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New Research Project with Muon Beams for Neutrino Nuclear Responses and Nuclear Isotopes Production
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DOI: 10.22661/AAPPSBL.2019.29.3.21

New Research Project with Muon Beams for Neutrino Nuclear Responses and Nuclear Isotopes Production



This is a brief report on the present status and perspectives of the research programs with negative muon beams. Intense negative muon beams provide new research opportunities for neutrino nuclear responses and nuclear isotope detection and their production. We started the program at the Research Center for Nuclear Physics (RCNP), Osaka University, and now it is continuing at RCNP, J-PARC and the Paul Scherrer Institute (PSI). Gamma rays following ordinary muon capture reactions are used for studies of nuclear responses for anti-neutrinos associated with double beta decays and astro-neutrinos, and for high-sensitivity nuclear-isotope detection and high-efficiency nuclear-isotope production. Recently, we have started a new research project for these subjects in collaboration with Universiti Teknologi Malaysia (UTM) Johor Bahru, RCNP Osaka and the Joint Institute for Nuclear Research (JINR) Dubna. We discuss briefly recent results and perspectives regarding the muon experiments.

Keywords: Ordinary muon capture, neutrino nuclear responses, nuclear-isotope detection, nuclear isotope production.


Muon beam experiments using negative muons are an alternative route for neutrino response studies, nuclear isotope detection and nuclear radioisotope production. Muon capture is a semi-leptonic reaction where a proton transforms into a neutron and a muon neutrino is emitted by the exchange of charged current (CC) weak W± boson. It is given as


Muon capture is an extension of an electron capture reaction where one able to probe more available excited states up to J± = 4± states. When muon capture occurs, the target nucleus is excited to about 100 MeV in energy and most of the energy is carried away by the muon neutrino. The remaining energy can be absorbed by the nucleus and starts many other cascade reactions. From reference [1], the capture in light nuclei produces the emission of neutrons, protons and alpha particles. However, for medium and heavy nuclei, neutrons are mainly emitted, with a very low possibility of proton and alpha emission due to the Coulomb barrier. These products have been observed through various experiments using neutron detectors and high-purity germanium (HPGe) detectors as discussed in the review paper and references therein [1].

The excitation region of the nuclei after muon capture corresponds to the neutrino nuclear response where the strength distributions of muon capture is observed. The neutrino nuclear response is important for studies of neutrino fundamental properties beyond the standard model. The neutrino response is crucial for neutrino studies by double beta decays (DBDs) [2-5]. Experimental data of single beta (β- or β+) decay (SBD), inverse beta decay (IBD) and electron capture (EC) are well-established probes for current works due to their sensitivity for nuclear weak coupling constants gA and gV. The analysis of SBD, IBD and EC is much simpler when compared to the two-stage process of DBD, which involves transformation from the parent to daughter nucleus through an intermediate nucleus [2]. Furthermore, nuclear structures associated with DBDs are quite complicated since DBDs include many multipole transitions in wide energy and momentum regions. All SBD, IBD, EC and DBD experiments investigate different areas of interest, where each respective area has important information for nuclear structure studies [3].

Various isotope production methods have been explored to detect nuclear isotopes with high sensitivity and to efficiently produce specific radioisotopes (RIs), especially for environmental and biomedical applications. Neutron-induced reactions have been extensively used for RI production due to the large neutron flux available at nuclear reactors. The typical reaction is the neutron capture (n,γ) reaction. Recently, the photon capture reaction has been shown to be very effective for selectively producing RIs. The reaction used is the (γ,xn) reaction with x being 1 or 2, depending on the photon energy. Here, the cross section is large at the E1 giant resonance region, and high-flux photons are available by Compton back scattering of laser photons scattered off GeV electrons in a storage ring. RIs produced from the target isotope are mainly and , respectively, in the case of neutron capture and photon capture reactions. They are RIs with different mass numbers but with the same atomic number as the target isotope. On the other hand, muon capture reactions provide mainly nuclear isotopes with an atomic number of Z-1.


The study of neutrino nuclear response by ordinary muon capture (OMC) focuses on the β+ side response of double beta decay (DBD) and the astro anti-neutrino response [2, 6]. OMC excites the nucleus up to 100 MeV in the excitation range. Allowed, first forbidden and second forbidden β-multipoles are excited, as shown in Figure 1. The strength functions B(μ,E) are very sensitive to nucleonic and non-nucleonic correlations [4]. The large energy and momentum regions are similar to those involved in neutrino-less DBDs [2, 7-9]. Our recent work focuses on the neutrino nuclear responses for medium heavy nuclei. First, we used 100Mo as a target due to its being a familiar candidate for supernova neutrino experiments. The study of Mo DBD responses is under way, using a 100Ru target.


Fig. 1: Ordinary muon capture (OMC) on 100Mo excites the target up to ~100 MeV in excitation energy by a charge exchange reaction given in eqn.(1). The excited state decays by multiple neutron and proton emissions are shown.

Investigations of OMC have been performed in a limited manner for stable and unstable nuclei and it will be a major undertaking to extend the investigations to neutrino nuclear response studies. Possibilities for realizing this goal are being investigated at the moment [9-11]. In the previous works, βγ-rays following such OMC RIs are shown to be very useful for studying DBD responses [2-5, 9, 12, 13] and also for studying fine nuclear isotopes of pure and applied science interests [10]. For medium-heavy nuclei, OMC is followed mainly (95%) by neutron emission with the remaining 5% coming from other particle emissions such as proton emission [9, 10, 14-16].

In reference [9], the extensive study of muon strength distribution using OMC on 100Mo has been studied. The negative muon beam from the D2 beamline of the Material Life Science Facility (MLF) J-PARC was used to irradiate 100Mo (94.5% in enrichment). RIs produced after the emission of up to 5 neutrons were observed. The total number of the stopped muons was around 108. The prompt and delayed γ-rays from RIs produced by (μ, xnν) reactions with x = 0, 1, …, 5 were measured by means of two Ge detectors. The irradiation of the target was made for 6.5 hours.

The prominent γ-ray peaks from Nb and Tc isotopes have been measured at 100Nb (535 keV), 99Nb (137keV), 99mTc: (140.5 keV, 181 keV and 735 keV), 98Nb: (722 keV and 787 keV), 97Nb: (658 keV) and 96Nb: (460 keV, 569keV and 778 keV) in the online and offline γ spectra. The peak yields undergo analysis to obtain the number of isotopes produced by (μ, xnν) reactions and thus RI mass distribution can be obtained. It shows that a (μ, ν) reaction with no neutrons is less probable compared to (μ, 1nν) and (μ, 2nν) reactions. The observed RI mass distribution for OMC on 100Mo is compared to the calculated mass distribution using the neutron emission model (NEM) to evaluate the strength distribution of the reactions. The obtained RI mass distribution is compared with the observed one in Figure 2(a).

The agreement with the observed data is quite good. Here, two giant resonance (GR) peaks EG1 and EG2 are observed at 12 MeV and 29 MeV (Fig. 2(b)) with an intensity ratio of EG1 / EG2 = 1/6. The OMC GR energy of 12 MeV is a bit smaller than the GR energy of 14 MeV for the photon capture reaction (PCR). The wider width of 8 MeV for the OMC GR is due to the mixed components of J 𝜋 = 1-, 1+, 2-, 2+, ... while PCR GR (5 MeV) has only one component, J 𝜋 = 1-.


Fig. 2: Output from a neutron emission model (NEM): (a) the comparison between calculated and experimental data from reference [9] and (b) the strength distribution to reproduces the RI mass distribution in (a).

The NEM was developed in 2014 [17] for the evaluation of relative capture strength from radioisotope production rates. Since then, various calculations have been made for understanding the formation of the giant resonance peak populated by muon capture reactions in 2 < A < 209 [18-21]. These calculations have been compared with previous experimental works by OMC on the nuclei reported in references [1, 11, 22]. From these observations, it can be noted that one neutron emission gives a major contribution of about 45% to 65% after muon capture. For lighter nuclei, the one neutron emission is greater than 50%. The mass distribution provides the relative capture strength of the reaction where captures on enriched nuclei populates almost 95% of the RIs by neutron emission. Natural targets with a wide isotope mass distribution can also be used for producing various residual isotopes in a wider mass range where high chances of proton emission can be observed.

The NEM shows preferential excitation of the GR region with EG1 = 10 - 20 MeV in the nucleus . The second GR is expected to reproduce the experimental data at peak around EG2 = 25 - 40 MeV. The strength distribution of B(μ, E) is given by the sum of the two giant resonance strengths of B1(μ, E) and B2(μ, E) [2, 9]



where EGi and Γi with i=1,2 are the resonance energy and the width for the ith giant resonance, and the constant Bi(μ) is expressed as Bi(μ) = σiΓi/(2𝜋) with σi being the total strength integrated over the excitation energy. The parameters of EG1 and EG2 as a function of A are given as EG1 = 25A-1/5 and EG2 = 75A-1/5 for OMC on 23Na, 24Mg, 27Al, 28Si, 40Ca, 56Ni, 76Se, 100Mo, 106Cd, 127I, 150Sm, 197Au and 209Bi. They are obtained from a comparison of NEM calculations with experimental data. As a final remark, the GR distribution obtained by this comparison provides information of the relative capture strength.

Primakoff derived absolute muon capture rates [23]. The capture rate is expressed as


From experimental data, Primakoff obtained X = 0.73 and X′ = 3. Later, total muon capture experiments [24] were performed at the M20 channel in TRIUMF for many light and heavy target nuclei, i.e., 12C, 18O H2O, LiF, CaF2, PbF, CCl4, Sc2O3, MnO2, GeO2, Br, I, BaO, NdO, W, and HgO. For all these nuclei, the impurities were less than l %. For heavy elements, higher order Pauli corrections become necessary and the equation (4) is modified as


Together with their experimental data and calculations using equation (4) and (5), they reported the mean lives τ and the total capture rates Λc for isotope with 1 < Z < 94.

The partial capture rate measured by [11, 14-16, 22] is deduced from muon disappearance rate ΛT expressed by,


where ΛC = ΛC(0n)+ΛC(1n)+ΛC(2n)+ΛC(1p)+…, Λfree is the free muon decay rate (0.4552 × 106 s-1) and H is the Huff factor from reference [24]. In the publication [11], the decay rates for natural Se, Kr, Cd and Sm and also enriched 48Ti, 76Se, 82Kr, 106Cd and 150Sm were observed by experiments at μE1 and μE4 beamlines at Paul Scherrer Institute (PSI).


The present OMC is also used for a non-destructive high-sensitivity detection (assay) of nuclear isotopes. It is interesting for basic and applied science. The sensitivities are of the orders of ppm-ppb by measuring nuclear gamma rays following muon capture reactions [10]. The feasibility test of this method was done at MuSIC, Osaka University in 2012 using NatMo targets. Low-energy negative muons are stopped in a bulk sample, where each muon is trapped in one of the atoms and promptly emits muonic x-rays. Then the muon either decays to electron directly or is captured into the nucleus. The muon is likely to be captured into the nucleus unless the atomic number is smaller than Z =10. Muon capture isotope detection (MuCID) uses OMC nuclear reactions to transmute isotopes X of interest to radioactive isotopes (RIs) X′ and the production of the X′ RIs is measured by observing nuclear γ rays with high-sensitivity Ge detectors. The low background (BG) and high energy-resolution measurements of the characteristic γ rays are key points of the present high-sensitivity detection/assay method.

The resonant photonuclear isotope detection (RPID) method using photon-capture reactions has been shown to be very useful for nuclear isotope detection [25]. It has similar sensitivities as the present MuCID, but the radioactive isotopes produced by muon and photon capture reactions are very different. Accordingly, sample forms and nuclear isotopes to be studied by the present MuCID are different from those by RPID. Neutron activation analysis has extensively been used for high-sensitivity isotope assay. It, however, is used mainly for isotopes with a large neutron-capture cross section. On the other hand, muon capture probabilities are almost 100 % for all isotopes with Z ≥ 10. Thus, MuCID is used for nuclei in a wide mass region if the reaction products are radioactive. The decay and capture scheme of MuCID, RPID and neutron activation is shown in Figure 3.


Fig. 3: Decay scheme comparing MuCID, RPID and neutron activation reactions.

MuCID and RPID are summarized in Table 1. In a MuCID method, more RIs could be observed mostly coming from the isotope where x = 0, 1, 2, ..., 5. In 100Mo [9] and NatMo [10], the total RI production rate is 95% and 43% respectively. Predominantly, the excited state with the excitation energy E in the after the μ-capture (μ,νμ) reaction de-excites by emitting neutrons at the pre-equilibrium (PEQ) and equilibrium (EQ) stages [26] if the state is neutron unbound, and de-excite by emitting γ rays to the ground state if it is particle bound.

MuCID, RPID and neutron activation are complementary to each other. They are often used to study rare and/or small components of nuclear isotopes, which are of great interest in astro-nuclear, particle and material sciences, geological and historical science and for other fields of science and technology, as shown in Table 1.

Muon capture isotope production (MuCIP) by using negative muon capture reaction can be used to provide efficiently various kinds of nuclear isotopes for studies of fundamental and applied science [27]. The large capture probability of a muon into a nucleus, together with the availability of a high intensity muon beam, makes it possible to produce nuclear isotopes of the order of 106-9 per sec. Radioactive isotopes (RIs) produced by MuCIP are complementary to those by photon and neutron capture reactions, and are used for various kinds of applications in science and technology.

MuCIP on Mo, using the RCNP MuSIC muon beam, was made at RCNP to demonstrate the feasibility of MuCIP [27]. Radioactive 99Mo isotopes and meta-stable 99mTc isotopes, which are used extensively for medical science, were produced by MuCIP on 100Mo.


Table 1: Radioactive isotopes to be studied by MuCID (μ, xn) reactions and comments (examples) on RIs by RPID (γ, n) reactions. Half-lives are given by d: day or h: hour.


μ reaction

RI (half life)

Comments on (γ, n)


(μ, 2n)

52Mn (5.59 d)

53Fe: short life


(μ, 0n)

56Mn (2.58 h)

55Fe: no γ


(μ, 0n)

65Ni (2.5h)

64Cu: 12.7 h


(μ, 0n)

90Y (64.1 h)

89Zr: 78.4 h


(μ, 0n)

92Y (3.54 h)

91Zr: stable


(μ, 0n)

99Mo (65.9 h)

98Tc: long life


(μ, 0n)

109Pd (13.7 h)

108Ag: short / long life


(μ, 1n)

127Sb (3.85 d)

127Te: 9.4 h, 109 d


(μ, 0n)

187W (23.7 h)

186Re: 90.6 h


(μ, 0n)

197Pt (18.3 h)

196Au: 6.18 d


(μ, 0n)

233Pa (27.0 d)

232U: long life


(μ, 1n)

234Pa (6.7 h)

234U: long life


(μ, 0n)

239Np (2.36 d)

238Pu: long life


(μ, 0n)

240Np (1.03 h)

239Pu: long life


The present report discusses briefly (1) a new method to study the neutrino nuclear response relevant to astro-antineutrino interactions and DBDs by OMC and (2) a new method for isotope detection and production by OMC. μ - γ spectroscopy is reliable for studying the higher momentum-transfer responses of the intermediate nuclei of DBDs in comparison with the lower momentum-transfer responses by electron capture. The neutron emissions following OMC on natural molybdenum and enriched molybdenum show that 1 neutron emission is the dominant process and 2 or 3 neutron emissions are appreciable. The statistical neutron emission model, with neutron emissions at the PEQ and EQ stages, is used to evaluate the muon-capture strength from the observed RI-mass distributions.

The characteristic delayed gamma rays following neutron and proton emissions have been well studied in order to evaluate the residual RI production rates. The dominant excitation by OMC is the muon giant resonance at a lower energy excitation of around 10 to 15 MeV. Some strength is located at around 20-30 MeV. The relative strength distribution, together with the muon capture lifetime, can be used to study the muon capture strengths, which are used to help determine the neutrino nuclear responses for astro-antineutrinos and DBDs.

Theoretical work on the 100Mo data [9] has recently been made using proton neutron quasi particle random phase approximation (pn-QRPA). The observed GR strength is well reproduced for the OMC on 100Mo. Neutrino nuclear responses calculated by pn-QRPA may be used to study weak coupling constants such as gA and gPP. [28].

A collaborative project involving three institutes-Universiti Teknologi Malaysia (UTM); the Research Center of Nuclear Physics (RCNP), Osaka University; and the Joint Institute for Nuclear Research (JINR), Russia - began in 2017. The collaboration aims to study the neutrino nuclear response by OMC through the observation of prompt and delayed gamma rays on various nuclei. The first joint program took place at the MuSIC facility, RCNP, Osaka University on Feb 2018. The results on muon absolute lifetimes on 100Mo, NatMo, NatRu and NatSe are currently under progress.


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Izyan Hazwani Hashim is a senior lecturer at the Universiti Teknologi Malaysia, Johor Bahru and a research fellow of the National Centre Particle Physics, Universiti Malaya and visiting researcher at the Research Center for Nuclear Physics (RCNP), Osaka University. She received her PhD in physics from Osaka University, Japan in 2014. She is an experimental physicist with research interests in neutrino nuclear physics, double beta decays and muon physics.

Hiro Ejiri is a research professor at the Research Center for Nuclear Physics (RCNP), Osaka University and an emeritus professor of Osaka University. He received his PhD in 1963 from the University of Tokyo. He has worked at the University of Tokyo, the University of Washington, the University of Copenhagen, the University of California and Osaka University. He is a former director of RCNP, Osaka University. His research fields include nuclear structures, nuclear reactions, hyper nuclear physics, neutrino nuclear physics, double beta decays, and dark matter.

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