K. Sakai

Nuclear spin maser with an artificial feedback mechanism

Abstract We describe a scheme for nuclear spin maser which has an artificial feedback to the nuclear spins to bring about a self-sustained nuclear spin precession and thus to allow unlimitedly long observation times for precession. This scheme can operate under lower applied fields and smaller spin densities than the conventional spin maser scheme. Its operation was experimentally tested by using a spin-polarized 129 Xe gas as a model substance. The frequency fluctuation and other basic properties were analyzed based on the modified Bloch equations.

Fabrication and thermoelectric characteristics of [(Bi,Pb)2Ba2O4±w]0.5CoO2 bulks with highly aligned grain structure

Here, we report fabrication and thermoelectric characteristics of polycrystalline bulks with aligned grains of the misfit-layered cobalt oxides, [(Bi1−xPbx)2Ba2O4±w]0.5CoO2. High-quality precursor powder was prepared with a spray-drying technique. The precursor was then sintered under a uniaxial pressure to yield dense bulk samples consisting of highly aligned grains. The electrical conductivity of such samples was four times higher than that of conventionally sintered bulks. Accordingly, the thermoelectric characteristics of the grain-aligned specimens were enhanced.

Pb-for-Bi substitution for enhancing thermoelectric characteristics of [(Bi,Pb)2Ba2O4±ω]0.5CoO2

We report strongly enhanced thermoelectric characteristics for a misfit-layered oxide, [Bi2Ba2O4±ω]0.5CoO2, in a wide temperature range, as achieved through substituting up to 20% of Bi by Pb. The Pb substitution kept the thermal conductivity (κ) unchanged but decreased the electrical resistivity (ρ) and increased the Seebeck coefficient (S) simultaneously, such that a three-fold enhancement in the thermoelectric figure of merit, Z (≡S2∕ρκ), was realized. At the same time x-ray absorption near-edge structure data indicated that the valence and spin states of Co are not affected by the Pb-for-Bi substitution.

MAGNETIC MOMENT AND ELECTRIC QUADRUPOLE MOMENT OF THE 18N GROUND STATE

Abstract The magnetic moment and electric quadrupole moment of a neutron-rich nucleus 18 N have been determined by the β-NMR method, taking advantage of the fragment spin polarization induced in the intermediate-energy projectile fragmentation reaction. To assure the existence of a sufficient polarization prior to the resonance search, a scheme of polarization measurement by means of an adiabatic field rotation has been developed and successfully incorporated in the β-NMR experiment. From the observed resonance in a Pt stopper the magnetic moment for the 18 N ground state has been determined to be |μ|=0.3279±0.0013μN. The 18 N fragments have also been implanted in a single crystal Mg stopper, and from the observed quadrupole frequency |eqQ/h|=73.2±1.4 kHz, the electric quadrupole moment |Q|=12.3±1.2 mb has been deduced. The obtained μ and Q are compared with shell model calculations.

Measurement of boron diffusivity in hydrogenated amorphous silicon by using nuclear reaction 10B(n,α)7Li

The method to measure boron profile by using the nuclear reaction 10B(n,α)7Li is applied to estimate the diffusivity of boron in the glow‐discharged hydrogenated amorphous–silicon (a–Si:H). It is found that the diffusivity of boron in a–Si:H is much larger than that in crystalline silicon (c–Si). For instance, it is about 2×1016 cm2/s at 330 °C and larger than in c–Si by twelve orders of magnitude. The activation energy of the diffusivity is also estimated to be 1.5 eV. These values are nearly equal to those of hydrogen in a–Si:H. The minimum duration, in which the characteristics of p–i–n type a–Si:H solar cells may start to change due to this boron diffusion, is roughly estimated using both boron diffusivity and its activation energy.

Structural model of sputtered fluorinated amorphous silicon

Structure of a fluorinated amorphous‐silicon alloy (a‐Si:F), which is produced by the sputtering of Si in mixtures of Ar and SiF4 gases, is studied by transmission electron microscope (TEM). As a special technique to study the localization of both atomic species and particular types of Si‐F configuration, the change of TEM micrograph due to anisotropic chemical etching of samples is observed, and it is compared with the similar changes in infrared absorption spectra and the Rutherford backscattering spectra. The evolved gases from heated a‐Si:F are also analyzed to determine the role of Ar atoms in the microstructure. It is found that a‐Si:F film consists of many small grains of a‐Si network, and that both Si‐F4 bonds and Ar atoms are localized at the grain boundary. Based on these results, the most possible structural model for a‐Si:F, which suggests key factors to improve the properties, is presented.

A resonance neutron-spin flipper for neutron spin echo at pulsed sources

Abstract We have developed a resonance neutron-spin flipper (RSF) with high frequency applicable to pulsed sources. This flipper is necessary for new neutron resonance spin echo spectrometers installed at pulsed neutron sources. The RSF works only for monochromatic neutron beams because of the dependence of its spin–flip probability on neutron velocity. The RSF can, however, be applied to pulsed neutron beam with wide band wavelength by varying the amplitude of the applied oscillating magnetic field since the pulsed neutrons are resolved into monochromatic components according to their velocity. In this paper, we describe the RSF system applicable to pulsed neutron sources, measure the flipping efficiency and discuss the experimental results.

Development of a solid-state detector with a 6Li/Ti multilayer converter for ultracold neutrons

A new type of solid-state detector for ultracold neutrons has been developed and confirmed to be applicable at 4.2 K. 6Li/Ti multilayers, in which the effective potential is compensated, were formed as a neutron converter by vacuum evaporation on the surface of a silicon-surface-barrier detector. The detector was tested using a cold-neutron beam, and the reaction products in 6+Li+n → α + t were clearly observed in the pulse-height spectrum with a detection efficiency of 0.28% for 4A neutrons. This is consistent with the estimated value, and implies that ultracold neutrons can be detected with an efficiency of greater than 60%. The detector enables us to study the characteristics of ultracold neutrons in liquid helium, which is necessary to discuss the feasibility of an intense ultracold neutron source based on the superthermal method.

Development of a high-resolution scintillator-based area detector for neutrons

We applied a scintillator-based area detector to the neutron time-of-flight method by developing a new electronics system for this detector, which detector consists of a thin plate of ZnS(Ag)+6Li scintillator optically coupled to wavelength shifter. The spatial resolution was about 0.4 mm which is consistent with a fiber width of 0.4 mm. The detection efficiency for thermal neutrons was about 6%. A good-resolution radiographic-image as a function of flight time was obtained.

Development of a spin flipper for an application of a neutron magnetic device

We have developed a radio-frequency gradient spin flipper for the effective utilization of the focused and polarized neutron beam obtained using a superconducting sextupole magnet developed for the practical application in the neutron-scattering experiment. The flipper was designed to be effective for the neutron beam with wavelengths longer than 4 A and the cross section of 50 mm in diameter. The flipper was tested using polarized cold neutrons, and good performance was obtained.