SEARCH FOR DARK MATTER USING THE SUPERHEATED SUERCONDUCTOR

我们已经完成了一个低温粒子探测器的研制工作，该探测器可用于探测宇宙线和/或中微子中的暗物质粒子，探测器中设置了一组过热超导锡微粒作为靶。1.直径为200微米的锡微粒对2 K α射线的响应：结果发现，单个颗粒对几百万个电子敏感，2.直径30微米的锡颗粒对30 mK γ射线的响应：我们能够测量由于K-和L-的光电效应引起的能量吸收的谱线。用60 keV的γ射线辐照锡原子，锡原子的壳层电子吸收率与粗略计算的结果一致。3.翻转现象的测量：在足够低的温度下，颗粒不仅可以从超导态翻转到正常导电态，而且还可以从正常导电态翻转到超导态。在这种情况下，颗粒的状态沿着磁场-磁化的相图中的磁滞曲线。精确的测量表明，这种现象发生的条件关键取决于颗粒材料的热性质。这种现象在锡的情况下并没有表现出来，锡的热性质是根据局部加热图来解释的。4.原型探测器：探测器具有1克的目标，该目标由在稀释冰箱的温度下设置的30微米锡颗粒的集合组成。很快就会开始试运行。

We have carried out a project of constructiog a cryogenic particle detector, which can be applied for the detection of dark-matter particles in cosmic rays and/or neutrinos, using a collection of superheated superconducting tin micro granules as a target set up in the detector.1.Responses of tin granules in 200micron diameter to alpha rays at 2 kelvins :It turned out that the single granule is sensitive to several-million-electron-volt energies deposited by an incident alpha particle.2.Responses of tin granules in 30micron diameter to gamma rays at 30mK :We were able to measure spectre lines of the energy absorption due to the photoelectric effect for the K-and L-shell electrons in tin atoms irradiated with gamma rays of 60keV.The absorption rates are consistent with roughly-calculated values. The heat energy induced by ionized electrons in the single granule was detected up to less than lkeV.3.Measurements of the flip-flop phenomenon :It is expected that a granule not only flips from a superconduting state to a normal conducting state receiving a sufficient heat energy, but also flops vice versa at a sufficiently low temperature. In this case, the state of a granule traces along a hysteresis curve in the phase diagram in magnetic field-magnetization. Precise measuremets showed that the condition for the phenomenon to take place depends critically on the thermal property of the material of a granule. The phenomenon turned out not to manifest itself in the case of tin of which thermal property is explained on the basis the local-heating picture.4.A prototype detector :The detector has a l-gram target consisting of a collection of 30micron tin granules set up set up at a temperature of a dilution refrigerator. A test run will start soon.