Kazuaki Ikeda

Development of a neutron detector based on a position-sensitive photomultiplier

A neutron scintillation detector based on a position-sensitive photomultiplier has been developed for neutron spin echo and small angle neutron scattering measurements. This photomultiplier has good spatial resolution, less than 1 mm2. The detection efficiency of gamma ray background is very low for using a thin ZnS/6LiF scintillator. The effective area of this detector is around 60 cm2.

Transport coefficients of InSb in a strong magnetic field

The improvement of a superconducting magnet makes induction of a strong magnetic field easier. This fact gives us a possibility of energy conversion by the Nernst effect. As the first step to study the Nernst element, we measured the conductivity, the Hall coefficient, the thermoelectric power and the Nernst coefficient of InSb, which is one of candidates of the Nernst elements. From this experiment, it is concluded that the Nernst coefficient is smaller than the theoretical values. On the other hand, the conductivity, the Hall coefficient and the thermoelectric power has values expected by the theory.

Transport Coefficients of Indium Antimonide in a Magnetic Field

Transport coefficients of indium antimonide in the presence of a temperature gradient and magnetic field were calculated numerically using the charge neutrality for a parabolic two-band model in classical statistics. Comparing theoretical with experimental results up to 4 T, the theoretical results are almost coincident with the experimental values except for the Nernst coefficient below 0.1 T. We assume that the parabolic two-band model is a criterion for estimating the power factor of the Nernst element.

Physical model of Nernst element

Generation of electric power by the Nernst effect is a new application of a semiconductor. A key point of this proposal is to find materials with a high thermomagnetic figure-of-merit, which are called Nernst elements. In order to find candidates of the Nernst element, a physical model to describe its transport phenomena is needed. As the first model, we began with a parabolic two-band model in classical statistics. According to this model, we selected InSb as candidates of the Nernst element and measured their transport coefficients in magnetic fields up to 4 Tesla within a temperature region from 270 to 330 K. In this region, we calculated transport coefficients numerically by our physical model. For InSb, experimental data are coincident with theoretical values in strong magnetic field.

Peltier current lead experiments with a thermoelectric semiconductor near 77 K [and HTSC]

Peltier current lead was proposed to reduce heat leak from the current lead. The temperature of the hot side of semiconductors was kept to be a room temperature and the liquid nitrogen was used to cool the system in the experiment. The experiment confirmed the principle of the Peltier current lead, and the reduction of the heat leak is calculated to be 30% for the liquid helium system and 40% for the liquid nitrogen system. We also proposed a new current lead system which is composed of semiconductors and high temperature superconducting material (HTS). This idea bases on the functionally gradient material (FGM), and the HTS is connected to the semiconductor directly. The temperature of the hot side of semiconductor is kept to be the liquid nitrogen temperature, the temperature of HTS can be expected to be lower than 77 K. Therefore, we can expect high current capacity of the HTS and/or high stability of the HTS. We use BiSb as a N-type semiconductor and BiTe as a P-type semiconductor in the experiment, and the temperature of the cold side of the semiconductor is 73 K in this experiment.

A fusion study of thermoelectric conversion in magnetic fields

We studied thermoelectric conversion and its application in a fusion reactor. We propose three new elements: first, a Nernst element; second, high-temperature thermoelectric materials; and third, a Peltier current lead. As these require new semiconductors, we conducted experimental and theoretical studies of semiconductor transport properties. Initially, we measured electrical resistivity, Hall coefficient, thermoelectric power, and Nernst coefficient of InSb. The Nernst coefficient is proportional to the mobility of the carrier, and InSb is a highly mobile semiconductor. The Nernst coefficient also depends on temperature, so a circular current is induced requiring Ohmic voltage considerations when estimating this coefficient. There is a microscopic effect, as electrical resistivity increases with the magnitude of the magnetic field, and a macroscopic effect, as a Hall voltage is induced, which creates a non-uniform current distribution.

Neutron Reflectometry for Observation of Machined Surface Roughness

Neutron reflectometry was applied for the characterization of machined roughness on iron surfaces, including a buried solid–solid contacting surface. The observed distribution of the reflected neutron beam could be explained using a structure factor estimated from the Fourier transform of the physical roughness measured by a contact-type roughness gauge, particularly in the case of a periodic surface structure. In the case of random roughness, the accumulation of beams specularly reflected from every small local region of the surface gave a better explanation of the broad distribution of the scattered beam, where each local region was assumed to be flat and differently inclined relative to the incident beam direction. When the surface was buried beneath another material, the width of the distribution of the reflected beam became slightly wider than that in air. Although neutron reflectometry for the characterization of surface roughness has various limitations, it is still an effective tool for nondestructively observing the surface roughness at a buried contacting surface.

New Estimations of Surface Properties for Tribological Studies by Neutron Reflectometry

Tribological phenomena usually occur at a solid surface or an interface of solids. To carefully examine a tribological phenomenon, information on the state at the surface or the interface must be tried to be gotten from different angles. Estimations of the state, however, are generally considered to be difficult because there are few methods suit for adequately picking up the information on profiles and/or properties of the only nearest region to the surface. Especially for the case that the target is at the interface of metals, the estimation is more difficult because a standard electron beam cannot penetrate into either of the metal. The authors have, accordingly, developed a new estimation method applying a ‘neutron reflectometry’ to such a metal surface or a metal-metal interface. The reflectometry is suitable for analysis of vertical structures of the nearest regions to a surface as less than 1 μm. In addition, since a neutron beam can penetrate into many kinds of common metals, the estimation by the neutron reflectometry can be useful even for the metal-metal interface. In this paper, we report some results obtained by some of several basic experiments on neutron reflectometry. Surface profiles and ultra-thin film thickness of PFPE lubricant at a metal surface and/or a metal-metal interface were estimated in the study.Copyright

Transport coefficients of Ge in magnetic fields

We measured the transport coefficients of InSb, Si and Ge in the magnetic field range of 0 to 4 Tesla near a room temperature. Transport coefficients obtained by measurements were electrical conductivity /spl sigma/, Hall coefficient R/sub h/, thermoelectric power /spl alpha/ and Nernst coefficient N. These experimental results have been verifying by the theoretical calculations based on the band model. We report the experimental results compared with theoretical values in the case of Ge. Theoretical values had qualitative agreements with experimental results.