ABOVEGROUND OBSERVATION OF DARK MATTER USING A CRYOGENIC PARTICLE DETECTOR

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By using a cryogenic detector with the target composed of about 344,000 tinmicrospheres of 0.0354g weight (in 30 micron diameter each), we have carried out a preliminary observation of dark-matter candidate particles in cosmic rays. It was performed at the temperature of 30mK and the applied field of 326 Gauss by use of the signal-readout system with a damped resonance circuit (the resonant frequency of about 1 MHz).By analysing the temporal spacings of signals and the pulse height, we confirmed what follows :(1) When an incident particle causes multiple spheres flip at a time, it is a charged cosmic one with certainty. For a single transition signal, however, we are unable to infer it whether due to the usual cosmic ray or a dark-matter particle without taking an event-coincidence procedure.(2) For the 5-micron variance of the size of the tin spheres used the super-heating superconducting field varies widely from 250 to 400G at 30mK,which reduces the effective mass of the target. The precision of the diameter within 1 micron is desired for the narrower allowance of the field.(3) The uniform distribution of the microspheres in the filling stuff is also critically important. A transition of a single sphere, one of the target ones in close vicinity each other, may trigger the nearest neighbor one and the chain reaction occurs successively. Provided that the first transition has owed to the very dark-matter particle, the resultant chain reaction is inferred to be caused by a charged particle.In conclusion, the particle detector based on the phase transition of the superheated superconducting spheres has some issues yet to overcome, the precise selection of the sphere size, the progress of the readout system with a higher sensitivity and the target of unifomly distributed spheres.