大俘获立体角的内靶超导螺线管表面muon源的设计研究

高通量μ子源是国际上μ子科学研究的重要条件。在中国散裂中子源的高能质子应用区中,运用蒙特卡罗工具Geant4和G4beamline软件设计了使用内靶超导螺线管俘获高通量表面μ子的束线。与传统的分离靶和基于四极磁铁的收集系统相比,大孔径超导螺线管可以将收集效率提高两个量级。通过对不同靶材的粒子产率进行分析得出石墨是最佳靶材,然后比较俘获螺线管与束流的不同偏转角度下收集的表面μ的产率,提出了合理的较高产率的俘获和输运螺线管的设计方案,并与常规磁铁方案比较,最终在衰变螺线管端口的表面μ通量高达108/s。     

Negative muon capture in very light atoms

The transition rates for unbound muons to be captured into atomic bound states are calculated as functions of (1) incident muon center-of-mass energy, (2) muon principal quantum number n, and (3) muon (final) angular momentum l, for the hydrogen, helium, and lithium atoms. These rates reflect differences in electron binding energies. At muon energies of several hundred electron volts, lithium K-shell electrons are more likely to be ejected than the L-shell electron, while this behavior is reversed for energies ? 10 eV. However, in each case when the capture rate is folded with a muon stopping power function, the result is that more than half of the unbound muons are absorbed above 75 eV. Implications for experiments which look at muon transfer processes are noted.     

The influence of kinetic processes at a negative muon track endpoint in doped nondegenerate silicon on the behavior of muon spin polarization

This work is aimed at creating a theoretical basis for analyzing and interpreting the results of μSR experiments in doped nondegenerate semiconductors at temperatures below 50 K, when the influence of kinetic processes at the endpoint of a muon stopping track on the behavior of its polarization is substantial. The effects related to the formation of free electrons and holes in a solid-state plasma at a muon track endpoint are shown to be responsible for the mere possibility of observing negative muon spin precession at the muon frequency in doped nondegenerate semiconductors at low temperatures. The Vangsness-Bloch equations are generalized to the case of parameters varying with time. A theory based on these generalized equations allowed us to interpret more correctly the available experimental results of mSR studies of semiconductors with the use of negative muons. We showed that the μSR method could be used to obtain information about the cross sections of exchange scattering of electrons and holes by impurity centers in the region of energies inaccessible to the other measurement techniques and to estimate the cross section of capture by a solitary charged Coulomb center at virtually all charge carrier concentrations and temperatures. Under the conditions when the Debye radius is larger than the mean distance between charged particles but smaller than the Thomson radius, the capture (recombination) cross section is described by a temperature dependence qualitatively different from that predicted by current theory.     

Recent advances in positive muon spin rotation; muonium as a probe of fullerenes

In non-metallic solids the positive muon often forms paramagnetic muonium centers which are characterized by the hyperfine interactions of the unpaired electron with the positive muon and with the surrounding nuclear spins. The static and fluctuating components of these hyperfine interactions provide information on local molecular dynamics and local electronic structure. Some recent results on C₆₀ and related compounds are presented to illustrate this.     

Electron transport in solids and liquids

We review our recent experimental studies of the excess electron states in insulating solids and liquids. We use a muon spin relaxation technique to explore the phenomenon of delayed muonium formation: excess electrons liberated in the μ⁺ ionization track converge upon the positive muons and form Mu (μ⁺e^?) atoms in which the μ⁺ polarization is partially lost. The spatial distribution of such electrons with respect to the muon is shown to be highly anisotropic: the μ⁺ thermalizes well “downstream” from the center of the electron distribution. Measurements in electric fields up to 30 kV/cm allow one to estimate the characteristic muon-electron distance in different insulators: the results range from 10^?6 to 10^?4 cm. Such a microscopic length scale enables the electron to sometimes spend its entire free lifetime in a state which may not be detected by conventional macroscopic techniques. This circumstance illustrates the potential of μ⁺SR techniques in studies of electron transport in matter. The muonium formation process in condensed matter is shown to depend critically upon whether the excess electron forms a polaron or remains in a delocalized state. Different mechanisms of electron transport in insulators are discussed.     

Muonium formation via charge transport in solids and liquids

We review our recent experimental studies on delayed muonium formation in insulators and semiconductors. This involves the positive muon capturing one of the excess electrons liberated in its own ionization track and competes with recombination or escape of the electrons. The muon is generally found to thermalise well “downstream” from the center of the electron distribution, so that the transport mechanism of the electrons is a crucial factor. This is discussed in terms of the different tendencies to localization (as polarons in solids or in bubbles in liquids) vs. band‐like propagation. Studies of Van der Waals cryocrystals and cryoliquids are reviewed and some preliminary results reported for sapphire and silicon. Transport distances and times are determined from the variation of μSR signal amplitudes with applied electric and magnetic fields, respectively, enabling the development of a new technique for measuring electron mobilities on a microscopic scale. This revised version was published online in August 2006 with corrections to the Cover Date.     

The study of prompt and delayed muon induced fission

The mean life times of negative muons bound to actinide nuclei have been measured by detecting the time difference between a stopped muon and the arrival of fragments from delayed fission after muon capture. The deduced capture ratesΛ _c are 1.392(4)·10⁷/s for²³⁷Np, 1.290(7)·10⁷/s for²⁴²Pu and 1.240(7)·10⁷/s for²⁴⁴Pu. The results are compared with published data for the fission and the neutron decay channels and for the electron decay of the bound muon. Including a former measurement ofΛ _c for²³⁹Pu, an isotopic dependence of the muon capture rates in the Pu isotopes is clearly observed.     

Giant magnetic dipole analog excitations by the allowed muon capture in 1p-shell nuclei

The distributions of the allowed muon capture strengths in 1p-shell nuclei are shown to exhibit a tendency to concentrate in a few low-lying daughter states. These strong excitations are analogs of the giant magnetic dipole transitions, predicted by Kurath, and observed in inelastic electron scattering. Similar excitations are expected in 1S radiative pion capture and low-energy charged pion photoproduction processes.     

Muon capture on nickel and tin isotopes

We calculate total and differential muon capture rates on nickel and tin isotopes ranging from the proton dripline to the neutron dripline. The total rates decrease as the neutron number increases due to the combined effect of gradual blocking of available final-state neutron levels and of decreased phase space. The ordering of single-particle levels determines when blocking becomes important. We show that the total capture rates thereby are sensitive to the evolution of nuclear structure along an isotope chain.Received: 28 January 2003, Revised: 7 March 2003, Published online: 4 June 2003PACS: 24.30.Cz Giant resonances - 23.40.-s Beta decay; double beta decay; electron and muon capture - 23.40.Hc Relation with nuclear matrix elements and nuclear structure     

Electron wind-induced muon drift in metals

A preliminary experiment was carried out by the present authors in collaboration with R. Keitel to search for electron wind-induced muon drift in copper at liquid helium temperatures. Drift was not observed, which is taken as support of Kondo's theory of nonadiabatic damping of light interstitials in metals. The present paper discusses the possibility of increased current and thinner foils and concludes that it should be possible with a carefully optimized experiment to detect muon drift. Weak longitudinal fields do not appear to offer any advantage over the transverse fields in which the original experiment was performed. It is proposed to look for wind-induced drift through muon trapping. Aluminum with manganese trapping centers offers a favorable opportunity for observing drift in this way.     

μ-介子在He3原子核上的俘获

本文计算了μ介子在He³核上俘获的几率、末态H³核的角分布和极化。所采用的理论是带有重正化效应(包含弱磁矩及赝标项)的V-A普适弱作用理论。在计算中考虑了μ和He³核在始态有极化及处于不同超精细态上的情况。在计算中假定了He³核的基态是纯S态,这时忽略了由张量力以及其他自旋轨道耦合力引起的其他态。介子交换电流的效应也没有考虑。在以上这两个假定下,我们证明了俘获几率中只包含一个未知的原子核矩阵元,这个矩阵元恰好是原子核密度函数的富氏分量。利用μ介子(或电子)与He³(或H³)原子核的散射可以确定这个未知矩阵元。     

Positive muon behavior in KCl with and without F centers

Positive muon behavior in KCl containing F centers has been studied. The muon spin depolarization rate showed a maximum near 120 K, which was not found in pure KCl. This is probably due to the fact that free positive muons are trapped by F centers in KCl. However, the binding energy between a positive muon and an F center is not large, so that muons detrap again above 150 K.     

Muon Physics Possibilities at a Muon-Neutrino Factory

New intense proton accelerators with above GeV energies and MW beam power, such as they are discussed in connection with neutrino factories, appear to be excellently suited for feeding bright muon sources for low-energy muon science. Muon rates with several orders of magnitude increased flux compared to present facilities will become available. This will allow higher precision in experiments which were statistics limited so far such as searches for rare decays, muonium spectroscopy, muon capture, muon catalyzed fusion, muon decay studies and measurements muon moments and parameters. Novel and most important experiments will become possible. For example a permanent electric dipole moment (edm_μ) of a muon could be searched with by far unprecedented accuracy and with a physics potential well beyond the possibilities of present electron, neutron and nuclear edm searches. Investigations of short lived radioactive nuclei using muonic atom spectroscopy would become feasible. This revised version was published online in August 2006 with corrections to the Cover Date.     

Uncertainties in underground muon flux calculations

Influence of various theoretical and experimental uncertainties on results of underground muon flux calculations is estimated on the basis of a semi-analytical solution of muon spectrum transport equation. Status and accuracies of theoretical formulae for muon interaction cross sections and energy loss rate are discussed. A brief review of recent improvements in theoretical treatment of muon interactions is given. A necessity of a comprehensive verification of different muon transport codes is stressed.     

Muon capture rates on 16O leading to bound states of 16N

The partial muon capture rates on ¹⁶O leading to all four particle-stable states in ¹⁶N have been measured. The results obtained are compared to previous measurements and are also used to extract a value for the induced pseudoscalar coupling constant in muon capture.     

Angular correlation for nuclear muon capture

Formulae for the capture rates, gamma-neutrino correlations, and angular distributions of recoil in muon capture processes J_i J_f by an unpolarized nucleus for J_i ≦ 3 and J_f ≦ 5/2, with and without change of parity of nuclear levels, are given explicitely in terms of the multipole amplitudes. Relation of helicity amplitudes for muonic atom decay to the multipole amplitudes and to amplitudes introduced by other authors is presented. The derivation is based on kinematics only, and the formulae are strictly valid for the muon capture by any nuclei. The formulae are convenient for model calculations of partial transitions with realistic nuclear and muon wave functions and for the phenomenological analysis of a weak interaction.     

The (α–d) cluster model of6Li and muon capture

The (α–d) cluster model with parameters determined from electron scattering and pion photoproduction processes is used to calculate the muon capture rate in⁶Li. The result is found to be better than the results calculated in other models and is in agreement with the experimental data.     

Meson-exchange currents and energetic particle emission from μ− capture

We consider corrections to the impulse approximation calculation of the excitation function generated by muon capture in complex nuclei, in the form of captures on pairs of nucleons induced via meson-exchange currents. Within the Fermi gas model for the nucleus, these multinucleon captures are found to account for 4–8% of the experimental muon capture rates and to produce high excitation energies. The emission of pre-equilibrium protons with energies between 40 and 90 MeV is calculated for several nuclei using the hybrid exciton model. The agreement with experiment on emission spectra and rates is generally good. The emission of neutrons at these energies is also discussed.     

Electronic structure, dynamics, and metastability of muonium in semiconductors

Muonium centers are light hydrogen-like centers formed when positive muons are stopped in crystalline semiconductors. Detailed information on the hyperfine structure, dynamics and metastability of muonium are obtained using a combination of muon spin rotation or relaxation, muon level-crossing resonance and related methods. The expected close similarity to hydrogen, especially with regard to electronic structure, is important since the equivalent information on isolated hydrogen is either less detailed or completely absent. There are also interesting differences between muonium and hydrogen. In particular muonium dynamics are expected to exhibit enhanced quantum mechanical effects since the muon has only 1/9th the proton mass. In this paper we review the current status of experiments.