For research of high energy radiation effects of some materials as superconductors at cryogenic temperature, a cryostat was constructed. In the cryostat, the samples immersed in liquid He are irradiated by electrons and X-rays of about 30meV from a linear accelerator. The irradiated samples and the surrounding materials can be activated and some part of the radioisotopes may contaminate the evaporated He gas. The contamination interferes reuse of He gas. In this paper, possibility of the activation was estimated and the contamination of the He gas was experimentally confirmed. A radiation safety system was installed in association with the cryostat. The system monitors radiation dose in the He gas evaporated from the cryostat in real time, and exhausts the contaminated He gas and transports only the clean He gas to a gas holder for the reuse.
The present paper describes monitors for the detection of β-ray and γ-ray radionuclides in He gas recovered from the cryogenic irradiation equipment reported in the preceding publication (I). The γ-ray monitor consists of a 2″×2″φ Nal (Tl) scintillation counter in the monitor vessel and the β-ray monitor is composed of thin box-type ionization chambers piled in three layers which can measure separately the radioactive concentrations of the groups consisting of low, medium and high energy β-emitters. The performance tests of these detectors revealed a typical sensitivity of 3.5×10-3Bq/cm3 for a low energy β-emitter such as tritium in He gas. In addition, satisfactory selection of the three energy groups of β-emitters was demonstrated using 133Xe in the β-ray monitor.
A fracture toughness test system of a turret disc type at cryogenic temperatures was designed and constructed in order to save the testing time and liquid helium consumption for measurement. It includes a computer system for data acquisition and real time analysis using the single specimen unloading method, It enabled us to carry out successive five JIC tests within eight hours including the time for setting and precooling. A necessary amount of liquid helium was about forty liters for five tests. A 22Mn-13Cr-5 Ni steel was tested at liquid helium temperature on trial and the obtained data were studied and compared with those reported and those obtained from an ordinary multispecimen method. These results showed a good performance of the new test system.
Acoustic emission (AE) from composite materials during tensile test have been studied in order to investigate the fracture process at cryogenic temperatures. Commercially available glass cloth reinforced plastics (epoxy) has been chosen and tested at room (RT), liquid nitrogen (LNT) and liquid helium temperature (LHeT). Specimen was loaded in tensile mode and AE was simultaneously detected by PZT sensor of which resonance frequency was 140kHz. The AE counts at cryogenic temperatures were large compared with those at RT. The decrease of temperature bring the shift of the point, at which AE count showed rapid rise, to lower strain level. The temperature decrease also brought the increase of AE intensity. different types of AE signals were found in different fracture modes. It was confirmed that the identification of fracture mode of composite materials was possible in terms of AE analysis referring stress-strain curves.
A Nb3Sn composite conductor with about 10 thousands submicron diameter filaments has been manufactured by the external diffusion process. Fine filament size of 0.53μm (design value), a tight twist pitch of 0.87mm for the wire diameter of 0.153mm and bronze matrix resulted in a strongly reduced a.c. losses: the hysteresis loss and the coupling current loss were observed to be 465kW/m3 and 96kW/m3 respectively in the a.c. field of 50Hz with an amplitude of 2.0T. A triplex conductor was constructed by cabling three strands at a twisting pitch of 3mm and a small coil (inner diameter 11mm, outer diameter 33mm, axial length 19mm) was wound from this cable. In the d.c. mode operation, the coil generates a field of 1.3T at the critical current of IC=37.4A. When the coil is operated at 50Hz with an exciting current of IC, the observed loss averaged over the windings was 240kW/m3. The quenching current in 50Hz operation was 53A at the maximum field of 1.8T, which is considerably higher than the critical value under d.c. condition. Preliminary studies have shown that, if this conductor is used in superconducting armature windings of rotating machines, economical benefits are obtained compared with conventional armatures.
A complex type of magnetic refrigerant for the Ericsson cycle has been investigated. For the magnetic refrigerant in the temperature range above -15K we have to use the ferromagnetic material, which has the internal magnetic field to arrange its spin system as far as using a magnetic field of -6T, because of increasing of the thermal excitation energy for spins -kT. Moreover, we have to select the Ericsson cycle instead of the Carnot cycle usually used in the low temperature range below -15K, since the lattice entropy of a solid material in this range is not negligible in comparison with the magnetic entropy. In order to form a high efficient Ericsson cycle that includes two kinds of iso-magnetic field processes, a new severe condition is required for the magnetic refrigerant, that is, the magnetic entropy change ΔSJ is constant in the refrigeration range. This condition, however, can not be satisfied by an usual homogeneous ferromagnetic substance. In the present paper we first made clear from the theoretical analysis that the complex of the ferromagnetic materials, for instance (ErAl2)0.312(HoAl2)0.198(Ho0.5Dy0.5Al2)0.490, well satisfies the above condition. On the basis of the analysis we have prepared a complex consisted of several RAl2.15 compound layers with different rare earths R, such as (ErAl2.15)0.312(HoAl2.15)0.198(Ho0.5Dy0.5Al2.15)0.490. For the complex we determine the magnetic entropy change ΔSJs and the entropy S from the specific heat measurement. From these results we can conclude that the layer type complex material is most hopeful not only for the Ericsson type magnetic refrigerant but also for the regenerative material.
Design features of a simple, inexpensive and versatile homemade vibrating-sample magnetometer for low temperature measurements are described. The vibrating-sample magnetometer measures the magnetic moment of a sample when it is vibrated perpendicularly to a uniform magnetizing field. This paper deals with some variation and improvement of the mechanical design on the original Foner design. In particular, the loudspeaker assembly (vibrator) and a method to reduce the positioning error from the sample displacement in the horizontal plane, are given. As a typical operating characteristic, the temperature dependence of the magnetization curves for the samples of amorphous Fe78B13Si9 are shown.