A. Matsushita

Evidence for muonium emission from an SiO2 layer on n-type Si and the positive muon state in Si

Evidence for the emission of slow muonium atoms from a 3.0-nm-thick SiO2 layer covered on an n-type Si is reported. Also, upon applying an rf-resonance technique at the muon frequency, a time-differential observation of a delayed state-change from muonium to diamagnetic muon at room temperature was observed. Combining results obtained by use of longitudinal field decoupling and transverse spin rotation methods, the conversion rate was estimated to be 5 to 10 μs−1. Both of the above results, namely the observation of the emission and state-change of muonium, suggest a process in which μ+ initially captures an electron from Si, then quickly converts to μ+ again during thermal diffusion in the Si towards the SiO2 layer. Within the oxide layer, muonium is again formed and subsequently is emitted from the SiO2 surface.

Quantum diffusion of muonium in GaAs with shallow donor impurities

Muonium diffusion was studied in silicon doped GaAs by means of muon spin relaxation in a longitudinal field. The muonium hopping frequencies in two samples with n-type carrier concentration of 1012∼14 cm−3 and 8×1016 cm−3 were deduced by using the model of fluctuating effective local fields. We found that muonium diffusion is strongly influenced by the dilute Si impurity in both samples in the temperature range belowTΔ≃30K. The absence of such a behavior in compensated high-resistivity samples indicates that the presence of shallow donor levels plays a decisive role for the tunneling diffusion of muonium in semiconductors.

Muon spin relaxation for Ar+NO2 systems in transverse and longitudinal magnetic fields

Surface muons produced in UT-MSL were introduced into argon gas of 4.0±0.2 atm with NO2 (0–30 ppm), and muonium signals were detected in the presence of a transverse (1.7–3.4 G) and a longitudinal magnetic field (0–3.5 kG) at 295±1 K. The cross section for the transverse relaxation was (11.0±1.0)×10−16 cm2. The relaxation rates in different longitudinal magnetic fields show that the rate does not follow the conventional equation which assumes that the relaxation occurs mainly by spin-exchange interaction. Similar measurements were performed for the Mu+O2 system. These findings indicate that chemical reactions contribute to these relaxation rates.

Relaxed excited states and anomalous hyperfine structure of muonium centers in KBr

We report on the luminescence induced by positive muons implanted (with 4 MeV) into KBr crystal, which evidences a long-lived (lifetime=13.3 μs excited state produced by muon radiolysis. The temperature dependence of the luminescence yield has a strong correlation with the amplitude of an “anomalous” muonium center: both are observed only below ∼50 K. This correlation strongly suggests that the muonium center is perturbed by the muon-induced excitons to cause the anomalous hyperfine structure. Moreover, the luminescence energy and decay time indicate that the observed luminescence is not associated with the intrinsic or impurity-related self-trapped excitons, but with a relaxed excited state specific to the muon(ium)-KBr system.

Muon radiolysis in alkali halides

In order to shed new light on the initial loss of muon spin polarization, or socalled “missing fraction”, which is commonly observed in non-metallic solids, we have studied muon-induced excitation in various alkali halides by measuring the luminescences associated with the radiative decay of the self-trapped excitons (STE). The result strongly suggests that the spin-exchange interaction between muonium and muon radiolysis products including STEs causes fast muon depolarization in those materials.

Muonium emission andreaction on platinum and gold powder surfaces revealed by MuSR

Muonium has been observed in powdered platinum (30 nm diameter) andgold (100 nm diameter), respectively, placed in vacuum by the muonium spin rotation (MuSR) technique at ambient temperature. Upon introducing gaseous oxygen up to 23 Torr into platinum powder, the muonium signal was eliminated, indicating that the observed muonium stays outside the platinum particles. The result on the platinum surface treated by hydrogen but exposed to oxygen gas suggests a reactive collision between muonium andoxygen adsorbed atomically on the surface.