Keiko Nishiyama

Ultra Slow Muon Microscope at MUSE / J-PARC

We report current constructing states of the Ultra Slow Muon Beam at U-line / MUSE / J-PARC, which are supported by thermal muonium (Mu, μ+e−) production with the most intense pulsed slow muon beam, laser resonant ionization, and transportation of Ultra Slow Muon Beam. A thermal Mu is produced by a hot tungsten foil in a Mu-production chamber. At the laser resonant ionization process, a thermal Mu is ionized by coherent vacuum ultraviolet radiation and coherent 355-nm radiation. The coherent radiation sources are developed at RIKEN, installed in a laser cabin, and connected via a VUV steering chamber with the Mu-production chamber.

Asymmetric F–μ–F interaction of the muon in polyfluorocarbons

Abstract The muon can be used to study the strong F–H–F bond in many chemicals including fluorine. In various metal fluorides a characteristic spin relaxation function has already been reported due to a symmetric F–μ–F bond and nuclear dipole coupling from the two neighboring fluorine nuclei. We have observed an asymmetric F–μ–F or μ–F bond in various ‘Fluorinert’ compounds below the freezing temperature. On the other hand in ‘Teflon’ we found strong F–μ–F signals at low temperature.

μ-H Interaction in hydrogen bonding systems

Muon spin rotation (μSR) experiments were performed on single crystal samples of KH2PO4(KDP) and KD2PO4(dKDP) to study the dynamics of hydrogen in hydrogen bonding systems. At low temperature, the nuclear dipole interaction of muon and proton was confirmed from the angular dependence of precession frequency of the muon spin under zero magnetic field. The muon occupation site was also determined. A clear change in μSR spectra was observed at the antiferroelectric transition temperature (123 K). At 90 K well below the transition temperature, the muon spin starts to relax, possibly due to muon dynamics.

Development of muon rotating target at J-PARC/MUSE

A pulsed muon beam with unprecedented intensity will be generated by a 3-GeV 333-micro A proton beam on a muon target made of 20-mm thick isotropic graphite at J-PARC. The current muon target with a fixed target method has been utilized without replacements since the first muon beam generation on September of 2008 till June of 2014. However, the proton irradiation damage to graphite is significant for our case. To extend the lifetime, the developments of the muon rotating target, in which the radiation damage is distributed to a wider area, have been performed.

Pilot experiment for muonium photo ionization in GaAs

Direct observation of muonium photo ionization in GaAs was tried for the first time, with wide range wave length from 1325nm to 800nm lasers in n-type GaAs at 15 K. Recently, Lichti et al. determined the energy levels in the band gap of T center muonium (as an acceptor) and BC muonium (as a donor) by reanalysis of the existing data obtained by various μSR techniques for several semiconductors like Si, Ge, GaAs, GaP etc. In these semiconductors, GaAs is the best sample to apply the muonium photo ionization method for the first time, because the energy level of T center muonium is above 0.54 eV from the valence band, therefore the ionization energy for MuT− → MuT0+e− is 0.98eV (corresponding laser wave length is 1260nm), which is within the region of present OPO laser system produced, which was installed RIKEN-RAL

Multiple phase transitions in CuO studied by μSR

Precise measurements of the local magnetic field at muon sites in CuO were performed. The temperature dependence of the local field including spatial orientation was obtained in the ordered state. Not only zero field μSR measurements were performed, but also external magnetic fields were applied to obtain an unambiguous determination of field direction. Multiple stages of muon signals were observed at 55 and 80 K. An incommensurate phase between 212 K and 227 K was confirmed from the line shapes of the local fields at the muon sites.