The Muon Science Facility at the JKJ Project

The muon science facility is one of the experimental arenas of the JKJ project, which was recently approved for construction in a period from 2001 to 2006, as well as neutron science, particle and nuclear physics, neutrino physics and nuclear transmutation science. The muon science experimental area is planned to be located in the integrated building of the facility for the materials and life science study. One muon target will be installed upstream of the neutron target in a period of phase 1. The beam line and facility are designed to allow the later installation of a 2nd muon target in a more upstream location. The detailed design for electricity, cooling water, primary proton beam line, one muon target and secondary beam lines (a superconducting solenoid decay muon channel, a dedicated surface muon channel, and an ultra slow muon channel) is underway. In the symposium, a latest status of the muon science facility at JKJ project will be reported. This revised version was published online in August 2006 with corrections to the Cover Date.     

Microscopic state of the positive muon in 2H-NbSe2

The microscopic state of the positive muon in the transition metal dichalcogenide 2H-NbSe₂ was studied using the muon spin relaxation method (μ⁺SR). We found that the μ⁺SR spectra consist of two components. The ratio of the two components and the dynamics of the muon change at 140 K, at the charge-density-wave transition temperature (32 K), and again at the superconducting transition temperature (7 K). We discuss the relation between conduction electron properties and the muon's behavior. This revised version was published online in August 2006 with corrections to the Cover Date.     

Dramatic suppression of antiferromagnetic coupling in nanoparticle CuO

Magnetic susceptibility and muon spectroscopy measurements are carried out on antiferromagnetic nanoparticles (AFN) of CuO that are directly prepared by ball-milling a single crystal. The recently reported ferromagnetic features in AFN CuO of sol-gel growth are confirmed. New and significant result of the present work is a direct observation of an unusual dramatic reduction of the antiferromagnetic transition temperature, T_N, from the bulk T_N=230 to 50 K in AFN CuO by the μSR measurement.     

Ultra slow muon microscopy by laser resonant ionization at J-PARC, MUSE

As one of the principal muon beam line at the J-PARC muon facility (MUSE), we are now constructing a Muon beam line (U-Line), which consists of a large acceptance solenoid made of mineral insulation cables (MIC), a superconducting curved transport solenoid and superconducting axial focusing magnets. There, we can extract 2 × 10⁸/s surface muons towards a hot tungsten target. At the U-Line, we are now establishing a new type of muon microscopy; a new technique with use of the intense ultra-slow muon source generated by resonant ionization of thermal Muonium (designated as Mu; consisting of a μ ^?+? and an e^???) atoms generated from the surface of the tungsten target. In this contribution, the latest status of the Ultra Slow Muon Microscopy project, fully funded, is reported.     

Negative muon capture ratios for nitrogen oxide molecules

Journal of Radioanalytical and Nuclear Chemistry - Per-atom muon capture ratios are precisely determined from characteristic muonic X-ray measurements of three nitrogen oxide samples (NO, N2O, and...     

Time-reversal symmetry-breaking superconductivity in heavy-fermion PrOs4Sb12 detected by muon-spin relaxation

We report on muon-spin relaxation measurements of the 4f(2)-based heavy-fermion superconductor filled-skutterudite Pr(Os4Sb12. The results reveal the spontaneous appearance of static internal magnetic fields below the superconducting transition temperature, providing unambiguous evidence for the breaking of time-reversal symmetry in the superconducting state. A discussion is made on which of the spin or orbital component of Cooper pairs carries a nonzero momentum.     

Muon science in J-PARC

The intensity of proton accelerator has attained to the order to mega-watt, and several MW-class proton accelerators start to operate in the world. J-PARC is a complex of three accelerators, and generates a variety of secondary beams, i.e. muon beam, neutron beam, meson beam and neutrino beam. The muon facility is established in order to provide a pulsed muon beam for various experimental programs. The first muon beam is transported to the experimental area in September 2008. Although the accelerator is still under commissioning, and the beam power doesn’t reach the design value of 1 MW yet, the world strongest pulsed muon beam will be provided shortly. In this paper, we review the muon beam line in J-PARC, and discuss evolved scientific programs.     

μSR Study of the Unusual Magnetic Ordering in the Frustrated Antiferromagnet Zn(Cr x Ga1−x )2O4

μSR spectra on the spin frustrating spinel antiferromagnet Zn(Cr _x Ga_1−x )₂O₄ (x=0.9,1.0) have been measured. For x=1.0 compound, both the relaxation rate and the initial asymmetry showed distinct anomalies at the Néel temperature. The magnetic susceptibility for the x=0.9 compound was known to have a faint peak at around 12 K, whose origin was not clear so far. Our μSR study revealed that this temperature is the onset temperature of development of the magnetic correlation accompanied by appreciable spin fluctuations. This revised version was published online in September 2006 with corrections to the Cover Date.     

A naturally occurring trap for antiprotons

The phenomenon of delayed annihilation of antiprotons in helium is the first instance of a naturally occurring trap for antimatter in ordinary matter. Recent studies of this effect at CERN are summarised, and plans are described for laser excitation experiments to test its interpretation in terms of metastable exotic helium atom formation.