si1−δMδ]1.74P1.44S11.1Br0.3O0.6 (M = Ge, Sn; 0 ≤ δ ≤ 1).
The researchers used a crystal with Ge = M and δ = 0.4 as a catholyte in an ASSLB with an 1- or 0.8- millimeter-thick cathode. The former and latter ASSLB exhibited discharge capacities of 26.4 mAh cm−2 at 25℃ (1 mm) and 17.3 mAh cm−2 at −10 ℃ (0.8 mm), respectively, with the area-specific capacity 1.8 and 5.3 times larger than those reported for previous state-of the-art ASSLBs, respectively. Theoretical calculations suggested that the enhanced conductivity of the solid electrolyte could be a result of the flattening of the energy barrier for ion migration, caused by a small degree of chemical substitution in the above-mentioned crystal.
This study provides a new way for preparing high-entropy solid electrolytes for millimeter-thick electrodes while preserving their superionic conduction pathways. "In effect, the proposed design rule lays a solid groundwork for exploring new superionic conductors with superior charge-discharge performance, even at room temperature," concludes Prof. Kanno.
Authors : Yuxiang Li1, Subin Song2, Hanseul Kim2, Kuniharu Nomoto1, Hanvin Kim1, Xueying Sun2, Satoshi Hori1, Kota Suzuki1, Naoki Matsui1, Masaaki Hirayama1,2, Teruyasu Mizoguchi3, Takashi Saito4,5,6, Takashi Kamiyama4,6, and Ryoji Kanno1,*
Title : A lithium superionic conductor for millimeter-thick battery electrode
Journal : Science
Affiliations : Yuta Yasui1 Masataka Tansho2, Kotaro Fujii1, Yuichi Sakuda1, Atsushi Goto2, Shinobu Ohki2, Yuuki Mogami2, Takahiro Iijima3, Shintaro Kobayashi4, Shogo Kawaguchi4, Keiichi Osaka5, Kazutaka Ikeda6,7,8, Toshiya Otomo6,7,8,9, Masatomo Yashima1*
* Corresponding author
Affiliations :
1 Research Center for All-Solid-State Battery, Institute of Innovative Research, Tokyo Institute of Technology
2 Department of Chemical Science and Engineering, School of Materials and Chemical Technology, Tokyo Institute of Technology
3 Institute of Industrial Science, the University of Tokyo
4 Neutron Science Division (KENS), Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK)
5 Department of Materials Structure Science, School of High Energy Accelerator Science, The Graduate University for Advanced Studies
6 Japan Proton Accelerator Research Complex (J-PARC) Center, Materials and Life Science Division
Determining the precise structure of a crystalline solid is a challenging endeavor. Materials properties such as ion conduction and chemical stability, are heavily influenced by the chemical (occupational) order and disorder. However, the techniques that scientists typically use to elucidate unknown crystal structures suffer from serious limitations.
For instance, X-ray and neutron diffraction methods are powerful techniques to reveal the atomic positions and arrangement in the crystal lattice. However, they may not be adequate for distinguishing different atomic species with similar X-ray scattering factors and similar neutron scattering lengths.
To tackle this issue, a research team led by Professor Masatomo Yashima of Tokyo Institute of Technology (Tokyo Tech) in Japan sought to develop a novel and more powerful approach to analyze the order and disorder in crystals. They combined four different techniques to analyze the crystal structure of an important ionic conductor, Ba7Nb4MoO20. "We chose Ba7Nb4MoO20 as Ba7Nb4MoO20-based oxides and related compounds are a class of emerging materials with interesting properties such as high ionic conduction and high chemical stability," explains Prof. Yashima. "However, given that both the Mo6+ and Nb5+ cations have similar scattering powers, all structural analyses of Ba7Nb4MoO20 until now have been performed assuming complete Mo/Nb disorder."
As described in their recent paper published in Nature Communications, the researchers used an approach that combined two experimental techniques, resonant X-ray diffraction (RXRD) and solid-state nuclear magnetic resonance (NMR) aided by computational calculations based on density functional theory (DFT). The NMR provided direct experimental evidence that the Mo atoms occupy only the crystallographic M2 site in Ba7Nb4MoO20, indicating the chemical order of Mo atoms.
Next, the researchers used RXRD to quantify the occupancy factors of Mo and Nb atoms. They found that the occupancy factor of Mo atoms was 0.5 at the M2 site but zero at all other sites. Interestingly, the M2 site is close to the oxide-ion conducting, oxygen-deficient layer of Ba7Nb4MoO20. This suggests that the Mo atoms at the M2 site have key role in the high ion conduction of Ba7Nb4MoO20. Furthermore, DFT calculations indicated that the Mo ordering stabilizes Mo excess composition exhibiting high ionic conductivity. Positions, occupancy, and atomic displacements of protons and oxide ions were also determined by neutron diffraction.
"Our results demonstrate that the Mo order affects the material properties of Ba7Nb4MoO20," highlights Prof. Yashima. "In this regard, our work represents a major advance in our understanding of the correlation between the crystal structure and the material properties of ionic conductors." Further, in contrast to single-crystal X-ray and neutron diffraction, the proposed approach can even be extended to other polycrystalline and powdered samples.
Overall, the methodology presented in this study can open up new avenues for an in-depth analysis of chemical order/disorder in materials. In turn, this could lead to the development of physics, chemistry, and materials science and technology.
Only time will tell what other hidden orders and disorders we will stumble upon!
Journal : Nature Communications
Title : Hidden chemical order in disordered Ba7Nb4MoO20 revealed by resonant X-ray diffraction and solid-state NMR
Authors : Yuta Yasui1 Masataka Tansho2, Kotaro Fujii1, Yuichi Sakuda1, Atsushi Goto2, Shinobu Ohki2, Yuuki Mogami2, Takahiro Iijima3, Shintaro Kobayashi4, Shogo Kawaguchi4, Keiichi Osaka5, Kazutaka Ikeda6,7,8, Toshiya Otomo6,7,8,9, Masatomo Yashima1*
* Corresponding author
Affiliations :
1 Department of Chemistry, School of Science, Tokyo Institute of Technology, 2-12-1-W4-17, O-okayama, Meguro-ku, Tokyo, 152-8551, Japan.
2 NMR Station, National Institute for Materials Science (NIMS), 3-13 Sakura, Tsukuba, Ibaraki 305-0003, Japan.
3 Institute of Arts and Sciences, Yamagata University, 1-4-12 Kojirakawa-machi, Yamagata, Yamagata 990-8560, Japan.
4 Diffraction and Scattering Division, Japan Synchrotron Radiation Research Institute (JASRI), SPring-8, 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5198, Japan.
5 Industrial Application and Partnership Division, Japan Synchrotron Radiation Research Institute (JASRI), SPring-8, 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5198, Japan.
6 Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), 203-1 Shirakata, Tokai, Ibaraki 319-1106, Japan.
7 J-PARC Center, High Energy Accelerator Research Organization (KEK), 2-4 Shirakata-Shirane, Tokai, Ibaraki 319-1106, Japan.
8 School of High Energy Accelerator Science, The Graduate University for Advanced Studies, 203-1 Shirakata, Tokai, Ibaraki 319-1106, Japan.
9 Graduate School of Science and Engineering, Ibaraki University, 162-1 Shirakata, Tokai, Ibaraki 319-1106, Japan.
DOI:10.1038/s41467-023-37802-4
The Standard Model of particle physics tells us that most particles we observe are made up of combinations of just six types of fundamental entities called quarks. However, there are still many mysteries, one of which is an exotic, but very short-lived, Lambda resonance known as ∧(1405) . For a long time, it was thought to be a particular excited state of three quarks-up, down, and strange-and understanding its internal structure may help us learn more about the extremely dense matter that exists in neutron stars.
Now, investigators from Osaka University were part of a team that succeeded in synthesizing ∧(1405) for the first time by combining a K- meson and a proton and determining its complex mass (mass and width) . The K- meson is a negatively charged particle containing a strange quark and an up antiquark. The much more familiar proton that makes up the matter that we are used to has two up quarks and a down quark. The researchers showed that ∧(1405) is best thought of as a temporary bound state of the K- meson and the proton, as opposed to a three-quark excited state.
In a study published recently in Physics Letters B, the group describe the experiment they carried out at the J-PARC accelerator. K- mesons were shot at a deuterium target, each of which had one proton and one neutron. In a successful reaction, a K- meson kicked out the neutron, and then merged with the proton to produce the desired ∧(1405) . "The formation of a bound state of a K- meson and a proton was only possible because the neutron carried away some of the energy," says an author of the study, Kentaro Inoue One of the aspects that had been perplexing scientists about ∧(1405) was its very light overall mass, even though it contains a strange quark, which is nearly 40 times as heavy as an up quark. During the experiment, the team of researchers was able to successfully measure the complex mass of ∧(1405) by observing the behavior of the decay products.
"We expect that progress in this type of research can lead to a more accurate description of ultra-high-density matter that exists in the core of a neutron star." says Shingo Kawasaki, another study author. This work implies that ∧(1405) is an unusual state consisting of four quarks and one antiquark, making a total of 5 quarks, and does not fit the conventional classification in which particles have either three quarks or one quark and one antiquark. This research may lead to a better understanding of the early formation of the Universe, shortly after the Big Bang, as well as what happens when matter is subject to pressures and densities well beyond what we see under normal conditions.
Fig. 1 The exotic baryon called ∧(1405) and a schematic illustration of the evolution of matter
Credit: Hiroyuki Noumi
Fig. 2 Schematic illustration of the reaction used to synthesize ∧(1405) by fusing a K- (green circle) with a proton (dark blue circle) , which takes place inside a deuteron nucleus
Credit: Hiroyuki Noumi
Fig. 3 (Top) Measured reaction cross-section. The horizontal axis is the K- and proton collision recoil energy converted into a mass value. Large reaction events occur at mass values lower than the sum of the K- and proton masses, which itself suggests the existence of ∧(1405). The measured data were reproduced by scattering theory (solid lines) . (Bottom) Distribution of K- and proton scattering amplitudes. When squared, these correspond to the reaction cross-section, and are generally complex numbers. The calculated values match with the measured data. When the real part (solid line) crosses 0, the value of the imaginary part reaches its maximum value. This is a typical distribution for a resonance state, and determines the complex mass. The arrows indicate the real part.
Credit: 2023, Hiroyuki Noumi, Pole position of Λ(1405) measured in d(K^-,n)π ∑ reactions, Physics Letters B
The article, "Pole position of ∧(1405) measured in d(K-,n)π ∑ reactions," was published in Physics Letters B at DOI: https://doi.org/10.1016/j.physletb.2022.137637.
The current work was performed by an international research collaboration, E31, involving scientists from Research Center for Nuclear Physics (RCNP), Osaka University together with RIKEN, KEK, JAEA, J-PARC, Tohoku University, INFN (Italy), SMI (Austria) and others.
Research representives:
Prof. Hiroyuki Noumi, RCNP, Osaka University/IPNS, KEK
Dr. Fuminori Sakuma, RIKEN Cluster for Pioneering Research, RIKEN
Dr. Tadashi Hashimoto, Advanced Science Research Center, JAEA
Prof. Hiroaki Ohnish, Research Center for Electron Photon Science, Tohoku University
Prof. Catalina Curceanu, Laboratori Nazionali di Frascati, INFN
Prof. Johannes Zmeskal, Stefan-Mayer-Institut für subatomare Physik
Stone samples brought back to Earth from asteroid Ryugu have had their elemental composition analyzed using an artificially generated muon beam from the particle accelerator in Japan Proton Accelerator Research Complex (J-PARC). Researchers found a number of important elements needed to sustain life, including carbon, nitrogen, and oxygen, but also found the oxygen abundance in asteroid Ryugu was different from all meteorites that have been found on Earth, reports a new study in Science.
In 2014, the unmanned asteroid explorer Hayabusa 2 was launched into space by the Japan Aerospace Exploration Agency (JAXA) with a mission to bring back samples from asteroid Ryugu, a type C asteroid that researchers believed was rich in carbon. After successful landing on Ryugu and collecting samples, Hayabusa 2 returned to Earth in December 2020 with samples intact.
Since 2021, researchers have been running the first analyses of the samples, led by University of Tokyo Professor Shogo Tachibana. Split into several teams, researchers have been studying the samples in different ways, including stone shapes, elemental abundance, and mineral composition.
Ryugu samples awaiting muon analysis
The Ryugu samples are in the area circled in white and wrapped in copper foil. During analysis, silver holder was removed, and samples were placed in a copper-only space except for the sample.
In this study, led by Tohoku University Professor Tomoki Nakamura, the Institute for Materials Structure Science(IMSS) of High Energy Accelerator Research Organization (KEK), Osaka University, the Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU) of University of Tokyo, Japan Atomic Energy Agency (JAEA), Kyoto University, International Christian University, Institute of Space and Astronautical Science (ISAS) of JAXA and Tohoku University have applied elemental analysis methods using negative muons, elementary particles produced by the accelerator at J-PARC. They applied the method to stones from the asteroid Ryugu (Figure 1), succeeding in nondestructively determining their elemental compositions.
This is important, because if asteroids were built at the beginning of the formation of the Solar System itself, then they would still withhold information about the average elemental composition at that time, and therefore that of the entire Solar System.
Analysis of meteorites that have fallen to Earth have been carried out in the past, but it is possible these samples have been contaminated by the Earth's atmosphere. So, until Hayabusa 2, no one knew what the chemical composition of C-type asteroid was for sure (S-type asteroid composition was revealed by Hayabusa mission).
Figure 1: a sample of the asteroid Ryugu. (Credit: JAXA)
But the researchers faced a challenge. Muon elemental analysis has been used for relatively large samples, such as archaeological samples, and has not been used for samples weighing less than 1 grams.
The team had developed a new method, which involved shooting a quantum beam, or specifically a beam of negative muons, produced by one of the world's largest high-energy proton accelerators J-PARC in Ibaraki prefecture, Japan, to identify the chemical elements of sensitive samples without breaking them.
Muons are one of the elementary particles in the universe. When a negative muon is captured by the irradiated sample, a muonic atom is formed (figure 2). The muonic X-rays emitted from the new muonic atoms have high energy, and so can be detected with high sensitivity. This method was used to analyze the Ryugu samples.
Figure 2: A muonic X-ray created after a muon is captured by an atom of irradiated material.
But there was another challenge. In order to keep the samples from being contaminated by the Earth's atmosphere, the researchers needed to keep the samples out of contact with oxygen and water in the air. Therefore, they had to develop an experimental setup, casing the sample in a chamber of helium gas (figure 3). The inner walls of the chamber were lined with pure copper to minimize the background noise when analyzing the samples.
Figure 3: The custom-made experiment setup developed to avoid the samples from being contaminated by the Earth's atmosphere. The interior is filled with helium gas, and the chamber is lined with pure copper to minimize background noise. (Credit: KEK IMSS)
In June 2021, 0.1 grams of Ryugu asteroid were brought into J-PARC, and the researchers ran their muonic X-ray analysis, which produced an energy spectrum (figure 4). In it, they found the elements needed to produce life, carbon, nitrogen and oxygen, but they also found the sample had a composition similar to that of carbonaceous chondrite (CI chondrite) asteroids, which are often referred to as the standard for solid substances in the Solar System. This showed the Ryugu stones were some of the earliest stones to have formed in our Solar System.
Figure 4: Muonic X-ray spectral comparison of asteroid Ryugu sample and CI chondrite Orgueil.
However, while similar in composition to CI chondrites, the Ryugu sample's oxygen abundance was about 25 per cent less than that of the CI chondrite. The researchers say this could indicate that the excess oxygen abundance in CI chondrites could have come from contamination after they entered Earth's atmosphere. Ryugu stones could become a new standard representative of the Solar System.
The team's results show the success of the muonic X-ray method, and that it can be used to analyze samples from future space missions.
Details of this study were published in Science on September 22.
Note : The Muon Analysis Team was formed to perform the muon analysis and belongs to the "Stone Material Analysis Team." Members include: KEK/J-PARC Materials and Life Science Experimental Facility Professor Emeritus Yasuhiro Miyake, Assistant Professor Izumi Umegaki, Assistant Professor Soshi Takeshita, Professor Koichiro Shimomura, Japan Atomic Energy Agency Chief Researcher Takahito Osawa, Osaka University Associate Professor Kazuhiko Ninomiya, Professor Kentaro Terada, Specially Appointed Researcher I-Huan Chiu, University of Tokyo Professor Tadayuki Takahashi, graduate student Shunsaku Nagasawa, Assistant Professor Shin'ichiro Takeda, Project Researcher Miho Katsuragawa, graduate student Takahiro Minami, Kyoto University Associate Professor Akihiro Taniguchi, International Christian University Professor Kenya Kubo, Japan Aerospace Exploration Agency Associate Professor Shin Watanabe, Tokyo University of the Arts part-time lecturer Kazumi Mizumoto, RIKEN Chief Scientist Toshiyuki Azuma, and Tohoku University Professor Tomoki Nakamura and graduate student Taiga Wada.
Group photo of the researchers (Credit: KEK IMSS)
The Japan Aerospace Exploration Agency (JAXA) has just completed the first year of its analytical campaign on samples returned from asteroid Ryugu. These cutting-edge studies have been undertaken by six sub-teams of the Initial Analysis Team and two Phase-2 curation teams. We are pleased to announce that a paper summarizing the research results of the "Stone Analysis Team" of the Hayabusa2 Initial Analysis Team has been published in the American scientific journal "Science" on September 23, 2022.
Title | Formation and evolution of the carbonaceous asteroid Ryugu: Direct evidence from return samples |
---|---|
Journal | Science |
DOI | 10.1126/science.abn8671 |
The samples from asteroid Ryugu returned to Earth by the asteroid explorer Hayabusa2 on December 6, 2020, initially underwent a cataloguing description (Phase-1 curation) at the facility established at JAXA's Institute of Space and Astronautical Science. Part of the returned sample was distributed to the "Hayabusa2 Initial Analysis Team", consisting of six sub-teams and two Phase-2 curation institutes. The initial analysis team is designed to reveal the multifaceted features of the sample through a plan of high-precision analysis, with specialized sub-teams assigned to tackle the science objectives of the Hayabusa2's mission. Meanwhile, the Phase-2 curation institutes have specific specialties that are utilized to catalog the samples based on a comprehensive analysis flow and clarify the potential impact of the sample through measurement and analysis appropriate to the characteristics of the returned particles.
Reports from the six teams involved in the initial analysis, as well as two Phase-2 curation institutes, will be announced separately as the results are published in scientific journals. After all the initial results have been released, a new overall summary of the Hayabusa2 science is planned.
∗ We found that Ryugu samples contain particles (such as Ca- and Al-rich inclusions 1) that were formed in high-temperature environments (>1000℃). These high-temperature particles are thought to have formed near the Sun and then migrated to the outer solar system, where Ryugu was formed. This indicates that large-scale mixing of materials occurred between the inner and outer solar system at the time of its birth.
∗ Based on the detection of the magnetic field left in the Ryugu samples, it is highly likely that the original asteroid from which the current Ryugu descended (Ryugu's parent body 2)) was born in the darkness of nebular gas 3), far from the Sun, where sunlight cannot reach.
∗ Liquid water trapped in a crystal in the sample was discovered. This water was carbonated water containing salts and organic matter, which was once present in the Ryugu parent body.
∗ Crystals shaped like coral reefs were growing from the liquid water that existed in the interior of Ryugu's parent body.
∗ In the parent body of Ryugu, the ratio of water to rock differed between the surface and the interior, with rocks deeper underground containing more water.
∗ The hardness, heat transfer, and magnetic properties of the samples were measured. The results showed that the Ryugu sample was soft enough to be cut with a knife. The sample also contained many small magnets, behaving like a natural hard-disk, recording the magnetic field of the past.
∗ A computer simulation of the process from the birth of the Ryugu parent body to its destruction by a catastrophic impact was performed. This is the first time that measurements of the hardness and thermal diffusivity of actual asteroid samples have been incorporated into a simulation of the formation and evolution of asteroids.
∗ The simulation shows that the Ryugu parent body accumulated about 2 million years after the formation of the solar system, and then heated up to about 50℃ over the next 3 million years, resulting in chemical reactions between water and rock The size of the impactor that destroyed the Ryugu parent body, which is about 100 km in diameter, is at most 10 km in diameter, and that the present-day Ryugu is composed of material from a region far from the impact point.
A research group led by Professor Tomoki Nakamura at the Graduate School of Science, Tohoku University analyzed samples from the asteroid Ryugu recovered by the asteroid explorer Hayabusa2 (17 particles including the third largest sample recovered by the explorer (Figure 1)) using cosmochemical and physical methods at universities and institutes including five synchrotron radiation facilities in Japan, United States, and Europe. As a result, the history of Ryugu from its formation to its collisional destruction (i. e., formation and location in the solar system, information on the source material, types of ice contained, chemical evolution through reactions with water on the surface and in the interior of the asteroid, effects of collisions, etc.) were determined. The Ryugu samples were found to contain a mixture of material near the surface of the parent body before impact destruction and material from the interior of the body. The hardness, heat transfer, specific heat, density, etc. of the Ryugu samples were measured, and numerical simulations of the temperature change due to heating in the interior of the parent body and the impact destruction process of the body were performed using these measured physical values to reproduce the formation evolution of Ryugu on a computer.
The formation history of Ryugu, as determined from the analysis of Ryugu samples, can be divided into the six phases shown below. Figure 2 show the results of the numerical simulation using the analysis results.
1. Formation of the Ryugu parent body,
2. Melting of ice due to decay heat of radioactive elements,
3. Progression of water-rock reactions due to further increase in the internal temperature of the body,
4. Cooling of the body due to depletion of radioactive elements,
5. Destruction of the parent body by a large-scale collision event,
6. Formation of Ryugu by reassembly of rock fragments generated by the collision.
The evidence for each of these stages of formation was obtained from Ryugu samples, as shown below.
Based on the remanent magnetization in the samples, it is highly likely that Ryugu's parent body was born in the primordial solar nebula3), which does not exist today. Ryugu was born in the darkness of nebular gas far from the sun, where sunlight cannot reach.
The Ryugu parent body was born at an extremely low temperature of -200℃ or lower. In that region, not only water ice but also dry ice (CO2 ice) existed. The Ryugu parent body was formed by incorporating the rock particles and ice that existed in the region.
We found particles (such as Ca- and Al-rich inclusions 1)) that were formed at high temperatures near the Sun (Fig. 3). The newly-born Ryugu contained, in addition to the low-temperature materials (ice and dry ice), a small amount of materials formed at high temperatures near the sun. These high-temperature particles are thought to have migrated from near the Sun to the outer solar system. This is evidence of a large-scale mixing of materials between the inner and outer solar system at the time of its birth.
The chemical composition of 10 particles of the Ryugu samples (126 mg in total) was determined for light elements using a muon4) beam (@J-PARC). The abundance of the light elements nitrogen and carbon is close to that of the most primitive meteorite (CI carbonaceous chondrites), indicating that the elemental abundance of Ryugu is very primitive.
The raw materials of Ryugu were ice and various aggregates of solid particles (Figures 2 and 4). These raw materials reacted with water and CO2 in the interior of the parent body (aqueous alteration) to form hydrous silicates and carbonate minerals that make up the majority of the sample.
The water temperature during aqueous metamorphism is estimated to have been approximately 25℃ based on the stability of the minerals formed during aqueous alteration.
Liquid water trapped in crystals in the sample was found (Figure 5). The water was held in micron-sized vacancies. Molecular species were determined by mass spectrometry, and the water was carbonated containing salts and organic matter.
The Ryugu sample was composed of small rock fragments (~1 mm in diameter). The diversity of minerals in these rock fragments can be explained by the different conditions for chemical reactions with water.
The rock fragments can be divided into two main types: materials formed in environments with a low water content (water/rock mass ratio 0.2) and materials formed in environments with a high water content (0.2 water/rock mass ratio 0.9). The former is rock fragments formed near the surface of the parent body where it was easy to cool and ice was difficult to melt (Figure 4), and the latter is considered to be material formed in the interior of the parent body. Therefore, the present-day Ryugu contains a mixture of materials from the surface and interior of its parent body.
In the interior of Ryugu's parent body, tabular coral-like crystals grew from liquid water (Fig. 6), suggesting that an environment similar to the Earth's oceans existed in Ryugu's interior.
Physical properties (hardness, heat transfer, specific heat, density, etc.) of the Ryugu samples were measured. The volume of the samples was precisely determined by synchrotron radiation CT analysis (@SPring-8) with a spatial resolution of less than 1 micron, and the mass was measured in an environment with no atmosphere to avoid the influence of adsorbed water on the samples. The average density of the sample was 1.79 ± 0.08 g/cm3, which is much higher than the density of the entire Ryugu asteroid (1.19 g/cm3). This suggests that interior of Ryugu contains more than 30% of pore spaces.
The hardness of Ryugu's samples is very low compared with that of igneous rocks on Earth, making them soft. The Ryugu stones were actually easily cut using a blade.
The Ryugu sample contains a large amount of magnetite, and a characteristic distribution of magnetic field lines (spiral magnetic domain structure: Figure 7) was confirmed in the interior of these crystals. This structure is more stable than ordinary hard disks and can record magnetic fields for more than 4.6 billion years. The magnetic fields inside and around magnetite record the magnetic field at the time when these crystals were formed, and it is highly likely that the solar nebula (with a magnetic field) was present when the Ryugu parent body was formed.
The authors have succeeded in reproducing the history of Ryugu from the birth of the parent body to the disruption of the parent body via a large-scale collision using a computer. This is the first time that the results of hardness and thermal diffusivity of an actual asteroid sample are used to simulate the formation and evolution of an asteroid.
A numerical simulation of the temperature change inside the asteroid due to the heat of decay of radioactive elements was performed. As a result, we were able to reproduce the process from the formation of the Ryugu parent body in an environment below -200℃ about 2 million years after the formation of the solar system, to the start of the water-rock reaction about 3 million years later, to the maximum temperature (~50℃) reached inside the body about 5 million years later, to the formation of the present Ryugu's constituent materials.
We performed numerical simulations of the collisional destruction of the parent body of Ryugu. It is thought that Ryugu once belonged to either the Polarna or Eularia family 5) of asteroid families 6), and that all asteroids belonging to these families were formed by the destruction of Ryugu's parent body. Based on this inference, the Ryugu parent body would have had a diameter of about 100 km. When another body with a diameter about 1/10th of the parent body collides with the parent body, the parent body is destroyed, forming a body with a maximum diameter of ~50 km (about the same size as Polana and Eularia) and numerous small rock fragments. The present-day Ryugu is thought to have been formed by the reassembly of some rock fragments produced by the collision.
Simulations of collisional disruption indicate that high pressures and temperatures are reached only near the epicenter of impact (only about 0.2 volume % of the parent body experiences impact pressures of 10 GPa or higher), and the majority of the parent body breaks up without experiencing high pressures and temperatures. We found little evidence of strong impacts in the Ryugu samples. This indicates that the rock fragments that formed the present-day Ryugu are materials that originated distant from the epicenter of the collision with Ryugu's parent body.
The present-day Ryugu is thought to have been formed through the above processes (Figure 2). Water-bearing asteroids such as Ryugu are more widely distributed in the solar system than water-free objects. This study shows how such asteroids formed, evolved, were collisionally destroyed to their present form in the low-temperature region outside Jupiter, far from the Sun. This has provided a pathway to solutions to some of the many unresolved questions regarding the formation of the solar system.
This research was supported by Grant-in-Aid for Scientific Research (20H00188 and 21H00159), of which Nakamura is a principal investigator, and many separate grants to coinvestigators by other agencies.
Fig.1(A) Optical micrograph of the largest sample C0002 analyzed and (B) CT view of the interior of the sample obtained by synchrotron radiation X-ray CT analysis at SPring-8. It can be seen that the entire sample is composed of fine-grained material (gray). (Credit: SPring-8, Tohoku Univ.)
Fig.2: Ryugu formation and evolution process inferred from the analysis of Ryugu samples. The temperature distribution, age, and collisional destruction process of the object were obtained by numerical simulation. (Credit: MIT, Chiba Tech, Tokyo Tech, Tohoku Univ.)
Figure 3: Particles formed in high temperature environments (>1000℃) found in the Ryugu samples (all images are taken by electron microscopes). (A, B) Ca- and Al-rich inclusions, (B-D) chondrules 7) consist of olivine (Ol), metallic iron (FeNi), and iron sulfide (FeS), (F) porous particles resembling amoeboid olivine aggregates. (Credit: Tohoku Univ)
Fig. 4: Rock fragments found in the C0002 sample that retain primitive features before aqueous alteration (images taken by electron microscopes). (A) Overall view of the fine-grained, porous rock fragment. (B) magnified view of a portion of the rock fragment. (C) elemental distribution in the same area as in B. Red particles indicate olivine or pyroxene, indicating that these minerals are abundant. (D) Overall view of a fine-grained, porous rock fragment. (E) magnified view of a portion of D. The main constituents are amorphous silicate and iron sulfide particles less than 1 micron in size (indicated as GEMS-like in the photograph), and olivine (Ol). (Credit: Tohoku Univ.)
Figure 5: Liquid consisting mainly of water and CO2 found inside a hexagonal iron sulfide crystal (iron sulfide) in a Ryugu sample. (A, B) CT images of vacancies in iron sulfide crystals. (C) Various ion species contained in the vacancies as measured by mass spectrometer (the two pictures of the same molecular species show the ion species contained in the upper part of the vacancy on the left and in the middle part on the right). The crystal temperature was set to -120℃ and the liquid in the vacancies was frozen for analysis. (D) After the analysis, the liquid in the vacancies was evaporated and the interior of the vacancies was observed. The results indicate that there are no solid components other than the liquid in the vacancy. (Credit: Tohoku Univ. NASA/JSC, SPring-8)
Figure 6: Crystals similar in shape to table corals found on the surface of the Ryugu sample (electron microscope image). The small, plate-like crystals are piled up to form the overall crystal. (Credit: Tohoku Univ.)
Figure 7: Paleomagnetic record remained in spherical magnetite (Fe3O4) crystals. (A) Transmission electron microscope image and (B, C) magnetic flux distribution images obtained by electron holography of magnetite cut from a Ryugu sample. Arrows and colors indicate the direction of magnetization. The concentric stripe pattern seen inside the particle indicates that the magnetic field wires wind in the direction of the arrow (called a spiral magnetic domain structure). Magnetic field wires seen on the outside of the particle are leakage fields from the particle, reflecting the magnetic field environment of Ryugu when the interior of the Ryugu parent body heated up and aqueous alteration reactions occurred. (Credit: Hokkaido Univ., Tohoku Univ.)
1) Ca,Al-rich inclusions
The oldest solid particles in the solar system. It is thought to have been formed by condensation from high-temperature nebular gas near the sun during the formation of the solar system.
2) Ryugu parent body
Original asteroid Ryugu at the time of its birth. The diameter is thought to have been about 100 km. This parent body was destroyed to form the current Ryugu.
3) Primordial solar nebula, nebular gas
A disk of gas surrounding the sun that is thought to have existed in the solar system 4.5 billion years ago. It does not exist in the present solar system and is thought to have disappeared early in the formation of the solar system.
4) Muons
Negatively charged particles with a mass about 200 times that of an electron.
5) Asteroid family
Asteroid families are groups of asteroids with similar intrinsic orbital elements such as orbital radius, eccentricity, and inclination. Asteroids belonging to the same family are considered to be a debris group formed by collisional destruction of a common parent body.
6) Sugita et al, (2019) Science 364, eaaw0442. doi
10.1126/science.aaw0422/
7) Chondrules
spherical or near-spherical morphology particles that are abundant in meteorites of asteroidal origin. They are thought to have been formed by rapid cooling after heating to more than 1200℃ in the solar nebula.
Journal Title | Science |
---|---|
DOI | 10.1126/science.abn8671 |
Title of paper | Formation and evolution of carbonaceous asteroid Ryugu: Direct evidence from returned samples |
Authors | T. Nakamura1, M. Matsumoto1, K. Amano1, Y. Enokido1, M. E. Zolensky2, T. Mikouchi3, H. Genda4, S. Tanaka5,6, M. Y. Zolotov7, K. Kurosawa8, S. Wakita9, R. Hyodo5, H. Nagano10, D. Nakashima1, Y. Takahashi11,12, Y. Fujioka1, M. Kikuiri1, E. Kagawa1, M. Matsuoka13,14, A. J. Brearley15, A. Tsuchiyama16,17,18, M. Uesugi19, J. Matsuno16, Y. Kimura20, M. Sato11, R. E. Milliken21, E. Tatsumi22,11, S. Sugita11,8, T. Hiroi21, K. Kitazato23, D. Brownlee24, D. J. Joswiak24, M. Takahashi1, K. Ninomiya25, T. Takahashi26,27, T. Osawa28, K. Terada29, F. E. Brenker30, B. J. Tkalcec30, L. Vincze31, R. Brunetto32, A. Aléon-Toppani32, Q. H. S. Chan33, M. Roskosz34, J.-C. Viennet34, P. Beck35, E. E. Alp36, T. Michikami37, Y. Nagaashi38,1, T. Tsuji39,40, Y. Ino41,5, J. Martinez2, J. Han42, A. Dolocan43, R. J. Bodnar44, M. Tanaka45, H. Yoshida11, K. Sugiyama46, A. J. King47, K. Fukushi48, H. Suga49, S. Yamashita50,51, T. Kawai11, K. Inoue48, A. Nakato5, T. Noguchi52,53, F. Vilas54, A. R. Hendrix54, C. Jaramillo-Correa55, D. L. Domingue54, G. Dominguez56, Z. Gainsforth57, C. Engrand58, J. Duprat34, S. S. Russell47, E. Bonato59, C. Ma60, T. Kawamoto61, T. Wada1, S. Watanabe5,26, R. Endo62, S. Enju63, L. Riu64, S. Rubino32, P. Tack31, S. Takeshita65, Y. Takeichi50,51,66, A. Takeuchi19, A. Takigawa11, D. Takir2, T. Tanigaki67, A. Taniguchi68, K. Tsukamoto1, T. Yagi69, S. Yamada70, K. Yamamoto71, Y. Yamashita69, M. Yasutake19, K. Uesugi19, I. Umegaki72,65, I. Chiu25, T. Ishizaki5, S. Okumura52, E. Palomba73, C. Pilorget32,74, S. M. Potin13,75, A. Alasli10, S. Anada71, Y. Araki76, N. Sakatani70,5, C. Schultz21, O. Sekizawa49, S. D. Sitzman77, K. Sugiura4, M. Sun17,18,78, E. Dartois79, E. De Pauw31, Z. Dionnet32, Z. Djouadi32, G. Falkenberg80, R. Fujita10, T. Fukuma81, I. R. Gearba43, K. Hagiya82, M. Y. Hu36, T. Kato71, T. Kawamura83, M. Kimura50,51, M. K. Kubo84, F. Langenhorst85, C. Lantz32, B. Lavina86, M. Lindner30, J. Zhao36, B. Vekemans31, D. Baklouti32, B. Bazi31, F. Borondics87, S. Nagasawa26, 27, G. Nishiyama11, K. Nitta49, J. Mathurin88, T. Matsumoto52, I. Mitsukawa52, H. Miura89, A. Miyake52, Y. Miyake65, H. Yurimoto90, R. Okazaki91, H. Yabuta92, H. Naraoka91, K. Sakamoto5, S. Tachibana11,5, H. C. Connolly Jr.93, D. S. Lauretta94, M. Yoshitake5, M. Yoshikawa5,6, K. Yoshikawa95, K. Yoshihara5, Y. Yokota5, K. Yogata5, H. Yano5,6, Y. Yamamoto5,6, D. Yamamoto5, M. Yamada8, T. Yamada5, T. Yada5, K. Wada8, T. Usui5,11, R. Tsukizaki5, F. Terui96, H. Takeuchi5,6, Y. Takei5, A. Iwamae97, H. Soejima5,97, K. Shirai5, Y. Shimaki5, H. Senshu8, H. Sawada5, T. Saiki5, M. Ozaki5,6, G. Ono95 T. Okada5,98, N. Ogawa5, K. Ogawa5, R. Noguchi99, H. Noda100, M. Nishimura5, N. Namiki100,6, S. Nakazawa5, T. Morota11, A. Miyazaki5, A. Miura5, Y. Mimasu5, K. Matsumoto100,6, K. Kumagai5,97, T. Kouyama101, S. Kikuchi8,100, K. Kawahara5, S. Kameda70,5, T. Iwata5,6, Y. Ishihara102, M. Ishiguro103, H. Ikeda95, S. Hosoda5, R. Honda104,105, C. Honda23, Y. Hitomi5,97, N. Hirata38, N. Hirata23, T. Hayashi5, M. Hayakawa5, K. Hatakeda5,97, S. Furuya11, R. Fukai5, A. Fujii5, Y. Cho11, M. Arakawa38, M. Abe5,6, S. Watanabe106, Y. Tsuda5. |
Affiliations |
1Department of Earth Sciences, Tohoku University, Sendai 980-8578, Japan. 2NASA Johnson Space Center; Houston TX 77058, USA. 3The University Museum, The University of Tokyo, Tokyo 113-0033, Japan. 4Earth-Life Science Institute, Tokyo Institute of Technology, Tokyo 152-8550, Japan. 5Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency (JAXA), Sagamihara 252-5210, Japan. 6Department of Space and Astronautical Science, The Graduate University for Advanced Studies (SOKENDAI), Hayama 240-0193, Japan. 7School of Earth and Space Exploration, Arizona State University, Tempe AZ 85287, USA. 8Planetary Exploration Research Center, Chiba Institute of Technology, Narashino 275-0016, Japan. 9Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge MA 02139, USA. 10Department of Mechanical Systems Engineering, Nagoya University, Nagoya 464-8603, Japan. 11Department of Earth and Planetary Science, The University of Tokyo, Tokyo 113-0033, Japan. 12Isotope Science Center, The University of Tokyo, Tokyo 113-0032, Japan. 13Laboratoire d'Etudes Spatiales et d'Instrumentation en Astrophysique (LESIA), Observatoire de Paris, Meudon 92195 France. 14Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, 305-8567, Japan. 15Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque NM 87131, USA. 16Research Organization of Science and Technology, Ritsumeikan University, Kusatsu 525-8577, Japan. 17Chinese Academy of Sciences (CAS) Key Laboratory of Mineralogy and Metallogeny, Guangdong Provincial Key Laboratory of Mineral Physics and Materials, Guangzhou Institute of Geochemistry, CAS, Guangzhou 510640, China. 18CAS Center for Excellence in Deep Earth Science, Guangzhou 510640, China. 19Scattering and Imaging Division, Japan Synchrotron Radiation Research Institute, Sayo 679-5198, Japan. 20Institute of Low Temperature Science, Hokkaido University,Sapporo 060-0819, Japan. 21Department of Earth, Environmental, and Planetary Sciences, Brown University, Providence, RI 02912, USA. 22Instituto de Astrofísica de Canarias, University of La Laguna, Tenerife 38205, Spain. 23Aizu Research Center for Space Informatics, The University of Aizu, Aizu-Wakamatsu 965-8580, Japan. 24Department of Astronomy, University of Washington, Seattle WA 98195 USA. 25Institute for Radiation Sciences, Osaka University, Toyonaka 560-0043, Japan. 26Kavli Institute for the Physics and Mathematics of the Universe (The World Premier International Research Center Initiative), The University of Tokyo, Kashiwa 277-8583, Japan. 27Department of Physics, The University of Tokyo, Tokyo 113-0033, Japan. 28Materials Sciences Research Center, Japan Atomic Energy Agency, Tokai 319-1195, Japan. 29Department of Earth and Space Science, Osaka University; Toyonaka 560-0043, Japan. 30Institute of Geoscience, Goethe University, Frankfurt, 60438 Frankfurt am Main, Germany. 31Department of Chemistry, Ghent University, Krijgslaan 281 S12, Ghent, Belgium. 32Institut d'Astrophysique Spatiale, Université Paris-Saclay, Orsay 91405, France. 33Department of Earth Sciences, Royal Holloway University of London, Egham TW20 0EX, UK. 34Institut de Minéralogie, Physique des Matériaux et Cosmochimie, Muséum National d'Histoire Naturelle, Centre national de la recherche scientifique (CNRS), Sorbonne Université, Paris, France. 35Institut de Planétologie et d'Astrophysique de Grenoble, CNRS, Université Grenoble Alpes, 38000 Grenoble, France. 36Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439, USA. 37Faculty of Engineering, Kindai University, Higashi-Hiroshima 739-2116, Japan. 38Department of Planetology, Kobe University, Kobe 657-8501, Japan. 39Department of Earth Resources Engineering, Kyushu University, Fukuoka 819-0395, Japan. 40School of Engineering, The University of Tokyo, Tokyo 113-0033, Japan. 41Department of Physics, Kwansei Gakuin University, Sanda 669-1330, Japan. 42Department of Earth and Atmospheric Sciences, University of Houston, Houston TX 77204, USA. 43Texas Materials Institute, The University of Texas at Austin, Austin TX 78712, USA. 44Department of Geoscience, Virginia Tech., Blacksburg VA 24061, USA. 45Materials Analysis Station, National Institute for Materials Science, Tsukuba 305-0047, Japan. 46Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan. 47Department of Earth Science, Natural History Museum, London SW7 5BD, UK. 48Institute of Nature and Environmental Technology, Kanazawa University, Kanazawa 920-1192, Japan. 49Spectroscopy Division, Japan Synchrotron Radiation Research Institute, Sayo 679-5198, Japan. 50Department of Materials Structure Science, The Graduate University for Advanced Studies (SOKENDAI), Tsukuba, Ibaraki 305-0801, Japan. 51Institute of Materials Structure Science, High-Energy Accelerator Research Organization, Tsukuba 305-0801, Japan. 52Division of Earth and Planetary Sciences, Kyoto University; Kyoto 606-8502, Japan. 53Faculty of Arts and Science, Kyushu University, Fukuoka 819-0395, Japan. 54Planetary Science Institute, Tucson AZ 85719, USA. 55The Pennsylvania State University, University Park, PA 16802, USA. 56Department of Physics, California State University, San Marcos, CA 92096, USA. 57Space Sciences Laboratory, University of California, Berkeley, California 94720, USA. 58Laboratoire de Physique des 2 Infinis Irène Joliot-Curie, Université Paris-Saclay, CNRS, 91405 Orsay, France. 59Institute for Planetary Research, Deutsches Zentrum für Luftund Raumfahrt, Rutherfordstraße 2 12489 Berlin, Germany. 60Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena CA 91125, USA. 61Department of Geosciences, Shizuoka University, Shizuoka 422-8529, Japan. 62Department of Materials Science and Engineering, Tokyo Institute of Technology, Tokyo 152-8550, Japan. 63Graduate School of Science and Engineering, Ehime University, Matsuyama 790-8577, Japan. 64European Space Astronomy Centre, 28692 Villanueva de la Cañada, Spain. 65High Energy Accelerator Research Organization, Tokai 319-1106, Japan. 66Department of Applied Physics, Osaka University, Suita, 565-0871, Japan. 67Hitachi, Ltd., Hatoyama 350-0395, Japan. 68Institute for Integrated Radiation and Nuclear Science, Kyoto University, Kumatori 590-0494, Japan. 69National Metrology Institute of Japan, AIST, Tsukuba 305-8565, Japan. 70Department of Physics, Rikkyo University, Tokyo 171-8501, Japan. 71Japan Fine Ceramics Center, Nagoya 456-8587, Japan. 72Toyota Central Research and Development Laboratories, Inc., Nagakute 480-1192, Japan. 73Istituto di Astrofisica e Planetologia Spaziali, Istituto Nazionale di Astrofisica, Rome 00133, Italy. 74Institut Universitaire de France, Paris, France. 75Faculty of Aerospace Engineering, Delft University of Technology, Delft, The Netherlands. 76Department of Physical Sciences, Ritsumeikan University, Shiga 525-0058, Japan. 77Physical Sciences Laboratory, The Aerospace Corporation, California 90245, USA. 78University of Chinese Academy of Sciences, Beijing 100049, China. 79Institut des Sciences Moléculaires d'Orsay, Université Paris-Saclay, CNRS, 91405 Orsay, France. 80Deutsches Elektronen-Synchrotron Photon Science, 22603 Hamburg, Germany. 81Nano Life Science Institute (The World Premier International Research Center Initiative), Kanazawa University, 920-1192, Japan. 82Graduate School of Life Science, University of Hyogo, Hyogo 678-1297, Japan. 83Institut de Physique du Globe de Paris, Université de Paris, Paris 75205, France. 84Division of Natural Sciences, International Christian University, Mitaka 181-8585, Japan. 85Institute of Geosciences, Friedrich-Schiller-Universität Jena, 07745 Jena, Germany. 86Center for Advanced Radiation Sources, The University of Chicago, Chicago, IL 60637, USA. 87Optimized Light Source of Intermediate Energy to LURE (SOLEIL) Synchrotron, L'Orme des Merisiers, Gif sur Yvette Cedex, F-91192, France. 88Institut Chimie Physique, Université Paris-Saclay, CNRS, 91405 Orsay, France. 89Graduate School of Science, Nagoya City University, Nagoya 467-8501, Japan. 90Department of Natural History Sciences, Hokkaido University, Sapporo 060-0810, Japan. 91Department of Earth and Planetary Sciences, Kyushu University, Fukuoka 819-0395, Japan. 92Graduate School of Advanced Science and Engineering, Hiroshima University, Higashi-Hiroshima 739-8526, Japan. 93Department of Geology, Rowan University, Glassboro NJ 08028, USA. 94Lunar and Planetary Laboratory, University of Arizona; Tucson AZ 85721, USA. 95Research and Development Directorate, JAXA, Sagamihara 252-5210, Japan. 96Department of Mechanical Engineering, Kanagawa Institute of Technology, Atsugi 243-0292, Japan. 97Marine Works Japan Ltd., Yokosuka 237-0063 Japan. 98Department of Chemistry, The University of Tokyo, Tokyo 113-0033, Japan. 99Faculty of Science, Niigata University, Niigata 950-2181, Japan. 100National Astronomical Observatory of Japan, Mitaka 181-8588, Japan. 101Digital Architecture Research Center, National Institute of Advanced Industrial Science and Technology, Tokyo 135-0064, Japan. 102JAXA Space Exploration Center, JAXA, Sagamihara 252-5210, Japan. 103Department of Physics and Astronomy, Seoul National University, Seoul 08826, Korea. 104Department of Information Science, Kochi University, Kochi 780-8520, Japan. 105Center for Data Science, Ehime University, Matsuyama 790-8577, Japan. 106Department of Earth and Environmental Sciences, Nagoya University, Nagoya 464-8601, Japan. |
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∗ Institute of Materials Structure Science, High Energy Accelerator Research Organization (IMSS-KEK) has been conducting experiments utilizing muons that are produced by irradiating proton beams on a graphite muon target at the Japan Proton Accelerator Research Complex (J-PARC).
∗ To cope with the world's most intense pulsed proton beam, a rotating target was developed under collaboration with Paul Scherrer Institute (PSI) in Switzerland, which had developed the target in advance. However, the target was suffering from the fact that the bearings supporting the rotating body would fail within a year.
∗ The group in IMSS-KEK achieved long-term operation of the high-intensity beam from 2014 to 2019 by adopting a new kind of bearing that can achieve a longer life.
∗ The same bearings were also used at PSI, and consequently one year of stable continuous operation was achieved in December 2021.
Institute of Materials Structure Science, High Energy Accelerator Research Organization (IMSS-KEK) has been conducting research in materials life science and fundamental physics by using muon beams of the world's highest quality and intensity. The muon beams are produced by irradiating proton beams on a graphite muon target at Japan Proton Accelerator Research Complex (J-PARC). Under the technical collaboration with Paul Scherrer Institute (PSI) of Switzerland, which had developed the target in advance, the development of a rotating target that can withstand high-intensity proton beam has been promoted at J-PARC as well. However, the bearings that support the rotating body fail within a year in Target E at the world most intense continuous proton beam facility HIPA of PSI, requiring the experiments to be stopped and the target to be replaced. Therefore, the group in IMSS-KEK developed the rotating target taking advantage of a new kind of bearing that could achieve a longer life. By adopting these bearings, the long-term operation from 2014 to 2019 was realized. Based on these results, the same bearings were adopted at PSI, and in December 2021, the stable continuous operation period reached one year, the best after a long period of several bearing failures at PSI. J-PARC and PSI, the world's two major muon facilities, are good rivals in science, and at the same time, they are striving to improve their technologies through mutual collaboration. This technology can be applied to various rotating target in the world's high-intensity accelerators.
Currently, 17 elementary particles are known, but the most familiar ones are photons and electrons, (protons and neutrons, which create matter, are also familiar, but they are composite particles made from quarks and are not elementary particles).
A Muon is an elementary particle classified as a lepton in the same family of an electron and was discovered in cosmic rays in 1936. Although the muon is about 200 times more massive than the electron, they have similar properties, such as having the same negative electric charge as the electron.
So far, we use photons and electrons in various purposes, and muons are also beginning to be utilized by taking advantage of their properties. For instance, muons contained in cosmic rays are used to see through the inside of pyramids and nuclear reactors because of their high transparency. At J-PARC, a world-class proton accelerator artificially produces large quantities of muons for research in material and life sciences and fundamental physics. Recently, it became a popular topic that components enclosed in "Kouan Ogata's medicine box", an ancient medicine box out of glass, was nondestructively identified by the muons produced at J-PARC.
There are technical challenges in keeping producing large quantities of muons for a long period in the high-intense proton accelerators. Muons are produced by the immediate decay of pions created by irradiation of the proton beam into a 20 mm thick graphite muon target. Normally, the beam path around the muon target is kept in an airless vacuum to prevent the proton beam, pions, and muon beams (Note 1) from colliding with air and being lost. Simultaneously, heat is generated on the muon target when exposed to the proton beam, and since there is no air to release the generated heat, it is important to find a way to cool the target.
In the Materials and Life Science Experimental Facility (MLF), IMSS-KEK is in charge of the development of the graphite muon target that produces muons. When the beam operation started in 2008, a fixed target is applied for the muon target (Figure 1 left). In the fixed target, since a graphite disk with a diameter of 70 mm is bonded to a copper frame, the heat on the center of the muon target is removed by thermal conduction in the graphite. J-PARC aims to increase the intensity of the proton beam to obtain more muons, and when the beam reaches its current designed intensity of 1 MW, 3.3 kW of heat and irradiation damage will occur on the graphite. This irradiation damage results in the loss of the graphite's ability to conduct heat and in localized shrinkage of the graphite. Due to this damage, the muon target is anticipated to fail within one year in the fixed target. Once the muon target fails, the MLF must be shut down for more than three weeks to replace it. Therefore, it was necessary to extend the lifetime of the muon target to continue the MLF experiments in a stable manner.
The problem of graphite being damaged by proton beam irradiation can be solved by avoiding irradiating the same spot with proton beams. However, the proton beam must pass through the exact center of the beamline, and it is not easy to move the proton beam itself. For this reason, a method called a rotating target has been introduced. In the rotating target, the proton beam position is not moved, but the graphite itself is rotated to disperse the damage on the graphite over a wide area (Fig. 1, right). Similarly, the heat is widely dispersed and is cooled by thermal radiation (emission of light).
PSI has been conducting research using muons as well as J-PARC and has been using a rotating graphite target as a meson target (Note 2) since 1985, before the construction of the J-PARC MLF. The members developing the muon target at J-PARC have visited the meson facility at PSI since the construction of the J-PARC MLF and have referred shared information from many researchers and engineers for development of the proton beamline and the muon target. PSI has been operating two types of rotating targets, Target M and Target E. At that time, the Target E, which is a high-intensity target, had a problem that the bearing supporting the rotation body broke down within a year, and the experiments had to be stopped and the target had to be replaced.
Bearings supporting rotating body are composed of an outer ring, an inner ring and multiple balls between the rings. Lubricants are used to reduce friction between the balls and the rings. Normally, organic functional material, grease, which has properties between those of a solid and a liquid, is used as a lubricant. However, the grease cannot be used for bearings in rotating targets because it releases a lot of gas into the vacuum at high temperature and is affected by high radiation environment due to proton beam irradiation. Therefore, it is necessary to use solid lubricant made of inorganic materials that are less affected by irradiation and release less gas. The life of a rotating target bearing is greatly affected by the type and shape of the solid lubricant, and PSI has used bearings coated with molybdenum disulfide or silver as solid lubricants for inner and outer rings, balls, and a ball-separator (Figure 2 left). However, if the coatings peel off, the bearings lose their lubricating performance. Therefore, J-PARC adopted a bearing (JTEKT Corporation: WS bearing) in which lumps of tungsten disulfide (bottom right of Fig. 2), an inorganic material, is inserted between the balls (top right of Fig. 2). In this case, the amount of lubricant is far greater than in the case of coating, and thus a longer life can be expected.
The bearings selected in this way were assembled into a prototype of the rotating target for validation, where they underwent a series of rotational tests with heating to high temperatures in a vacuum. According to the results of these rotational tests, it was decided to use bearings specially designed for the J-PARC MLF with an improved solid lubricant shape. Finally, the rotating target #1 was installed in the proton beamline in 2014 and had stably operated for 5 years. The rotating target #2, which was replaced in 2019, has achieved stable operation with a beam intensity of 0.83 MW until now in 2022, while the beam intensity has been gradually increased. It also successfully operated at the beam power of 1 MW for 32 hours. Because of the stable operational record of this bearing at J-PARC, we offered PSI to adopt the bearing with the same lubricant and geometry, and after two years of rotational tests, the Target E implementing the bearings was installed in the beamline. As a result, in December 2021, one year of stable operation was achieved, which is the best after a long period of several bearing failures at Target E of PSI. This achievement has given us the prospect of providing muon beams for experiments without losing beamtime.
J-PARC and PSI, the world's two major muon facilities, are good rivals in science, and at the same time they are striving to improve new technologies through mutual collaboration. This technology can be applied to rotating targets used in accelerators around the world that are aiming for higher intensities.
It can realize continuous production for large quantities of muons in the future project, which is expected to accelerate research in material life science, particle physics, and other fields.
[1] S. Makimura et al., "Muon production target at J-PARC MLF", J. Particle Accelerator Society of Japan, Vol. 18, NO. 4, 2021, p202-209, written in Japanese.
[2] S. Makimura, et al., "Perspective of Muon Production Target at J- PARC MLF MUSE", Proceedings of the 14th International Conference on Muon Spin Rotation, Relaxation and Resonance (μSR2017), JPS Conf. Proc. 21, 011058 (2018)
DOI: 10.7566/JPSCP.21.011058
[3] Things run smoother without lubricants
https://www.psi.ch/en/science/scientific-highlights/things-run-smoother-without-lubricants
[4] D Kiselev, et al., "The Meson Production Targets in the high energy beamline of HIPA at PSI", SciPost Phys. Proc. 5, 003 (2021)
DOI: 10.21468/SciPostPhysProc.5.003
J-PARC and PSI, the world's two major muon facilities, are good rivals in science, and at the same time they are striving to improve new technologies through mutual collaboration. This technology can be applied to rotating targets used in accelerators around the world that are aiming for higher intensities.
It can realize continuous production for large quantities of muons in the future project, which is expected to accelerate research in material life science, particle physics, and other fields.
(Note 1) In the following, "pions and muons" are abbreviated as "muons" if there is no need to distinguish between them.
(Note 2) According to category of pions, which are the source of muons, PSI call them meson targets.
Figure 1: Fixed target (left) and rotating target (right). The rotating target disperses the damage and heat generated by the proton beam. The bearings supporting the rotating body determine the life of the target.
Figure 2: Conventional bearings using solid lubricants are coated with molybdenum disulfide or silver on the balls, an outer ring, an inner ring, and a ball separator (left); J-PARC uses a bearing with lumps of tungsten disulfide (bottom right) inserted between the balls (top right).
"We knew that chromium possesses strong corrosive resistance, but the superelasticity, flexibility, and significant wear resistance of the cobalt-chromium-based material surprised us," added Xu.
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Details of their research were published in the journal Advanced Materials on May 9, 2022.
With the elderly population increasing across the globe, the need for improved biomaterials that can replace or support damaged bones has risen. For this purpose, metals are widely used because of their strength and ductility. However, as a consequence of their strength, their flexibility diminishes.
To date, most metallic biomaterials are stiffer than human bones, and using them as implants leads to bone atrophy-a condition where bone density is reduced because of a breakdown in bone substance and structure. Meanwhile, biomaterials with elevated flexibility lose their wear resistance.
Although superelastic materials made from nickel-titanium (Ni-Ti) alloys, which are commonly used in stents and orthodontic wires, maintain high flexibility and the ability to recover from strain, Ni is an allergic element. Ni-free alloys have not replicated the superelasticity of Ni-Ti alloys, rendering them impractical.
The research group, which comprised researchers from Tohoku University's Graduate School of Engineering and Institute for Materials Research (IMR), the J-PARC Center, the Japan Atomic Energy Agency, and the Czech Academy of Sciences, focused on lessening the Young's modulus gap between metal implants and human bones. When a material is flexible, it has a low Young's modulus. When it is stiff, it has a high Young's modulus.
"Since the Young's modulus hinges on crystal orientation, we grew single crystals with a specific crystal orientation," said Xiao Xu, corresponding author and assistant professor at Tohoku University's Graduate School of Engineering.
<Figure 1> Single crystals of the flexible and tough CoCr-based alloys. ©Xiao Xu et al.
Using a cyclical heat treatment technique, Xu and his colleagues successfully prepared large single crystals sized several centimeters. The developed Co-Cr-Al-Si (CCAS) alloy demonstrated a 17% strain recovery rate-twice that of commercial Ti-Ni shape memory alloys. Moreover, the CCAS's Young's modulus was extremely low, resembling the flexibility of human bones.
"We knew that chromium possesses strong corrosive resistance, but the superelasticity, flexibility, and significant wear resistance of the cobalt-chromium-based material surprised us," added Xu.
<Figure 2> The novel Co-Cr-based biomaterial not only has a low Young's modulus (10-30 GPa) similar to human bones, but also has high wear resistance, disrupting the trade-off relation in conventional metallic biomaterials. These alloys also show a huge recoverable superelastic strain up to 17.0%-twice that of commercial Ti-Ni. ©Takumi Odaira et al.
Title | Flexible and Tough Superelastic Co-Cr Alloys for Biomedical Applications |
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Authors | Takumi Odaira, Sheng Xu, Kenji Hirata, Xiao Xu, Toshihiro Omori, Kosuke Ueki, Kyosuke Ueda, Takayuki Narushima, Makoto Nagasako, Stefanus Harjo, Takuro Kawasaki, Lucie Bodnárová, Petr Sedlák, Hanuš Seiner, Ryosuke Kainuma |
Journal | Advanced Materials |
DOI | 10.1002/adma.202202305 |
Press release in Japanese
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A large, unconventional anomalous Hall resistance in a new magnetic semiconductor in the absence of large-scale magnetic ordering has been demonstrated by Tokyo Tech materials scientists, validating a recent theoretical prediction. Their findings provide new insights into the anomalous Hall effect, a quantum phenomenon that has previously been associated with long-range magnetic order .
Charged particles such as electrons can behave in interacting ways when moving under the influence of electric and magnetic fields. For instance, when a magnetic field is applied perpendicular to the plane of a current-carrying conductor, the electrons flowing within start to deviate sideways due to magnetic force and soon enough, a voltage difference appears across the conductor. This phenomenon is famously called the "Hall effect." However, the Hall effect does not necessarily require fiddling with magnets. In fact, it can be observed in magnetic materials with long-range magnetic order, such as ferromagnets, for free!
Named "anomalous Hall effect" (AHE), this phenomenon appears to be a close cousin of the Hall effect. However, its mechanism is way more involved. Currently, the most accepted one is that the AHE is produced by a property of the electronic energy bands called "Berry curvature," which results from an interaction between the electron's spin and its motion inside the material, more commonly known as "spin-orbit interaction."
Is magnetic ordering necessary for AHE? A recent theory suggests otherwise. "It has been theoretically proposed that a large AHE is possible even above the temperature at which the magnetic order vanishes, especially in magnetic semiconductors with low charge carrier density, strong exchange interaction between electrons, and finite spin chirality, which relates to the spin direction with respect to the direction of motion," explains Associate Professor Masaki Uchida from Tokyo Institute of Technology (Tokyo Tech), whose research focus lies in condensed matter physics.
Curious, Dr. Uchida and his collaborators from Japan decided to put this theory to the test. In a new study published in Science Advances, they investigated the magnetic properties of a new magnetic semiconductor EuAs that is only known to have a peculiar distorted triangular lattice structure and observed an antiferromagnetic (AFM) behavior (neighboring electron spins aligned in opposite directions) below 23 K. Furthermore, they observed that the material's electrical resistance dropped dramatically with temperature in the presence of an external magnetic field, a behavior known as "colossal magnetoresistance" (CMR). However, more interestingly, the CMR was observed even above 23 K, where the AFM order vanished.
"It is naturally understood that the CMR observed in EuAs is caused by a coupling between the diluted carriers and localized Eu2+ spins that persist over a wide range of temperatures," comments Dr. Uchida.
What really stole the show, however, was the rise in Hall resistivity with temperature, which peaked at a temperature of 70 K, far above the AFM ordering temperature, demonstrating that large AHE was indeed possible without magnetic order. To understand what caused this unconventionally large AHE, the team performed model calculations, which showed that the effect could be attributed to a skew scattering of electrons by a spin cluster on the triangular lattice in a "hopping regime" where the electrons did not flow but rather "hopped" from atom to atom.
These results bring us one step closer to understanding the strange behavior of electrons inside magnetic solids. "Our findings have helped shed light on triangular-lattice magnetic semiconductors and could potentially lead to a new field of research targeting diluted carriers coupled to unconventional spin orderings and fluctuations," comments an optimistic Dr. Uchida.
Indeed, new discoveries in the endlessly fascinating quantum world of electrons might be on the horizon!
Authors | Masaki Uchida,1,2,3,* Shin Sato,2 Hiroaki Ishizuka,1 Ryosuke Kurihara,4,5 Taro Nakajima,4 Yusuke Nakazawa,2 Mizuki Ohno,1,2 Markus Krienersup,5 Atsushi Miyake,4 Kazuki Ohishi,7 Toshiaki Morikawa,7 Mohammad Saeed Bahramy,2 Taka-hisa Arima,5,6 Masashi Tokunaga,4,5 Naoto Nagaosa,2,5 Masashi Kawasaki2,5 |
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Title of original paper: | Above-ordering-temperature large anomalous Hall effect in a triangular-lattice magnetic semiconductor |
Journal | Science Advances |
DOI | 10.1126/sciadv.abl5381 |
Affiliations | 1 Department of Physics, Tokyo Institute of Technology 2 Department of Applied Physics and Quantum-Phase Electronics Center (QPEC) , the University of Tokyo 3 PRESTO, Japan Science and Technology Agency (JST) 4 Institute for Solid State Physics, the University of Tokyo 5 RIKEN Center for Emergent Matter Science (CEMS) 6 Department of Advanced Materials Science, University of Tokyo 7 Neutron Science and Technology Center, Comprehensive Research Organization for Science and Society (CROSS) |
*Corresponding author's email: m.uchida[at]phys.titech.ac.jp
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Tokyo Tech stands at the forefront of research and higher education as the leading university for science and technology in Japan. Tokyo Tech researchers excel in fields ranging from materials science to biology, computer science, and physics. Founded in 1881, Tokyo Tech hosts over 10,000 undergraduate and graduate students per year, who develop into scientific leaders and some of the most sought-after engineers in industry. Embodying the Japanese philosophy of "monotsukuri," meaning "technical ingenuity and innovation," the Tokyo Tech community strives to contribute to society through high-impact research.
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An international research group [see latter:【International research team】] led by Senior Professor Kazuma Nakazawa and Postdoctoral Fellow Masahiro Yoshimoto (Graduate School of Education and Engineering, Gifu University), Assistant Professor Junya Yoshida (Graduate School of Science, Tohoku University) and Professor Toshiyuki Takahashi (Institute of Particle Physics and Nuclear Studies (IPNS), High Energy Accelerator Research Organization (KEK)) has observed of a novel hypernucleus called the Xi nucleus. The Xi nucleus contains a Xi-minus particle (i.e. negatively charged Xi particle) 1 ) with two strange quarks 1 ) (Fig. 1). This Xi nucleus was produced in the Japan Proton Accelerator Research Complex (J-PARC) E07 experiment. The event has been named the Irrawaddy event after the majestic river in Myanmar, the home country of the Gifu University student who detected it. Analysis shows that the Irrawaddy event was produced by the strong attraction (i.e. high binding energy) between a Xi-minus particle and a nitrogen-14 nucleus. Further analysis indicates that the binding energy was five times that of an event announced by the present research group in February 2021 the (Ibuki event [*]). The Irrawaddy observation confirmed the attraction between the Xi particle and the nitrogen-14 nucleus due to the 'strong interaction' 2 ) (i.e. nuclear force), which is much stronger than the electromagnetic interaction (i.e. Coulomb force). Furthermore, successful observation of the inner level structure 3 ) of the Xi nucleus was achieved for the first. The discovery of the Xi nucleus was unexpected according to many current theories, and it is hoped that this discovery will lead to the development of a new theory in the future. Since strange-quark-containing hyperons 1 ) such as Xi-minus particles are thought to appear in the cores of neutron stars 4 ), which are the densest objects in the universe and are considered giant nucleus, the results of this research will play an important role in our understanding of neutron stars.
The results of this research will be published in the online journal : Progress of Theoretical and Experimental Physics (PTEP) on 23rd July, 2021.
Figure 1. Micrograph and images of a new Xi nuclear event (Irrawaddy event) observed in a photographic emulsion sheet. A Xi-minus (Ξ-) particle was absorbed by a nitorogen-14 nucleus at point A to form Xi nucleus, which decayed into two helium-5-Lambda nuclei (#1 and #2), a helium-4 nucleus (#3) and a neutron. The neutron is invisible on the micrograph because it has no electric charge. At points B and C, the two helium-5-Lambda nuclei decayed into two hydrogen nuclei each, as well as several uncharged particles, namely neutrons and neutral pions.
∗ In the largest-ever Xi nucleus search experiment at J-PARC, a Xi nuclear event (Irrawaddy event) of unprecedentedly high binding energy was observed. The binding energy was more than five times that of a Xi nuclear event (Ibuki event) reported by our group in February 2021.
∗ The Irrawaddy event shows that the Xi-minus particle and the nucleus were in the most deeply bound s-state 3 ). This state, together with the Ibuki event showing the p-state 3 ), reveals the level structure inside the Xi nucleus.
∗ The level structure of the Xi nucleus reveals the magnitude of the force binding Xi-minus particles to protons and neutrons. This insight is important for understanding how nuclei are formed from quarks and for understanding the internal structure of neutron stars, where the densest matter in the universe resides.
The research team observed a novel event of the Xi nucleus (Irrawaddy event) in the J-PARC E07 experiment conducted at the J-PARC Hadron Experimental Facility. Detailed analysis reveals that this event was due to the binding of a Xi-minus particle to a nitrogen-14 nucleus at a very large energy of 6.27 ± 0.27 MeV. Like negatively charged electrons, Xi-minus particles are bound to positively-charged nuclei by the Coulomb force, however the Coulomb force acting alone could not generate a binding energy over 1 MeV. The observed event thus represents the formation of the s-state of the Xi-nucleus, which is even more strongly bound by the attractive nuclear force of the strong interaction with the nucleus in addition to action of the Coulomb force, than the previously detected p- and d-states of Xi nuclei. Taken together, these results reveal the level structure of the Xi nucleus, as seen in Figure 2, and specify the magnitude of the attractive force due to the strong interaction.
The Xi nucleus decays via a reaction in which a Xi-minus particle reacts with a proton in the nucleus to convert it into two Lambda particles 1 ). If the likelihood of this conversion reaction were higher, then the nucleus would typically break down before the Xi-minus particles could enter the nucleus and form the Xi-nuclear state. Therefore, the existence of Xi nuclei also implies a relatively low likelihood for the conversion reaction.
The strong interaction between Xi-minus particles and nuclei is important for understanding the origin of nuclei, as are the forces and conversions between Xi particles 1 ) and protons or neutrons. These phenomena are also crucial for understanding the inner state of neutron stars, which can be regarded as at astronomical scale.
Figure 2. Schematic diagram of the binding energy (BΞ-) between the Xi-minus particle and the nitrogen-14 nucleus (14N) and the corresponding states in the Xi nucleus measured thus far. The singly measured BΞ- value for both the Irrawaddy and Ibuki events allows two different BΞ- values of the Kinka and Kiso events to be interpreted as s- and p-orbits, respectively. In the case of the nitrogen-14 nucleus, the d-orbit after s and p is denoted as a capital 'D' because electromagnetism is the dominant force in this case, with little influence from the strong interaction. As the level becomes shallower than the s-state, the distance from the Xi-minus particle to the center of the nucleus increases.
A Xi nucleus is a nucleus that contains a Xi particle added to the normal nucleus. Like protons and neutrons, which make up a normal nucleus, each Xi particle is made up of three quarks. A Xi particle includes two strange quarks and characteristically decays within about 10 billionths of a second. Hypernuclei, i.e. nuclei containing strange quark(s), do not naturally exist on the earth and their study leads to a deeper understanding of the mechanisms of nucleus formation. The study of hypernuclei, especially Xi nucleus, is also important for understanding neutron stars, which are regarded as giant nucleus with the size of celestial bodies. Neutron stars are ultra-dense objects formed after the supernova explosion of a star, and neutron star merging events have attracted much attention in recent years as a source of gravitational waves and heavy elements in the universe. To understand the properties of neutron star, such as maximum mass, radius, internal density and pressure, scientists must consider the conditions under which particles are produced in the interior of neutron stars including the Xi particles. Since the conditions for Xi particle production depend on the magnitudes of forces between a Xi particle and a proton or neutron, it is necessary to determine these magnitudes in earth-based experiments. Scientists have eagerly awaited additional experimental data on Xi nucleus for some time.
Previous experiments had suggested that the strong interaction between a Xi particle and a nucleus was attractive. In 2015, the group of senior professor Kazuma Nakazawa reported the world's first Xi nuclear event (Kiso event[**]) at the KEK 12 GeV proton synchrotron (E373). This report revealed that the strong interaction between a Xi particle and a nucleus is indeed an attractive force. However, the binding energy could be interpreted in two ways. In order to measure in detail, the force acting between a Xi-minus particle and a nucleus, it was necessary to detect a large number of Xi nuclear events for each of which the binding energy could be uniquely determined. Therefore, the research team planned and carried out an international collaborative experiment (J-PARC E07) aiming to increase the number of observed events by an order of magnitude by developing a new technology at J-PARC, where a high-intensity beam was available.
In the E07 experiment at the J-PARC Hadron Experimental Facility, Xi-minus particle was produced using an accelerator beam of high-intensity, high-purity, negatively charged K mesons (K-) 1 ). A large number of Xi-minus particles were injected onto a total of 1500 special photographic emulsion sheets to record Xi nuclear events. The emulsion sheets were then developed, and dedicated optical microscope systems searched them for Xi nuclei. In order to enhance the efficiency of the search, the reactions that produced Xi-minus particles were identified and located by using a set of detectors placed in front of and behind the emulsion sheets. These measured positions were used to search for Xi-minus particles on the sheets. As a result of this search, an event was observed in which a Xi-minus particle was absorbed by a nucleus in the photographic emulsion sheet and split into two Lambda nuclei and other particles. The analysis uniquely determined that the event was caused by the absorption of a Xi-minus particle by a nitrogen-14 nucleus in the emulsion sheet and the decay of this Xi nucleus into two helium-5-Lambda nuclei, a helium-4- nucleus and a neutron, with a binding energy of 6.27 ± 0.27 MeV.
The event was named the 'Irrawaddy event' after the majestic river in Myanmar, the home country of the international student at Gifu University who led the microscopic search and discovered the event. Only a few Xi nuclear events have yet been observed, and it is customary for research teams to name each new discovery after a place associated with discoverer or with the institution that contributed to its discovery.
By revealing the level structure in the Xi nucleus, in which a Xi-minus particle bound, this study will lead to an understanding of how matter is formed from the elementary particles, called quarks. It will also bring us one step closer to understanding the internal structure of neutron stars, which are regarded as giant nuclei.
The research group responsible for the Irrawaddy event is now developing a new search method for Xi nuclei, called the 'overall-sheet scan' method. This method will facilitate the detection of Xi nuclei that the current searching method cannot identify. It is estimated that the number of Xi-nuclear events observable with the new technique will exceeded that of the present detection limit by an order of magnitude, thus exceeding those of past experiments' detection limits by 2 orders of magnitude. The observation of many more Xi nuclei will allow us to measure the various types and energy states of Xi nuclei in a more systematic and precise way, while also elucidating the details of the strong interaction that acts on the Xi particle.
Journal Name | Progress Theoretical and Experimental Physics |
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Title | irst observation of a nuclear s-state of Ξ hypernucleus, 15ΞC |
Authors | M. Yoshimoto et al. |
DOI number | 10.1093/ptep/ptab073 |
Published URL | https://www.doi.org/10.1093/ptep/ptab073 |
This research was carried out by an international research team formed by the following universities and institutes (20 agencies, 49 students and researchers):
1) Strange quark, Lambda particle, Xi particle, hyperon, kaons (K meson)
A baryon is a particle composed of three quarks, like a proton or a neutron. Some baryons other than the proton and neutron can be formed by considering the third quark, the strange 's' quark, in addition to the up 'u' and down 'd' quarks. A Lambda particle (composition of uds) contains a strange quark, whereas each type of Xi particle contains two strange quarks: Xi-minus (dss) or Xi-zero (uss). A baryon that contains any number of strange quarks is called a hyperon.
A nucleus that contains one or more hyperons is called a hypernucleus. A Lambda hypernucleus contains a Lambda hyperon, whereas a double-Lambda hypernucleus contains two Lambda hyperons. A Xi hypernucleus contains a Xi hyperon.
A meson is composed of a quark and an anti-quark. A negatively charged kaon K- (us) is a meson which has a strange quark, where u is an anti-quark of the up quark. The Xi-minus particle is produced along with a K+ (u s) meson in the reaction of the K- beam with a proton in the diamond target.
A Xi-minus particle reacts with a proton and converts it into two Lambda particles. This conversion reaction causes the Xi hypernucleus to decay. At this point, the two Lambda particles may happen to bind together to form two Lambda hypernuclei; the Irrawaddy event observed here is just such an event.
2) Strong interaction
This interaction is often called the 'strong force' or the 'nuclear force' and is one of the four elementary interactions in the universe, the others are weak, electromagnetic, and gravitational interactions. The strong force acts between quarks and forms a nucleon (i.e. a proton or neutron) from quarks or a nucleus from nucleons. The properties of the strong interaction are among the most important subjects in modern physics. Crucial keys to such questions are offered by measurement of the forces in the following interactions: hyperon-hyperon, hyperon-nucleon and hyperon-nucleus. Thus, hyperons and hypernuclei, which do not exist naturally on the earth and are produced only by using accelerator beams, are studied intensively at J-PARC and other accelerator facilities around the world
3) Level structure
The electrons in an atom are in orbits of fixed energy (K-shell, L-shell, etc.). The X-rays emitted from the excitation (the rise of its energy) of an atom thus have a fixed energy, since the electrons move between orbital state in a phenomenon called a transition. Similarly, the protons and neutrons in a normal nucleus occupy such states, called levels. The energy of the gamma rays emitted from an excited nucleus is determined by the transitions of proton or neutron between these levels. In nuclei, the levels are named s, p, d, etc., in descending order of binding energy, just as the shells are named K, L, etc., in the case of atoms. In this study, the levels of the Xi-minus particle have been observed in a Xi nucleus for the first time.
4) Neutron stars
Neutron stars are the densest celestial objects in the universe, and are thought to be born in supernova explosions that occur in the final stage of the evolution of massive stars that are more than eight times the mass of the Sun. Neutron stars have the 1.5 - 2 times mass of the Sun, but their radius is only about 10 km, then their centers are extremely dense, reaching 1 billion tons per 1 cm3, which corresponds to one spoonful, where such density is 5 to 7 times the density of normal nuclei. Neutron stars, as their name implies, are composed primarily of neutrons. At very high density areas, such as in the center, the energy of those neutrons becomes so high that the particle motion inside is calmer when they are converted to hyperons, which are even heavier particles than neutron. Thus, the hyperons should emerge. As the motion becomes calmer with this appearance, the pressure at the center decreases. Since the hyperon-nucleon and hyperon-hyperon interactions are attractive as in our experimental results, then even at lower densities, conversion to hyperons will occur, and the pressure will drop further. Then the heavier neutron star would not be able to support its own weight and would not be able to exist. Astronomers have found neutron stars with masses twice that of the Sun, which contradicts the attractive hyperon interaction and has become a major topic of debate, known as the 'Hyperon Puzzle'.
[*] Ibuki event
https://j-parc.jp/c/en/press-release/2021/03/02000662.html
https://www2.kek.jp/ipns/en/release/20210302/
[**] Kiso event
https://www.doi.org/10.1093/ptep/ptv008
The authors would like to thank MARUZEN-YUSHODO Co., Ltd. [ About research ] [ About Gifu University ] [ About Tohoku University ] [ About J-PARC ] [ About KEK] ] *Please replace "[at]" with "@"
( https://kw.maruzen.co.jp/kousei-honyaku/ ) for the English language editing.
Media contacts for further inquiries
Socionext Inc. has collaborated with researchers from Institute of Materials Structure Science at High Energy Accelerator Research Organization, Institute for Integrated Radiation and Nuclear Science at Kyoto University and Graduate School of Information Science and Technology at Osaka University, and has successfully demonstrated, for the first time, that the soft errors of semiconductor devices induced by muons and neutrons have different characteristics. The research group conducted experiments to irradiate semiconductor devices with negative and positive muon beams at Muon Science Facility (MUSE) of Materials and Life Science Facility (MLF), Japan Proton Accelerator Research Complex (J-PARC), thermal neutron beams at Kyoto University Research Reactor (KUR), and high-energy neutron beams at Research Center for Nuclear Physics (RCNP) of Osaka University, respectively. By using multiple types of quantum beams, the research group has achieved a comprehensive measurement of the effects of cosmic-ray muons and neutrons in environmental radiation. The encouraging results are expected to drive the development of effective evaluation method and countermeasures for soft errors caused by environmental radiation. The results are also expected to lead to the creation of highly reliable semiconductor devices that will support the future infrastructure.
The results of the research work have been published online in IEEE Transactions on Nuclear Science on May 21, 2021.
Part of this research work was supported by the "Program on Open Innovation Platform with Enterprises, Research Institutes and Academia (OPERA)" of the Japan Science and Technology Agency (JST).
As semiconductor devices become more highly integrated and their operation voltage becomes lower, they are more prone to soft errors, which occur when electronic information is unexpectedly altered by radiation. There are concerns that soft errors by environmental radiation will cause more serious problems.
Previously, cosmic-ray neutrons in environmental radiation were considered as the main source of the problems to cause soft errors. On the other hand, for advanced semiconductor devices, which are highly integrated and use lower voltage, soft errors caused by muons, which are also derived from cosmic rays, have become a concern. Muons account for about three-quarters of all particles in cosmic rays that fall to the Earth, and it has been pointed out that they may cause a bigger problem than neutrons. However, there have been very few reports of soft errors caused by muons, and the difference between soft errors caused by neutrons and by muons has not been well understood.
In this study, to understand the difference in soft errors caused by cosmic-ray muons and neutrons, the research group performed comparative evaluations by irradiating semiconductor devices with muons and neutrons. An SRAM circuit fabricated with 20-nm CMOS process technology has been used in this experiment. The SRAM was irradiated with each quantum beam, and the rate and the trend of soft error occurrence were analyzed by each of the particles.
It was found that there are clear differences between muons and neutrons in terms of supply voltage dependency of the error rate, the ratio of multiple-bit errors, as well as the characteristics of the multiple-bit error patterns. The result has been obtained for the first time in the world.
Figure 1: Supply voltage dependency of soft error rate (left) and ratio of multiple-bit errors (right)
The results will lead to a development of technologies to effectively solve the problems caused by environmental radiation which includes muons. The difference of effects between muons and neutrons, which have been discovered in this study, will help establish an optimal design method to prevent soft errors. The results of this study are also expected to contribute to the evolution of evaluation method by numerical simulations.
In the future, the reliability of the infrastructure will depend on a vast number of semiconductor devices, and it is expected that evaluation and countermeasures against soft errors caused by environmental radiation will become even more important. Development of soft error evaluation using quantum beams, as was done in this study, is expected to lead to the creation of safer, securer, and more reliable semiconductor devices.
Subject | Muon-Induced Single-Event Upsets in 20-nm SRAMs: Comparative Characterization with Neutrons and Alpha Particles |
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Authors | Takashi Kato1, Motonobu Tampo2, Soshi Takeshita2, Hiroki Tanaka3, Hideya Matsuyama1, Masanori Hashimoto4, Yasuhiro Miyake2 1 Reliability Engineering Department, Socionext Inc. 2 Muon Science Laboratory, Institute of Materials Structure Science, High Energy Accelerator Research Organization / J-PARC 3 Institute for Integrated Radiation and Nuclear Science, Kyoto University 4 Graduate School of Information Science and Technology, Osaka University |
Publication | IEEE Transactions on Nuclear Science |
DOI | https://doi.org/10.1109/TNS.2021.3082559 |
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