VI. RNC ACTIVITIESRIKEN Accel. Prog. Rep. 55 (2022)

Subnuclear System Research Division
RIKEN Facility Oﬃce at RAL
1. Abstract
Our core activities are based on the RIKEN-RAL Muon Facility located at the ISIS Neutron & Muon Source at the Rutherford
Appleton Laboratory (UK), which provides intense pulsed-muon beams. The RIKEN-RAL Muon Facility is a signiﬁcant and longstanding collaboration between RIKEN and RAL in muon science—with 2020 being the 30th years of continuous agreements between
RIKEN and RAL. The Facility enables muon science throughout Japan and other ﬁeld—it continues to attract proposals from a wide
variety of Japanese universities and institutions (with over 80 groups having now used the facility), and including industrial users suchat Toyota, and has been instrumental in establishing scientiﬁc links with other Asian universities.
Muons have their own spins with 100% polarization, and can detect local magnetic ﬁelds and their ﬂuctuations at muon stoppingsites very precisely. The method to study the characteristics of materials by observing time dependent changes of muon spin polarization is called “Muon Spin Rotation, Relaxation and Resonance” (μSR method), and is applied to study electro-magnetic properties
of insulating, metallic, magnetic and superconducting systems. Muons reveal static and dynamic properties of the electronic state ofmaterials in the zero-ﬁeld condition, which is the ideal magnetic condition for research into magnetism. For example, we have carriedout μSR investigations on a wide range of materials including frustrated pyrochlore systems, which have variety of exotic groundstates of magnetic spins, so the magnetism study of this system using muon is quite unique.
The ultra-cold muon beam can be stopped in thin foil, multi-layered materials and artiﬁcial lattices, which enables us to apply theaSR techniques to surface and interface science. The development of an ultra-cold muon beam is also very important as a source of
pencil-like small emittance muon beam for muon g-2/EDM measurement. We have been developing muonium generators to create
more muonium atoms in vacuum even at room temperature to improve beam quality compared with the conventional hot-tungsten
muonium generator. We have demonstrated a strong increase in the muonium emission eﬃciency by fabricating ﬁne laser drill-holeson the surface of silica aerogel. We are also developing a high power Lyman-alpha laser in collaboration with the Advanced Photonics
group at RIKEN. The new laser will ionize muoniums 100 times more eﬃciently for slow muon beam generation.
Over the past 2–3 years, a signiﬁcant development activity in muon elemental analysis has taken place, proton radius experiments
have continued and been developed, and chip irradiation experiments have also continued.

2. Major Research Subjects
(1)
(2)
(3)
(4)

Materials science by muon-spin-relaxation method and muon site calculation
Development of elemental analysis using pulsed negative muons
Nuclear and particle physics studies via muonic atoms and ultra-cold muon beam
Other muon applications

3. Summary of Research Activity(1) Material science at the RIKEN-RAL muon facility
Muons have their own spins with 100% polarization, and can detect local magnetic ﬁelds and their ﬂuctuations at muon stoppingsites very precisely. The μSR method is applied to studies of newly fabricated materials. Muons enable us to conduct (1) material
studies under external zero-ﬁeld condition, (2) magnetism studies with samples without nuclear spins, and (3) measurements of muon
spin relaxation changes at wide temperature range with same detection sensitivity. The detection time range of local ﬁeld ﬂuctuations
by μSR is 10−6 to 10−11 second, which is an intermediate region between neutron scattering method (10−10 –10−12 second) and Nuclear
Magnetic Resonance (NMR) (longer than 10−6 second). At Port-2 and 4 of the RIKEN-RAL Muon Facility, we have been performing
μSR researches on strong correlated-electron systems, organic molecules, energy related materials and biological samples to study
electron structures, superconductivity, magnetism, molecular structures and crystal structures.
Among our scientiﬁc activities on μSR studies from year 2018 to 2021, following subjects of material sciences are most importantachievements at the RIKEN-RAL muon facility:(1) Quasi magnetic monopole state in the Ru-based pyrochlore systems, R2 Ru2 O7 ;(2) Novel magnetic and superconducting properties of nano-size La-based high-TC superconducting cuprates;(3) Determination of muon positions estimated from density functional theory (DFT) and dipole-ﬁeld calculations;(4) Chemical muonic states in DNA molecules.
Result-1) Quasi magnetic monopole state in pyrochlore systems gave us new interpretations to understand exotic phenomena,
like the quantum spin liquid and spin fragmentation states. Result-2) The same nano-size eﬀect was examined on the La-based
high-TC superconducting oxide changing the electronic state from insulating to superconducting. We conﬁrmed the reduction in themagnetic interaction and the disappearance of the superconducting state leading the increase in the ferromagnetic interaction within
the wide-range of the hole concentration. Result-3) Well known and deeply investigated La2 CuO4 has opened a new scheme of the
Cu spin. Taking into account quantum eﬀects to expand the Cu-spin orbital and muon positions, we have succeeded to explain newly
found muon sites and hyperﬁne ﬁelds at those sites. Result-4) Chemical states of the muon which attaches to DNA molecules were
investigated by the avoided level-crossing muon-spin resonance experiments. In conjunction with DFT calculations, we are trying toreveal the electron motion though the DNA molecule.
We have been continuing to develop muon-science activities in Asian countries. We enhanced international collaborations toorganize new μSR experimental groups and to develop muon-site calculation groups using computational method. We are creating- 333 -

VI. RNC ACTIVITIESRIKEN Accel. Prog. Rep. 55 (2022)

new collaborations with new teams in diﬀerent countries and also continuing collaborations in μSR experiments on strongly correlated
systems with researchers from Taiwan, Indonesia, China, Thailand and Malaysia including graduate students. We are starting tocollaborate with the new Chinese muon group who are developing the Chinese Muon Facility and trying to develop more muon
activities in the Asian area.
(2) Development of elemental analysis using pulsed negative muons
There has been signiﬁcant development of elemental analysis using negative muons on Port 4 and Port 1 over the past coupleof years. Currently, elemental analysis commonly uses X-ray and electron beams, which accurately measure surfaces. However, asigniﬁcant advantage of muonic X-rays over those of electronic X-rays is their higher energy due to the mass of the muon. These highenergy muonic X-rays are emitted from the bulk of the samples without signiﬁcant photon self-absorption. The penetration depth ofthe muons can be varied by controlling the muon momentum, providing data from a thin slice of sample at a given depth. This can beover a centimeter in iron, silver and gold or over 4 cm in less dense materials such as carbon.
Some techniques for elemental analysis are destructive or require the material under investigation to undergo signiﬁcant treatment
and some of the techniques are only sensitive to the surface. Therefore, negative muons oﬀer a unique service in which they canmeasure inside, beyond the surface layer and completely non-destructively.
The areas of science that have used negative muons for elemental analysis have been very diverse. The largest area is the cultural
heritage community as the non-destructive ability is particularly important and will become more so. This community have investigated swords from diﬀerent eras, coins (Roman gold and silver, Islamic silver and from the Tudor Warship Mary Rose), miniatureboats from Sardinia, reliefs on Baptist church gate, Bronze Age tools and cannon balls. In addition, energy materials (Li composition
for hydrogen storage), bio-materials (search for iron to potentially help understand Alzheimer’s), engineering alloys (manufacturing
processes for new materials for jet engines), and functional materials (surface eﬀects in piezo electrics) have also been investigated.
The study was extended to see the diﬀerence by isotopes of silver and lead, which may give hint on the source of the material.
(3) Ultra-cold (low energy) muon beam generation and applications
Positive muon beam with thermal energy has been produced by laser ionization of muonium (bound system of μ+ and electron)
emitted from a hot tungsten surface with stopping surface muon beam at Port-3. The method generates a positive muon beam withacceleration energy from several 100 eV to several 10 keV, small beam size (a few mm) and good time resolution (less than 8 nsec). By
stopping the ultra-cold muon beam in thin foil, multi-layered materials and artiﬁcial lattices, we can precisely measure local magnetic
ﬁeld in the materials, and apply the μSR techniques to surface and interface science. In addition, the ultra-cold muon is very importantas the source of pencil-like small emittance muon beam for muon g − 2/EDM measurement. It is essential to increase the slow
muon beam production eﬃciency by 100 times for these applications. There are three key techniques in ultra-cold muon generation:
production of thermal muonium, high intensity Lyman-alpha laser and the ultra-cold muon beam line.
A high-power Lyman-alpha laser was developed in collaboration with the Advanced Photonics group at RIKEN. The new laser
system is used at J-PARC U-line and, upon completion, will ionize muoniums 100 times more eﬃciently for slow muon beam
generation. In this development, we succeeded to synthesize novel ceramic-based Nd:YGAG and Nd:YSAG crystals, which realized
a highly eﬃcient and stable laser system. We are working hard to improve the homogeneity of large size crystals in order to achieve
full design power.
We also succeeded in developing an eﬃcient muonium generator, laser ablated silica aerogel, which emits more muoniums into
vacuum even at room temperature. Study has been done at TRIUMF utilizing positron tracking method of muon decay position.
We demonstrated in 2013 at least 10 times increase of the muonium emission eﬃciency by fabricating ﬁne laser drill-holes on thesurface of silica aerogel. Further study was carried out in 2017 to ﬁnd the optimum fabrication that will maximize the muonium
emission. From the analysis, we found the emission has large positive correlation with the laser ablated area rather than with anyother parameters. We also conﬁrmed the muon polarization in vacuum. An alternative detection method for muonium emission using
muonium spin rotation, which will be sensitive even to muoniums near the surface, was tested at RIKEN-RAL in 2018 and was foundsuccessful. The study was further applied to the measurement of the temperature dependence.
In RIKEN-RAL Port 3, the ultra-cold muon beam line, which had been designed with hot tungsten, was completely rebuilt to use
advantage of the new room temperature silica aerogel target. The equipment was tested with surface muon beam and basic data suchas muon stopping in aerogel were taken. Based on the design, an ultimate cold muon source is under preparation for muon g − 2/EDM
at J-PARC.
(4) Other fundamental physics studies
We proposed the measurement of the proton radius by using the hyperﬁne splitting of the muonic hydrogen ground state. This
hyperﬁne splitting is sensitive to the Zemach radius, which is a convolution of charge and magnetic-dipole distributions inside proton.
We are planning to re-polarize the muonic hydrogen by a circularly polarized excitation laser (excites one of the F = 1 states andregenerates the muon spin polarization), and detect the recovery of the muon decay-asymmetry along the laser.
Preparation using muon beam is in progress. We measured the muon stopping distribution in low-density hydrogen-gas cell, whichgave us consistent results with beam simulation. Another key is the lifetime of the upper hyperﬁne state of the muonic hydrogen thatwill contribute the polarization. We successfully observed the clear muon spin precession of muonic deuterium atom in 2018 for theﬁrst time in the world. The measurement with muonic protium was carried out in 2019 and the data is being analyzed.

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VI. RNC ACTIVITIESRIKEN Accel. Prog. Rep. 55 (2022)(5) Facility operation and refurbishment
The research activity at RIKEN-RAL had severe restriction in FY2021 also since the COVID-19 pandemic continued. It was
impossible for users from outside RAL to carry out the experiment. Even though, we managed to carry out several μSR experimentswith mailed samples. Since July 2021, RIKEN-RAL muon facility is undergoing major refurbishment, ﬁrst time since its operation in
1994. The work covers many important components, such as the cooling water circuit, vacuum system, radiation shielding, magnet
power supplies and power cables, detectors and beamline control system.

Members
Director
Philip KING
Senior Research Scientist
Isao WATANABE
Contract Researcher
Katsuhiko ISHIDA

List of Publications & Presentations
Publications[Original Papers]
C. Pizzolottoa, A. Sbrizzia, A. Adamczak, D. Bakalov, G. Baldazzie, M. Baruzzoa, R. Benoccih, R. Bertoni, M. Bonesinih, H. Cabrera,
D. Cirrincione, M. Clemenza, L. Colace, M. Danailov, P. Danev, A. deBari, C. DeVecchi, M. DeVincenzik, E. Fasci, F. Fuschino,
K. S. Gadedjisso-Tossou, L .Gianfrani, K. I shida, C. Labanti, V. Maggi, R. Mazza, A. Menegolli, E. Mocchiutti, S. Monzani, L. Moretti,
G. Morgante, J. Niemela, A. Pullia, R. Ramponi, L. P. Rignanese, M. Rossella, M. Stoilov, L. Stoychev, J. J. Suárez-Vargas, L. Tortora,
E. Vallazza, and A. Vacchi, “Measurement of the muon transfer rate from muonic hydrogen to oxygen in the range 70–336 K,” Phys.
Lett. A 403, 127401 (2021).
T. Yamashita, K. Okutsu, Y. Kino, R. Nakashima, K. Miyashita, K. Yasuda, S. Okada, M. Sato, T. Oka, N. Kawamura, S. Kanda,
K. Shimomura, P. Strasser, S. Takeshita, M. Tampo, S. Doiuchi, Y. Nagatani, H. Natori, S. Nishimura, A. D. Pant, Y. Miyake, and
K. Ishida, “Time evolution calculation of muon catalysed fusion: Emission of recycling muons from a two-layer hydrogen ﬁlm,”
Fusion Eng. Des. 169, 112580 (2021).
A. D. Pant, K. Nagamine, E. Torikai, I. Shiraki, K. Shimomura, Francis L. Pratt, H. Ariga-Miwa, K. Ishida, and J. S. Schultz, “Muonium
response to low oxygen levels in haemoglobin and other biological aqueous solutions and potential application towards monitoring
hypoxia,” Nucl. Instrum. Methods Phys. Res. A 1010, 165561 (2021).
M. D. Umar, K. Ishida, R. Murayama, D. P. Sari, U. Widyaiswari, M. Fronzi, I. Watanabe, and M. Iwasaki, “Muon-spin motion at thecrossover regime between gaussian and lorentzian distribution of magnetic ﬁelds,” Prog. Theor. Exp. Phys. 2021, 083101 (2021).
K. Okutsu, T. Yamashita, Y. Kino, R. Nakashima, K. Miyashita, K. Yasuda, S. Okada, M. Sato, T. Oka, N. Kawamura, S. Kanda,
K. Shimomura, P. Strasser, S. Takeshita, M. Tampo, S. Doiuchi, Y. Nagatani, H. Natori, S. Nishimura, A. D. Pant, Y. Miyake, and
K. Ishida, “Design for detecting recycling muon after muon-catalyzed fusion reaction in solid hydrogen isotope target,” Fusion Eng.
Des. 170, 112712 (2021).
A. Green, K. Ishida, Bethany V. Hampshire, K. Butcher, M. Pollard, and A. D. Hillier, “Understanding roman gold coinage inside out,” J.
Archaeol. Sci. 134, 105470 (2021).
J. Sugiyama, K. Ohishi, O. K. Forslund, M. Månsson, S. P. Cottrell, A. D. Hillier, and K. Ishida, “How Li diﬀusion in spinel
Li[Ni1/2 Mn3/2 ]O4 is seen with μ± SR,” Z. Phys. Chem. 236, 799 (2022).
W. N. Zaharim, S. N. A. Ahmad, S. Sulaiman, H. Rozak, D. F. H. Baseri, N. A. M. Rosli, S. S. Mohd-Tajudin, L. S. Ang, and I. Watanabe,“Density functional theory study of 12mer single-strand guanine oligomer and associated muon hyperﬁne interaction,” ACS Omega 6,29641–29650 (2021).
A. Jamaludin, W. N. Zaharim, S. Sulaiman, H. Rozak, A. L. Sin, and I. Watanabe, “Density functional theory investigation of muon
hyperﬁne interaction in guanine-cytosine double-strand DNA,” J. Phys. Soc. Jpn. 91, 024301 (2022).
S. Yoon, W. Lee, S. Lee, J. Park, H. Lee, Y. S. Choi, S. -H. Do, W. -J. Choi, W. -T. Chen, F. Chou, D. I. Gorbunov, Y. Oshima,
A. Ali, Y. Singh, A. Berlie, I. Watanabe, and K. -Y. Choi1, “Quantum disordered state in the J1 -J2 square-lattice antiferromagnet
Sr2 Cu(Te0.95 W0.05 )O6 ,” Phys. Rev. Res. 5, 014411 (2021).
M. Miyajima, F. Astuti, T. Fukuda, M. Kodani, S. Iida, S. Asai, A. Matsuo, T. Masuda, K. Kindo, T. Hasegawa, T. C. Kobayashi,
T. Nakano, I. Watanabe, and T. Kambe, “Spin-gap formation due to spin-Peierls instability in π-orbital-ordered NaO2 ,” Phys. Rev. B
104, L140402 (2021).

[Review Articles]
M. Cataldo, M. Clemenza, K. Ishida, and A. D. Hillier, “A novel non-destructive technique for cultural heritage: Depth proﬁling andelemental analysis underneath the surface with negative muons,” Appl. Sci. 12, 4237 (2022).
A. D. Hillier, S. J. Blundell, I. McKenzie, I. Umegaki, L. Shu, J. A. Wright, T. Prokscha, F. B. K. Shimomura, A. Berlie, H. Alberto, andI. Watanabe, “Muon spin spectroscopy,” Nat. Rev. Meth. Primers 2, 4 (2022).

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VI. RNC ACTIVITIESRIKEN Accel. Prog. Rep. 55 (2022)

I. Watanabe, D. P. Sari, R. Ramadhan, B. Adiperdana, and S. Sulaiman, “Muon-site determination in materials by using computational
methods,” J. Comput. Chem. Jpn. 19, 115–123 (2020).

[Book]
A. D. Hillier, B. Hampshire, and K. Ishida, “Depth-dependent bulk elemental analysis using negative muons,” in “Handbook of Cultural
Heritage Analysis,” S. D’Amico and V. Venuti (Eds.), Springer, 2022, pp. 23–44.

Presentations[International Conference/Workshop]
K. Ishida, “Muon g-2 experiment at J-PARC,” 14th European Research Conference on Electromagnetic Interactions with Nucleons andNuclei (EINN2021), Online, November 2–6, 2021.

[Domestic Conferences/Workshops]
石田勝彦, 「ミュオン原子分子とスピン状態」, 新学術領域「宇宙観測検出器と量子ビームの出会い. 新たな応用への架け橋. 」, 第 1
回超低速負ミュオンビーム研究会, オンライン, 2021 年 12 月 20 日.
石田勝彦, 「RIKEN-RAL 施設」, 第 12 回「Muon 科学と加速器研究」, 第 6 回「文理融合シンポジウム 量子ビームで歴史を探る—
加速器が紡ぐ文理融合の地平—」, 「ミュオンで見る磁性・超伝導物質研究の最前線」合同研究会, 豊中市（大阪大学）, 2022 年
1 月 6–8 日.
石田勝彦, 「パルスミュオン施設でのミュオン科学研究」, 「ミューオン X 線 γ 線分光 非破壊分析, 化学, 原子核物理への新展開」,「ミューオン原子核捕獲反応による原子核関連研究の可能性」, 合同研究会, 豊中市（大阪大学 RNCP）&オンライン, 2022 年 3月 24 日.

Press Release
神田聡太郎, 下村浩一郎他, 「『理想の水素原子』で未知の物理現象を探索する, ミュオニウムのマイクロ波分光実験がスタート」,
高エネルギー加速器研究機構, J-PARC センター, 理化学研究所, 東京大学大学院理学系研究科, 東京大学大学院総合文化研究科,
2021 年 4 月 16 日. https://www.riken.jp/press/2021/20210416_1/index.html .

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