We are developing a new type of photon detector in preparation of an experiment to search for muons decaying into positrons and gamma rays. In the experiment, the photon detector will utilize liquid xenon (Xe) as the scintillation material because of its fast response, large light yield, and high density. The scintillation light emitted in liquid Xe will be observed directly by positioning photomultipliers (PMTs) in the liquid without the use of a transmission window. In order to determine proper experimental procedures and study the detector response to gamma rays, we constructed a prototype utilizing a 100-liter volume of liquid Xe. Current status and future prospects of detector development are reported in this article.
The US-Japan collaboration in the constraction of a BNL muon storage ring for the g-2 experiment (E821) is described from the viewpoint of Japanese collaborators. Japan has contributed in the production of the pole pieces made of a vacuum-melted Ultra-Low Carbon Steel, Al-stabilized Nb/Ti superconductors for the superferric storage ring dipole coils, including technology transfer, and the development of a sophisticated superconducting inflector for muon injection. All of above items seem to be essential techniques to pursue accurate and detailed muon g-2 experiments. Recent experimental results are also mentioned in the latter part of this report.
Multi-strand superconducting cables are utilized for large superconducting machines and/or alternative or pulse current devices, such as fusion machines and superconducting magnetic energy storage (SMES) devices. In the numerical evaluation of AC loss, especially inter-strand coupling loss, it is important to establish a model for crossover contact resistance distribution among strands and/or sub-cables. For cables with multiple cabling stages, the contact resistance between two strands (or two sub-cables) changes along the conductor axis due to the related traces. Good contact is attained at locations where they crossover. On the other hand, when they are separated, there is no direct contact between them. The relationship between AC loss and contact resistance was evaluated for a short sample, a 3×3 twisted cable of Cr-plated Nb3Sn strands. The AC loss was measured using the calorie metric method at LHe temperature. The contact resistance between the two strands was also measured for the same sample under the same condition. The contact resistance distribution, which was expected as the result of the contact resistance measurement, was adopted to a numerical model and the AC loss was calculated using the non-uniform constant electric circuit. By comparing the calculated and measured losses, the contact resistance variation model was modified suitably. Finally, the relation between inter-strand coupling loss and contact resistance distribution are discussed from the viewpoint of constitution of the cable using established numerical model.
Evaluation of the Peltier current lead (PCL) system at the half-wave-rectified current (HWRC) mode was evaluated numerically, and compared with the results of Conventional current lead (CCL) and PCL at DC mode. Characteristics of the PCL system at HWRC mode were evaluated with reduction of the Seebeck coefficient of thermoelectric element (TE). Calculation results show that the heat leak is reduced by introducing the PCL although the conducting current has the large ripple. However, reduction of the heat leak was small in this calculation. The reason of small reduction is due to the gas cooling. Comparing to the effect of gas cooling and the Peltier effect, it was found that these effects have two same characteristics. They are 1) Both effects are the effect and heat transport from the lower temperature side to higher temperature side and depend on the temperature difference. 2) Both effects increase with current conduction. The former is due to the increase of current, and the latter is due to the increase of the heat leak by conducting the current.
Nb/Al precursor wire with a composition of Nb-25at%Al is converted into a bcc supersaturated solid solution (Nb(Al)SS) through RHQ treatment, of which the maximum heating temperature is around 2,000°C. However, the solubility limit of the bcc phase is 21.5at%Al. Therefore, the solid solution phase does not exist in a state of equilibrium at 25at%Al. Why 25at%Al bcc Nb(Al)SS forms is not clear. Accordingly, we studied the high-temperature state of the solid solution phase at around 2,000°C. We have made four kinds of samples with different Al compositions, ranging from 22.6at% to 30.0at%, and subjected them to RHQ treatment at temperatures from 1,900°C to 2,400°C. We measured variations in the microstructure and superconducting properties at the maximum heating temperature for each composition. We found that the temperature range where only the solid solution phase forms decreases as Al concentration increase. This clearly indicates that the solid solution phase extends to at least 27at%Al.
It is known that a piezo-resistive pressure sensor, FPS-51B manufactured by Fujikura Ltd., is available for ‘in situ’ pressure measurement in superfluid helium. The sensor covers a pressure range of zero to 103.4 kPa. The maximum rated pressure is 202.6 kPa at room temperature. The characteristics of the pressure sensor in a pressure range up to approximately 0.2 MPa were reported in detail for use in superfluid helium3,4). We measured the pressure characteristics of this sensor up to 1.5 MPa to determine its availability to be used under much higher pressure. Measurements were taken using a cryostat, which can be pressurized up to 1.5 MPa at room temperature and superfluid helium temperatures. It was proven that the sensor could be used in a superfluid helium environment of up to 1.5 MPa without any damage, and good reproducibility was shown for high pressures of up to 1.5 MPa.