Muon Capture Research Articles

The muon capture rate of 48Ca

The muon nuclear capture rate on 48Ca has been measured to be 1.214(8)×10 6 s −1 at the PSI, Switzerland. An isotopically enriched target was used, and the sensitivity increased further by tagging on known γ-rays observed following the capture. Possible systematic errors in this procedure are discussed. The muon capture rate is compared to theoretical predictions and the isotopic variation of the capture rate is shown to deviate from the Primakoff estimate.

Thunderstorms' atmospheric electric field effects in the intensity of cosmic ray muons and in neutron monitor data

Theoretical and experimental results on the influence of thunderstorms' atmospheric electric field on cosmic ray secondary components are presented. On the basis of the approach proposed by Dorman [1987], theoretical models for a correct numerical evaluation of these effects on hard muon, soft muon, and neutron monitor component are developed. For hard and soft muons the validity of the models are checked by their comparison with experimental results of the Baksan muon detector. For the first time, the effect of thunderstorms' atmospheric electric field on cosmic rays is investigated by simultaneous measurements of one‐minute neutron monitor intensity and of atmospheric electric field at the Emilio Segre' Observatory on Mt. Hermon (Israel). A series of large thunderstorms during February 2000 is investigated; for each thunderstorm the maximum atmospheric electric field intensity was ranging from 10 to about 100 kV/m. Clear correlation between field intensity and neutron monitor intensity variations is presented for total intensity and for different detected multiplicity channels. This correlation is quantitatively in agreement with the developed model which takes into account the formation of short‐living meso‐atoms by the capture of slow negative muons in the lead of the monitor. The effect is relevant only for neutron events with detected multiplicity m = 1 and evident for multiplicity m = 2; the other multiplicity channels are not influenced by neutrons from meso‐atoms.

An analysis of the models for the radiative muon capture on a proton

The recent TRIUMF experiment for μ−p → nνμγ gave a surprising result that the induced pseudoscalar coupling constant gP was larger than the value obtained from μ−p → nνμ experiment by as much as 44%. Subsequent debates on the result gave rise to many theoretical calculations. Most of them were skeptical for the enlarged coupling constant gP. Therefore, one needs to reexamine the theoretical analysis of the matrix element of Beder and Fearing which was exploited for the extraction of the value in the experiment. In this report we analyse the matrix element from the viewpoint of electro-magnetic coupling schemes and suggest an additional term. This additional term plays an important role in restoring the standard value of gP.

Muon capture on35Cl

We report measurements of $\gamma$--ray spectra from muon capture on $^{35}$Cl. For the allowed Gamow--Teller transitions to the $^{35}$S$(2939, 3/2^+)$ state and the $^{35}$S$(3421, 5/2^+)$ state we obtained their capture rates, hyperfine dependences and $\gamma$--$\nu$ correlation coefficients. The capture rates and hyperfine dependences were compared to shell model calculations using the complete 1s--0d space and the universal SD interaction. With $g_p / g_a = 6.7$ and $g_a = -1.00$ (or $g_a = -1.26$) we found agreement of the model and the data at the 1--2 $\sigma$ level. However, we caution that the transitions are sensitive to $\ell = 2$ forbidden matrix elements.

On the muon–nucleus integrals entering the neutrinoless μ− → e− conversion rates

The muon–nucleus integrals which determine the nuclear-structure dependence of the branching ratio Rμe, i.e. the rate of the flavour-changing muon–electron conversion divided by the total rate of the ordinary muon capture, are extensively studied. Precise muon wavefunctions are employed which are obtained by solving the Schrödinger and Dirac equations. To this aim a method based on modern neural network techniques is developed which gives the radial muon wavefunctions as a linear combination of sigmoid functions.

Microscopic study of muon-capture transitions in nuclei involved in double-beta-decay processes

Total and partial ordinary muon-capture (OMC) rates to 1 + and 2 − states are calculated in the framework of the proton–neutron quasiparticle random-phase approximation (pnQRPA) for several nuclei involved in double-beta-decay processes. The aim is to obtain information on intermediate states involved in double-beta-decay transitions having these nuclei as either daughter or parent nuclei. It is found that the OMC observables, just like the 2 νββ-decay amplitudes, strongly depend on the particle–particle part of the proton–neutron interaction. First experiments measuring the partial OMC rates for nuclei involved in double beta decays have recently been performed.

Antiproton, kaon, and muon capture by atomic hydrogen

Capture of negatively charged, heavy particles by hydrogen atoms, i.e., ${X}^{\ensuremath{-}}+\stackrel{\ensuremath{\rightarrow}}{H}{X}^{\ensuremath{-}}p+e,$ where ${X}^{\ensuremath{-}}=\overline{p}$ (antiproton), ${K}^{\ensuremath{-}}$ (kaon), and ${\ensuremath{\mu}}^{\ensuremath{-}}$ (muon), is investigated by carrying out a rigorous full quantum-mechanical (QM) wave-packet calculation and a semiclassical (SC) calculation. An empirical law for the capture probabilities, found by the present author [Phys. Rev. A 65, 012706 (2002)], is examined extensively by using the QM and SC results. The empirical law is useful to obtain reasonably accurate capture cross sections at center-of-mass translational energies less than 10 eV. Furthermore, a local-complex-potential (LCP) model is employed to discuss a quantum-mechanical effect of the relative motion at very low energies. The LCP calculation shows that a resonance structure is seen in the capture cross section.

Beta-decay and exotic nuclei

Selected examples are given of how beta-decay can provide information on nuclei far from the line of beta-stability. Emphasis is put on beta-delayed particle emission, in particular processes with emission of several particles. The physics questions addressed include halo nuclei and multi-particle emission mechanisms. Muon capture on radioactive nuclei is discussed as an alternative way of accessing the region of high excitation energy.

RPA puzzle in 12C weak decay processes

We explain the origin of the difficulties that appear in a straightforward application of the QRPA in 12C, and we demonstrate that it is imperative to use the projected QRPA (PQRPA). Satisfactory results, not only for the weak processes among the ground states of the triad { 12 B, 12 C, 12 N} , but also for the inclusive ones are obtained. We sketch as well a new formalism for the neutrino–nucleus interaction that furnishes very simple final formulae for the muon capture rate and neutrino induced cross sections.

Radiative muon capture by3He

The rate of the nuclear reaction $^3He+\mu^- \to ^3H+\gamma+\nu_{\mu}$ has been calculated using both the elementary particle model (EPM) approach and the impulse approximation (IA) approach. Using the EPM approach, the exclusive statistical radiative muon capture (RMC) rate for photon energy greater than 57 MeV is found to be $0.245 s^{-1}$ and the ordinary muon capture (OMC) rate to be $1503 s^{-1}$. The IA calculation exhibits a slight dependence on the type of trinucleon wave functions used. The difference between the IA and EPM calculation is larger for RMC than for OMC. To resolve the difference between the two approaches a more detailed investigation including meson exchange corrections will be required.

Capture of negative muons and antiprotons by noble-gas atoms

Cross sections for capture of negative muons (μ - ) and antiprotons (p) by helium, neon. argon, krypton, and xenon atoms are calculated using the fermion molecular dynamics method. These cross sections are used to estimatethe capture ratios in mixtures.

Ordinary and radiative muon capture in liquid hydrogen reexamined

A simultaneous analysis is made of the measured rates of ordinary muon capture (OMC) and radiative muon capture (RMC) in liquid hydrogen, using theoretical estimates for the relevant atomic capture rates that have been obtained in chiral perturbation theory with the use of the most recent values of the coupling constants. We reexamine the basic formulas for relating the atomic OMC and RMC rates to the liquid-hydrogen OMC and RMC rates, respectively. Although the analysis is significantly influenced by ambiguity in the molecular state population, we can demonstrate that, while the OMC data can be reproduced, the RMC data can be explained only with unrealistic values of the coupling constants; the degree of difficulty becomes even more severe when we try to explain the OMC and RMC data simultaneously.

The μ−d capture rate in effective field theory

Muon capture on the deuteron is studied in a framework that essentially incorporates heavy baryon chiral perturbation theory (HBχPT). It is found that by far the dominant contribution to μd capture comes from a region of the final three-body phase-space in which the energy of the two neutrons is sufficiently small for HBχPT to be applicable. The single unknown low-energy constant having been fixed from the tritium beta decay rate, our calculation contains no free parameter. Our estimate of the μd capture rate is consistent with the existing data. The relation between μd capture and the νd reactions, which are important for the SNO experiments, is briefly discussed.

Radiative muon capture in hydrogen

We analyze the radiative capture of the negative muon in hydrogen using amplitudes derived within the chiral Lagrangian approach. Besides the leading and next to leading order terms, given by the well-known Rood-Tolhoek Hamiltonian, we extract from these amplitudes the corrections of the next order in $1/M$ $(M$ is the nucleon mass). In addition, we estimate within the same formalism also the $\ensuremath{\Delta}(1232)$ isobar excitation effects and processes described by an anomalous Lagrangian. The model we consider allows us to put the \ensuremath{\Delta} isobar off-shell. Our calculations show sensitivity of capture rates and photon spectra to Z, one of the off-shell parameters, related to the $\ensuremath{\pi}N\ensuremath{\Delta}$ vertex. We have found that the model can provide the photon spectra, which are in the interval $60 \mathrm{MeV}<~k<~{k}_{\mathrm{max}}$ $(k$ is the photon momentum) close to the experimental one.

In situ produced 14C by cosmic ray muons in ablating Antarctic ice

Samples of a core (52 m) of ablating Antarctic ice were analysed for 14CO and 14CO2 by accelerator mass spectrometry. The data were compared with a 14C in situ production model that includes muon capture in addition to oxygen spallation by neutrons. The analysis reveals significant in situ 14C at depths below 10 m, which we attribute to 14C production by cosmic ray muons. The age of the ice was determined as 9.3±0.4 14C ka BP.

Microscopic calculation of total ordinary muon capture rates for medium-weight and heavy nuclei

Total ordinary muon capture (OMC) rates are calculated on the basis of the quasiparticle random phase approximation for several spherical nuclei from 90Zr to 208Pb. It is shown that total OMC rates calculated with the free value of the axial-vector coupling constant gA agree well with the experimental data for medium-size nuclei and considerably exceed the experimental rates for heavy nuclei. The sensitivity of theoretical OMC rates to the nuclear residual interactions is discussed.

In situ produced 14C by cosmic ray muons in ablating Antarctic ice

Samples of a core (52 m) of ablating Antarctic ice were analysed for 14CO and 14CO2 by accelerator mass spectrometry. The data were compared with a 14C in situ production model that includes muon capture in addition to oxygen spallation by neutrons. The analysis reveals significant in situ 14C at depths below 10 m, which we attribute to 14C production by cosmic ray muons. The age of the ice was determined as 9.3±0.4 14C ka BP.

Inclusive neutrino and antineutrino reactions in 56Fe

We obtain cross sections for the inclusive neutrino and antineutrino reactions in 56Fe for the processes νe + 56Fe ⟶ e− + X, ν̄e + 56Fe ⟶ e+ + X, νμ + 56Fe ⟶ μ− + X and ν̄μ + 56Fe ⟶ μ+ + X. For the former two reactions we do this for incident neutrino energies from threshold to 240 MeV and for the latter two reactions from threshold to 300 MeV. We make use of a tensor model, which depends upon data obtained from the total muon capture rate in 56Fe, to calculate our results. We also obtain the electron neutrino cross section averaged over the Michel spectrum and the muon neutrino cross section averaged over the LAMPF muon neutrino spectrum. We find that these cross sections are substantial as would be expected from the neutron content of the 56Fe nucleus and might therefore be a serious background for nuclear neutrino experiments with substantial iron or steel shielding. We compare our calculation with the experimental results of the KARMEN Collaboration and discuss our results in light of planned experiments in iron.

Study of high-pressure hydrogen-operated wire chambers designed for a precision measurement of the singlet μp capture rate

This work was carried out as part of a project aiming at a greatly improved measurement of the muon capture rate from the singlet state of the μp atom. The experiment will be performed at the intense muon beam of PSI using a new experimental method allowing high precision measurements of the lifetime of muons stopped in ultra-pure deuterium-depleted hydrogen (protium). The basic element of the detector is a time projection chamber operating in hydrogen gas at 10 bar pressure. The arrival times and trajectories of the incoming muons and the outgoing decay electrons are measured with this device providing effective suppression of background. The system of chambers and electronics is designed for the large muon stop rates required for attaining high statistical accuracy. During four beam periods at PSI, data were taken. Also, various studies of the MWPC performance in hydrogen were made including ageing studies of the chambers under irradiation with stopped muons and with alpha and beta sources. It was demonstrated that the MWPCs can operate in pure hydrogen under 10 bar pressure with gas gains up to 5000, which is sufficient for the detection of relativistic electrons.

Weak interaction rates involving12C,14N,and16O

Muon capture and neutrino scattering cross sections in ${}^{12}\mathrm{C},$ ${}^{14}\mathrm{N},$ and ${}^{16}\mathrm{O}$ are calculated within a large shell-model basis. The effects of quenching of the isovector strength on these quantities are studied.