Muon Capture Research Articles

Non-Destructive Composition Identification for Mixtures of Iron Compounds Using a Chemical Environmental Effect on a Muon Capture Process

Abstract The non-destructive investigation of the chemical state of elements within a material is urgently needed in various scientific research fields. In recent years, non-destructive elemental analysis methods using muons have been developed. These methods identify elements by measuring muonic X-rays emitted from muonic atoms formed by the muon irradiation of the material. Interestingly, muonic atom formation processes are slightly influenced by the chemical state of the muon-capturing atom, and as a result, the muon capture probability of each element and the muonic X-ray emission intensity change depending on the chemical state. By utilizing this effect, it may be possible to know the chemical state at the same time as elemental analysis. In this study, the compositions of γ-Fe2O3 and Fe3O4 in an ironsand sample were determined using two approaches: muonic X-ray intensity ratios and muon capture ratios. The mixing ratios obtained from the two approaches were consistent with each other and consistent with results of the Mössbauer technique, a completely different analysis method. In this study, non-destructive chemical state analysis using muons was successfully demonstrated, and this method is promising for applications in various research fields.

Determination of coupling patterns by parallel searches for μ− → e+ and μ− → e− in muonic atoms

We investigate a possibility that the μ−→e+ conversion is discovered prior to the μ−→e− conversion, and its implications to the new physics search. We focus on the specific model including the mixing of the SU(2)L doublet- and singlet-type scalar leptoquarks, which induces not only the lepton flavor violation but also the lepton number violation. Such a structure is motivated by R-parity violating (RPV) supersymmetric models, where a sbottom mediates the conversion processes. We formulate the μ−→e+ rate in analogy with the muon capture in a muonic atom, and numerically evaluate it using several target nuclei. The lepton flavor universality test of pion decay directly limits the μ−→e+ rate, and the maximally allowed μ−→e+ branching ratio is ∼10−18 under the various bounds on RPV parameters. We show that either μ−→e− or μ−→e+ signals can be discovered in near future experiments. This indicates that parallel searches for these conversions will give us significant information on the pattern of coupling constants.

Nuclear Excitation by Free Muon Capture

Efficient excitation of nuclei via exchange of a real or virtual photon has a fundamental importance for nuclear science and technology development. Here, we present a mechanism of nuclear excitation based on the capture of a free muon into the atomic orbits (NEμC). The cross section of such a proposed process is evaluated using the Feshbach projection operator formalism and compared to other known excitation phenomena, i.e., photoexcitation and nuclear excitation by electron capture (NEEC), showing up to 10 orders of magnitude increase in cross section. NEμC is particularly interesting for MeV excitations that become accessible thanks to the stronger binding of muons to the nucleus. The binding energies of muonic atoms have been calculated introducing a state of the art modification to the Flexible Atomic Code. An analysis of experimental scenarios in the context of modern muon production facilities shows that the effect can be detectable for selected isotopes. The total probability of NEμC is predicted to be P≈1×10^{-6} per incident muon in a beam-based scenario. Given the high transition energy provided by muons, NEμC can have important consequences for isomer feeding and particle-induced fission.

EMuS – A Pulsed Muon Facility for Multidisciplinary Research

A pulsed muon facility (the so‐called EMuS) at the China Spallation Neutron Source (CSNS) has been studied since 2007. It aims for multidisciplinary applications but with a focus on those based on muon spin rotation/relaxation/resonance techniques. As a standalone facility, EMuS will take about 5% or 25 kW of the total beam power (500 kW) from the CSNS‐II accelerator complex. Two schemes have been designed: the baseline scheme is based on an inner conical target in graphite and superconducting solenoids for the capture and transport of pions and muons; the simplified scheme is based on a conventional thick target and room‐temperature magnets for transport. With the former, multiple kinds of muon beams can be provided, from surface muons, decay muons, negative muons, to low‐energy muons. Mainly surface muons are available with the simplified scheme. With a number of novel design concepts such as forward capture of pions/muons from a target station based on superconducting solenoids and triple spatial beam splitting of a muon beam, the design aspects of EMuS are presented here. The wide application potential and the R&D progress are also included.

Partial muon capture rates in A=3 and A=6 nuclei with chiral effective field theory

Searches for neutrinoless double-$\ensuremath{\beta}$ decay rates are crucial in addressing questions within fundamental symmetries and neutrino physics. The rates of these decays depend not only on unknown parameters associated with neutrinos, but also on nuclear properties. In order to reliably extract information about the neutrino, one needs an accurate treatment of the complex many-body dynamics of the nucleus. Neutrinoless double-$\ensuremath{\beta}$ decays take place at momentum transfers on the order of $100\phantom{\rule{0.28em}{0ex}}\mathrm{MeV}/c$ and require both nuclear electroweak vector and axial current matrix elements. Muon capture, a process in the same momentum transfer regime, has readily available experimental data to validate these currents. In this Letter, we present results of ab initio calculations of partial muon capture rates for $^{3}\mathrm{He}$ and $^{6}\mathrm{Li}$ nuclei using variational and Green's function Monte Carlo computational methods. We estimate the impact of the three-nucleon interactions, the cutoffs used to regularize two-nucleon $(2N)$ interactions, and the energy range of $2N$ scattering data used to fit these interactions.

Muon to Positron Conversion

Lepton-flavor violation (LFV) has been discovered in the neutrino sector by neutrino oscillation experiments. The minimal extension of the Standard Model (SM) to include neutrino masses allows LFV in the charged sector (CLFV) at the loop level, but at rates that are too small to be experimentally observed. Lepton-number violation (LNV) is explicitly forbidden even in the minimally extended SM, so the observation of an LNV process would be unambiguous evidence of physics beyond the SM. The search for the LNV and CLFV process μ−+N(A,Z)→e++N′(A,Z−2) (referred to as μ−→e+) complements 0νββ decay searches, and is sensitive to potential flavor effects in the neutrino mass-generation mechanism. A theoretical motivation for μ−→e+ is presented along with a review of the status of past μ−→e+ experiments and future prospects. Special attention is paid to an uncertain and potentially dominant background for these searches, namely, radiative muon capture (RMC). The RMC high energy photon spectrum is theoretically understudied and existing measurements insufficiently constrain this portion of the spectrum, leading to potentially significant impacts on current and future μ−→e+ work.

Measurement of proton, deuteron, triton, and α particle emission after nuclear muon capture on Al, Si, and Ti with the AlCap experiment

Background: Heavy charged particles after nuclear muon capture are an important nuclear physics background to the muon-to-electron conversion experiments Mu2e and COMET, which will search for charged lepton flavor violation at an unprecedented level of sensitivity. Purpose: The AlCap experiment aimed to measure the yield and energy spectra of protons, deuterons, tritons, and α particles emitted after the nuclear capture of muons stopped in Al, Si, and Ti in the low-energy range relevant for the muon-to-electron conversion experiments. Methods: Individual charged particle types were identified in layered silicon detector packages and their initial energy distributions were unfolded from the observed energy spectra. Results: The proton yields per muon capture were determined as Yp(Al)=26.64(28stat.)(77syst.)×10−3 and Yp(Ti)=26.48(35)(80)×10−3 in the energy range 3.5–20.0 MeV, and as Yp(Si)=52.5(6)(18)×10−3 in the energy range 4.0–20.0 MeV. Detailed information on yields and energy spectra for all observed nuclei are presented in the paper. Conclusions: The yields in the candidate muon stopping targets, Al and Ti, are approximately half of that in Si, which was used in the past to estimate this background. The reduced background allows for less shielding and a better energy resolution in these experiments. It is anticipated that the comprehensive information presented in this paper will stimulate modern theoretical calculations of the rare process of muon capture with charged particle emission and inform the design of future muon-to-electron conversion experiments.12 MoreReceived 23 October 2021Accepted 4 March 2022DOI:https://doi.org/10.1103/PhysRevC.105.035501©2022 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasElectron & muon captureLepton induced nuclear reactionsPhysical SystemsMuonic atoms & moleculesParticles & FieldsNuclear Physics

Search for Muon-to-Electron Conversion with the COMET Experiment

Charged Lepton Flavor Violation is expected to be one of the most powerful tools to reveal physics beyond the Standard Model. The COMET experiment aims to search for the neutrinoless coherent transition of a muon into an electron in the field of a nucleus. Muon-to-electron conversion has never been observed, and can be, and would be, clear evidence of new physics if discovered. The experimental sensitivity of this process, defined as the ratio of the muon-to-electron conversion rate to the total muon capture rate, is expected to be significantly improved by a factor of 100 to 10,000 in the coming decade. The COMET experiment will take place at J-PARC with single event sensitivities of the orders of 10−15 and 10−17 in Phase-I and Phase-II, respectively. The ambitious goal of the COMET experiment is achieved by realizing a high-quality pulsed beam and an unprecedentedly powerful muon source together with an excellent detector apparatus that can tolerate a severe radiation environment. The construction of a new beam line, superconducting magnets, detectors and electronics is in progress towards the forthcoming Phase-I experiment. We present the experimental methods, sensitivity and backgrounds along with recent status and prospects.

MuSun - Muon Capture on the Deuteron

The MuSun experiment is a precision measurement of the rate for nuclear muon capture on the deuteron, designed to resolve a long-standing disagreement between experiment and theory, and to determine an important low-energy constant relevant for a variety of weak and strong dynamics. The experiment is based on a novel active target method employing a pure deuterium cryogenic time-projection chamber. The data taking was completed in two main campaigns and the analysis is well advanced. The unique challenges and corresponding strategy of the experiment as well as the status of the analysis are presented.

A Study of $\tau^-$ and $\mu^-$ in the Field of Nuclei Using Neural Network Techniques

The rate of a heavy lepton (muon or tau) capture by nuclei as well as the heavy lepton to electron conversion rate can be calculated when the heavy lepton wavefunction is known. Analytical calculation of the wavefunction of any of these leptons around any nucleus is not feasible owning to their small Bohr radii, on the one hand, and to the finite nuclear extend on the other. A new numerical calculation algorithm is proposed hereby, which makes use of the concept of neural networks. The main advantage of this new technique is that the wave function is produced analytically as a sum of sigmoid functions.

Self-destructing atomic dark matter

Self-destructing dark matter (SDDM) is a class of dark sector models in which the collision of a dark sector particle with the earth induces its prompt decay into Standard Model particles, generating unique signals at neutrino detectors. The inherent fragility of SDDM makes its survival from the early Universe unlikely, implying a late time production mechanism. We present an efficient late time production mechanism for SDDM based on atomic rearrangement, the mechanism responsible for muon or antiproton capture in hydrogen. In this model, an atomic rearrangement process occurs in our Galaxy, converting dark atoms into highly excited bound states---our SDDM candidates. While the resulting SDDM is only a small fraction of the dark matter flux, its striking self-destruction signals imply a significant discovery reach in the existing data from the Super-Kamiokande experiment.

Ordinary Muon Capture for Double Beta Decay and Anti-Neutrino Nuclear Responses

This is a brief review on ordinary muon capture (OMC) experiments at Research Center for Nuclear Physics (RCNP) Osaka University relevant for the study of double beta decays (DBDs) and astro anti-neutrinos (neutrino) nuclear responses. OMC usually leaves the nucleus in highly excited unbound state. OMC is a charge exchange reaction via the charged weak boson as given by (μ,vμ) reactions with μ and vμ being the muon and muon neutrino. Subjects discussed include 1) unique features of OMC for studying DBDs and astro anti-neutrino (neutrino) nuclear responses, 2) experiments of OMCs on 100Mo and natMo to study neutrino nuclear responses for DBDs and astro anti-neutrinos, 3) impact of the OMC results on neutrino nuclear responses for DBDs and astro anti-neutrinos. Remarks and perspectives on OMC experiments for neutrino nuclear responses are briefly described.

Muon-Induced Single-Event Upsets in 20-nm SRAMs: Comparative Characterization With Neutrons and Alpha Particles

Negative and positive muon-induced single-event upsets (SEUs) are studied in 20-nm bulk planar SRAMs. Muon irradiation is performed using a mono-energetic source with varying the muon energy. The energy dependence of the cross sections (CSs) of SEUs and multiple-cell upsets (MCUs) shows the significant contribution of muon capture reactions for the negative muon, as reported in previous studies. Interestingly, MCU events are found for the positive muon, in contrast to the previous studies. The CSs for the negative and positive muons are compared with that for the other terrestrial radiations: high-energy neutrons, thermal neutrons, and alpha particles. The voltage dependence of the SEU CS, together with the empirical model for charge collection, demonstrates the difference in the contributing secondary ions among the negative muon, the high-energy neutron, and the thermal neutron. The MCU events are thoroughly analyzed in terms of their ratio to the total events and their fail bit patterns. The results reveal that the MCU characteristics for the negative muon are different from that for the other terrestrial radiations due to the muon capture reactions, where parasitic bipolar effects and the isotropic emission of secondary ions are important factors.

Comparative Analysis of Nuclear Matrix Elements of 0νβ+β+ Decay and Muon Capture in 106Cd

Comparative analyses of the nuclear matrix elements (NMEs) related to the 0νβ+β+ decay of 106Cd to the ground state of 106Pd and the ordinary muon capture (OMC) in 106Cd are performed. This is the first time the OMC NMEs are studied for a nucleus decaying via positron-emitting/electron-capture modes of double beta decay. All the present calculations are based on the proton-neutron quasiparticle random-phase approximation with large no-core single-particle bases and realistic two-nucleon interactions. The effect of the particle-particle interaction parameter gpp of pnQRPA on the NMEs is discussed. In the case of the OMC, the effect of different bound-muon wave functions is studied.

Machine-learning-based inversion of nuclear responses

A microscopic description of the interaction of atomic nuclei with external electroweak probes is required for elucidating aspects of short-range nuclear dynamics and for the correct interpretation of neutrino oscillation experiments. Nuclear quantum Monte Carlo methods infer the nuclear electroweak response functions from their Laplace transforms. Inverting the Laplace transform is a notoriously ill-posed problem; and Bayesian techniques, such as maximum entropy, are typically used to reconstruct the original response functions in the quasielastic region. In this work, we present a physics-informed artificial neural network architecture suitable for approximating the inverse of the Laplace transform. Utilizing simulated, albeit realistic, electromagnetic response functions, we show that this physics-informed artificial neural network outperforms maximum entropy in both the low-energy transfer and the quasielastic regions, thereby allowing for robust calculations of electron scattering and neutrino scattering on nuclei and inclusive muon capture rates.

High energy spectrum of internal positrons from radiative muon capture on nuclei

The Mu2e and COMET collaborations will search for nucleus-catalyzed muon conversion to positrons (${\ensuremath{\mu}}^{\ensuremath{-}}\ensuremath{\rightarrow}{e}^{+}$) as a signal of lepton number violation. A key background for this search is radiative muon capture where either (1) a real photon converts to an ${e}^{+}{e}^{\ensuremath{-}}$ pair ``externally'' in surrounding material, or (2) a virtual photon mediates the production of an ${e}^{+}{e}^{\ensuremath{-}}$ pair ``internally.'' If the ${e}^{+}$ has an energy approaching the signal region then it can serve as an irreducible background. In this work we describe how the near end point internal positron spectrum can be related to the real photon spectrum from the same nucleus, which encodes all nontrivial nuclear physics.

Chemical effect on muonic atom formation through muon transfer reaction in benzene and cyclohexane samples

Abstract To investigate the chemical effect on the muon capture process through a muon transfer reaction from a muonic hydrogen atom, the formation rate of muonic carbon atoms is measured for benzene and cyclohexane molecules in liquid samples. The muon transfer rate to carbon atoms of the benzene molecule is higher than that to the carbon atoms of the cyclohexane molecule. Such a deviation has never been observed among those molecules for gas samples. This may be because the transfers occur from the excited states of muonic hydrogen atoms in the liquid system, whereas in the gas system, all the transfers occur from the 1s (ground) state of muon hydrogen atoms. The muonic hydrogen atoms in the excited states have a larger radius than those in the 1s state and are therefore considered to be affected by the steric hindrance of the molecular structure. This indicates that the excited states of muonic hydrogen atoms contribute significantly to the chemical effects on the muon transfer reaction.

Exploiting Dirac equations solution for exact integral calculations in processes of muonic atoms

Precise muon wave functions are calculated by solving the Dirac equations within a method based on modern neural networks and stochastic optimization techniques. The reduced radial wave functions (up and bottom) of a bound muon in muonic atoms are then given as linear combinations of the well behaved sigmoid functions. As concrete applications, the known muonnucleus overlap integrals, which determine the structure and evolution of muonic atoms (twobody quantum systems bound together due to the fundamental electromagnetic interactions), may accurately be calculated. In this work, the muon-nucleus integrals for a set of nuclei including the 28Si and 64Zn entering the ordinary muon capture rates are exactly obtained through numerical integrations using Gauss integration algorithms.

Construction of the Gaseous and Solid-State Targets for the Muon Capture Measuring System in 130Xe, 82Kr, and 24Mg

The description of the muon capture system units with different targets (gas and solid-state) is presented. Using the muon-event selection system and high-purity germanium (HPGe) detectors, precise measurements of the time-energy distributions following ordinary muon capture in the isotopically-enriched $^{130}$Xe, $^{82}$Kr, and $^{24}$Mg nuclei were performed. The optimal parameters of the muon capture measuring system to ensure maximum muon stops in the targets under study and to collect effective statistics during measurements are reported. The system makes it possible to separate muonic X-ray events from gamma radiation with high accuracy, which is critical in such studies, and it is also suitable for future measurements with other targets.

Theoretical study of Nb isotope productions by muon capture reaction on Mo100

The isotope $ {}^{99} \rm{Mo} $, the generator of $ {}^{99m} \rm{Tc} $ used for diagnostic imaging, is supplied by extracting from fission fragments of highly enriched uranium in reactors. However, a reactor-free production method of $ {}^{99} \rm{Mo} $ is searched over the world from the point of view of nuclear proliferation. Recently, $ {}^{99} \rm{Mo} $ production through a muon capture reaction was proposed and it was found that about $ 50 \, \% $ of $ {}^{100} \rm{Mo} $ turned into $ {}^{99} \rm{Mo} $ through $ {}^{100} \rm{Mo} \left( \mu^-, n \right) $ reaction [arXiv:1908.08166]. However, the detailed physical process of the muon capture reaction is not completely understood. We, therefore, study the muon capture reaction of $ ^{100} \rm{Mo} $ by a theoretical approach. We used the proton-neutron QRPA to calculate the muon capture rate. The muon wave function is calculated with considering the electronic distribution of the atom and the nuclear charge distribution. The particle evaporation process from the daughter nucleus is calculated by a statistical model. From the model calculation, about $ 38 \, \% $ of $ {}^{100} \rm{Mo} $ is converted to $ {}^{99} \rm{Mo} $ through the muon capture reaction, which is in a reasonable agreement with the experimental data. It is revealed that negative parity states, especially $ 1^- $ state, play an important role in $ {}^{100} \rm{Mo} \left( \mu^-, n \right) {}^{99} \rm{Nb} $. The feasibility of $ {}^{99} \rm{Mo} $ production by the muon capture reaction is also discussed. Isotope production by the muon capture reaction strongly depends on the nuclear structure.