Ⅵ. ORGANIZATION AND ACTIVITIES OF RNC

RIKEN Accel. Prog. Rep. 56 (2023)

Nuclear Science and Transmutation Research Division
Nuclear Transmutation Data Research Group
Muon Data Team
1. Abstract

Dr. Yoshio Nishina observed muons in cosmic rays in 1937. The muon is an elementary particle similar to electron and classiﬁed
to lepton group. The muon has positive or negative electric charge, and the lifetime is 2.2 µsec. The negative muon (µ− ) is 207 times
heavier than the electron and behaves as a “heavy electron” in materials. The negative muon is captured by atomic orbits of nuclei
to form a muonic atom and cascades down to the 1s orbit to make muon nuclear capture. The muon is combined with a proton in
the nucleus to convert to a neutron and a neutrino. The muon nuclear capture reaction on a nucleus (ZA N) with the atomic number Z
A−x
N (x = 0, 1, 2, 3, 4) by emitting some neutrons in the reaction. The phenomenon isand mass number A generates the isotopes of Z−1
A−x
N reactions (x = 0, 1, 2, 3, 4) is one of importantcalled “muon nuclear transmutation.” The reaction branching ratio of ZA N(µ− , xnν)Z−1
factors toward various applications with nuclear transmutation technique. From a viewpoint of the nuclear physic, the muon nuclear
capture reaction is very unique and interesting. A high-energy compound nuclear state is suddenly generated in the nuclei associatedwith a weak conversion process of proton to neutron and neutrino. Many experimental results have been so far reported, however, thereaction mechanism itself is not well clariﬁed. The research team aims at obtaining the experimental data to investigate the reaction
mechanism of muon nuclear capture, and also at theoretical understanding on the nuclear capture reaction.

2. Major Research Subjects(1) Experimental clariﬁcation on the mechanism of nuclear muon capture reaction(2) Theoretical understanding on the nuclear muon capture reaction(3) Interdisciplinary applications with the nuclear transmutation technique

3. Summary of Research Activity

There are two experimental methods to study the muon nuclear capture reaction. The ﬁrst one is “muon in-beam spectroscopy
A−x
N nuclei are prompt events and are observed by the “muonmethod.” The neutron and γ-ray emissions from the excited states of Z−1
in-beam spectroscopy method” with a DC muon beam. The reaction branching ratio is directly determined by measuring the neutron
multiplicity in the reaction. The DC muon beam is available at the MuSIC (Muon Science Innovative Channel) muon facility in the
Research Center for Nuclear Physics (RCNP) at Osaka University. The second one is “muon activation method” with the pulsed
A−x
N make β+/− decays. The γ-rays associated with β+/− decays to the daughter nuclei aremuon beam. The produced unstable nuclei Z−1
observed in the experiment. The build-up curve of γ-ray yield at muon beam-on and the decay curve at beam-oﬀ are measured. Since
A−x
N nuclei are determined
the half-lives and decay branching ratios of β+/− -γ decays are known, the reaction branching ratios to the Z−1
by the γ-ray yield curves. The pulsed muon beam is available at the RIKEN-RAL Muon Facility in the UK and J-PARC muon facility.
Muon nuclear capture reactions are studied on ﬁve isotope-enriched palladium targets (104, 105, 106, 108, 110 Pd) and ﬁve isotopeenriched zirconium targets (90, 91, 92, 94, 96 Zr) employing two experimental methods. By obtaining the experimental data on the Pd and Zr
targets, the reaction mechanism is investigated experimentally, and the results are compared with appropriate theoretical calculations.
The 107 Pd is classiﬁed to a long-lived ﬁssion product (LLFP) and is contained in a spent nuclear fuel. The study of muon nuclear
capture on the Pd and Zr targets is aiming at exploring a possible reaction path to make the nuclear transmutation of the Pd and Zr
metal extracted from the spent nuclear fuel without an isotope separation process. This research was funded by the ImPACT Programof Council for Science, Technology and Innovation (Cabinet Oﬃce, Government of Japan).
(1) Experiments with “muon in-beam spectroscopy method”
Muon nuclear capture reactions were investigated on ﬁve palladium targets (104, 105, 106, 108, 110 Pd) by employing the DC muon
beam at MuSIC. The γ-ray and neutron in the muon nuclear capture reaction were measured with the time information relative toA−x
Rh reactions, wheremuon beam arrival. The measured neutron multiplicity gives the reaction branching ratio of 46A Pd(µ− , xnν)45
A = 104, 105, 106, 108, 110 and x = 0, 1, 2, 3, 4.
Employing a newly built neutron spectrometer, the neutron was measured to obtain the reaction branching ratios of muon capture
reactions on the Pd targets. We have constructed a neutron spectrometer named “Seamine”: Scintillator Enclosure Array for Muon
Induced Neutron Emission. The spectrometer consists of 21 liquid scintillation counters, 2 Ge γ-ray detectors, 7 BaF2 counters. The
Pd target, muon beam counters and muon degraders are placed at the center of spectrometer. The neutron counter is a BC-501A
liquid scintillation counter with 20 cm diameter and 5 cm depth and is connected to a 5” photo multiplication tube (H4144-01). The
total neuron detection eﬃciency is estimated 5%, where the distance is 4 cm from the target to neutron counters. The Ge γ-ray
detectors are placed at 10 cm form the target, and the typical detection eﬃciency is 0.5% for 200 keV γ-ray. The BaF2 counters arelocated beneath the target to detect fast γ-rays emitted from the compound nucleus formed in the reactions. Signals from the liquid
scintillation counters are processed in a CAEN V1730B waveform digitizer (16 channel, 14 bit, 500 M samplings/sec.). The neuron-γ
discrimination is performed on-line during the experiment, and the detailed data analysis is conducted oﬀ-line after the experiment.
The neutron energy spectrum is constructed in the digitizer. Signals from Ge detectors are also processed in the digitizer to obtain theenergy and time spectrum of γ-rays associated with the reaction. Signals from the BaF2 counters and muon beam counters are sent to
the digitizer to make the fast timing signals.
We have established the muon in-beam spectroscopy method employing the “Seamine” spectrometer. The neutron data analysis- 282 -

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RIKEN Accel. Prog. Rep. 56 (2023)

is in progress to obtain the multiplicity, the energy and the TOF spectrum using start signals given by γ-rays detected in the BaF2
counters. The γ-ray data gives the energy spectrum of prompt γ-rays and muonic X-rays originated from the 104, 105, 106, 108, 110 Pdtargets.
(2) Experiments with “muon activation method” at the RIKEN-RAL Muon Facility
We conducted the experiments on the muon nuclear capture employing the muon activation method at the RIKEN-RAL Muon
Facility in the UK. The pulsed muon beam was irradiated on the 104, 105, 106, 108, 110 Pd targets. The γ-rays were detected by a Ge detector
located at the downstream of the Pd targets to maximize the detection eﬃciency. The build-up and decay curves of γ-ray intensities
were measured associated with β+/− decays of produced unstable nuclei to daughter nuclei. The γ-ray-yield curves give the absolute
A−x
Rh reactions. The
radiation activity produced by the reaction, and the reaction branching ratios are determined for 46A Pd(µ− , xnν)45
decay curves of γ-rays from the produced nuclei with long half-lives were measured under low γ-ray background at an experimental
apparatus built in a separated room. The detailed oﬀ-line data analysis is in progress.
(3) Experiments with “muon activation method” at J-PARC muon facility
The experiments employing the muon activation method were performed at J-PARC muon facility. The ﬁve isotope-enriched Pdtargets (104, 105, 106, 108, 110 Pd) were irradiated by the pulsed muon beam, and the build-up and decay curves of γ-ray intensities were
measured.
In addition to the Pd targets, the experiments on ﬁve isotope-enriched Zr target (90, 91, 92, 94, 96 Zr) were conducted to obtain theA−x
Y reactions, where A = 90, 91, 92, 94, 96. The obtained reaction branching ratios on thereaction branching ratios of 40A Zr(µ− , xnν)39
Pd and Zr targets are important to understand the reaction mechanism of muon nuclear capture. The 93 Zr is one of the LLFP and iscontained in a spent nuclear fuel. The experiment on the Zr targets is to explore a possibility to realize the nuclear transmutation of
the Zr metal extracted from the spent nuclear fuel.
−107−x
Rh reactions, the muon activation experiment was performed
In order to obtain the reaction branching ratio of 107
46 Pd(µ , xnν)45
employing a Pd target containing 107 Pd of 15.3%. The γ-ray intensities associated with β+/− decays of produced unstable nuclei were
measured to obtain the build-up and decay curves. Once the branching ratios of the reactions on the 104, 105, 106, 108, 110 Pd targets are
obtained, these contributions are extracted from the branching-ratio data obtained for the Pd target with 107 Pd. The reaction branching−107−x
Rh reactions is ﬁnally determined. The detailed oﬀ-line data analysis is in progress.
ratio of 107
46 Pd(µ , xnν)45(4) Comparison with theory
The muon activation method gives the reaction branching ratios. The muon in-beam spectroscopy method gives the neutron
multiplicity and the neutron energy spectrum. These experimental results are important to understand the compound nuclear state andneutron emission mechanism. The reaction branching ratios obtained by the muon activation method are compared with the results ofneutron multiplicity measurements. The neutron energy spectrum is considered to be reﬂected by the energy distribution of compound
nuclear state and neutron emission mechanism. The experimental results are compared with the appropriate calculations employing theneutron emission mechanisms due to an evaporation, a cascade and a direct emission processes with assuming the energy distribution
at compound nuclear state.

Members
Team Leader
Hiroyoshi SAKURAI
Contract Researcher
Megumi NIIKURA
Senior Visiting Scientist
Teiichiro MATSUZAKI

List of Publications & Presentations
Publications[Original Papers]
T. Nishi et al., “Chiral symmetry restoration at high matter density observed in pionic atoms,” Nat. Phys. 19, 788 (2023).
A. Yagi et al., “Various nuclear structures in 140 Xe studied by beta decay of ground and isomeric states in 140 I,” Phys. Rev. C 105, 044325(2022).
B. Moon et al., “First observation of the π0h(11/2) × ν0h(9/2) partner orbital conﬁguration in the odd-odd 138 I nucleus,” Phys. Rev. C 105,
034334 (2022).
B. Moon et al., “Gamow-Teller decay of 142 Te to 142 I,” Phys. Rev. C 107, 014311 (2023).

[Proceeding]
S. Manabe et al., “Emissions of hydrogen isotopes from the nuclear muon capture reaction in nat Si,” EPJ Web Conf. 284, 01029 (2023).

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Presentations[International Conferences/Workshops]
M. Niikura (oral), “Nuclear physics with muon,” Physics of RI: Recent progress an dperspective, RIKEN, Wako, May, 2022.
M. Niikura (oral), “Study of muon-induced ﬁssion for spectroscopy of the muonic atom with unstable nuclei,” Open CHRISP Users
Meeting, PSI, Villigen, Switzerland, January, 2023.
R. Mizuno et al. (poster), “Measurement of muon-induced nuclear transmutation for Si isotopes,” Transscale Quantum Science, Univ. ofTokyo, November, 2022.
R. Mizuno et al. (oral), “Measurement of nuclear transmutation via muon capture reaction for Si isotopes,” VIth Topical Workshop on
Modern Aspects in Nuclear Structure, Bormio, Italy, February, 2023.

[Domestic Conferences/Workshops]

新倉潤 (口頭発表), “Study of muon-induced ﬁssion for spectroscopy of the muonic atom with unstable nuclei,” Fission Workshop, 和光
市 (理科学研究所), 2023 年 1 月.
新倉潤 (招待講演), 「ミューオン原子核物理」, 中間子科学の将来検討会, 和光市 (理化学研究所), 2022 年 11 月.
水野るり惠, 他 7 名 (口頭発表), 「1 MeV 陽子ビームを用いた広ダイナミックレンジ光子検出器の性能評価実験」, タンデム加速器
及びその周辺技術の研究会, オンライン, 2022 年 7 月.
水野るり惠, 他 7 名 (口頭発表), 「Ge 検出器を用いた光子検出器システムの広ダイナミックレンジにおける性能評価実験」, 放射
線化学討論会, オンライン, 2022 年 9 月.

[Seminar]

M. Niikura, “Development of muonic X-ray spectrometer for element analysis,” LMU seminar at Paul Scherrer Institut (PSI), Online,
April, 2022.

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