Development of a high-resolution large-mass cryogenic particle detector

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A dedicated dilution refrigerator has been constructed. It is capable of cooling one-kilogram of absorber material down to 10mK.It is made of specially selected raw materials to reduce natural radiation background from them. Especially, the inner and outer cylinders of the dewar vessel as well as the inner vacuum chamber of the refrigerator are all made of oxigen free copper.It is also made so that a lead radiation shield can be installed inside it. It has a specification suitable for a long time operation in an underground laboratory.A bolometric particle detector with an absorber of lithium fluoride crystal has been constructed with a newly developed germanium thermistor, which has a high sensitivity at around 20mK.It has been operated successfully in the dilution refrigerator.In order to manufacture many thermistors with nealy identical sensitivities, two methods were adopted. The one is to use germanium crystals melt-doped with antimony, and the other is so called neutron transmutation doping method. For the latter method, neutron irradiation has been done at the JRR-3 reactor of the Japan Atomic Energy Research Institute.We also developed bolometers with absorbers of molybdenum, nickel oxide, zinc oxide, zinc sulfide and cadmium telluride crystals for the neutrinoless double positronium decay experiment. It is found that the zinc sulfied and cadmium telluride are suitable for the bolometers of this purpose.