Study of Neutrino and Dark Matter by means of CaF_2

本研究项目的目的是利用CaF_2探测器研究暗物质和中微子的质量。人们强烈认为暗物质是一种与原子核弱相互作用的新粒子。CaF_2探测器中的^<19>F由于其自旋具有频繁的相互作用，因此可以通过观测其反冲来探测暗物质。由于CaF_2探测器具有^<48>的Ca，因此也可以用来研究^<48>Ca的双β衰变，这是研究中微子性质的一种很有前途的方法。构造了有源屏蔽和无源屏蔽的CaF_2探测器。整个系统现在都在运行。由于去年的大地震，我们的探测系统受到了严重的破坏。它已经恢复，我们的研究项目的延迟最小。要搜索上述稀有进程，必须有一个低背景系统。我们达到了相当低的背景水平，这与世界上其他实验室相当。应该指出的是，我们的探测系统位于海平面上，而其他探测系统则位于地下。我们未来在地下实验室进行的实验是很有希望的，因为通常期望将背景电平降低1到2个数量级。该结果发表在1995年“原子核中的弱相互作用和电磁相互作用”国际研讨会上。我们的成就总结如下。(1)该探测器系统在同类系统中具有最低的能量阈值（-3 keV），其效率和能量分辨率得到了充分的研究。(2)中央CaF2 （Eu）晶体由纯CaF2晶体观察，既起到主动屏蔽作用，又起到导光作用。它是第一个具有4pi有源屏蔽的探测器。(3)对探测器中的放射性污染进行了研究，建立了一种降低α辐射源本底的方法。我们将测量海平面的背景水平，直到我们达到系统的极限。然后我们会把探测器移到地下实验室开始采集全部数据。

The purpose of this research project is to study Dark Matter and mass of Neutrino by means of CaF_2 detector. Dark matters are strongly suggested to be new particles interacting weakly wiht nuclei. The ^<19>F in the CaF_2 detector has frequent interaction due to its spin thus the dark matter can be detected by obseving its recoil. Since CaF_2 detector has ^<48>Ca it can also be used to study double beta decay of ^<48>Ca which is one of promising methods to study properties of neutrinos.We constructed CaF_2 detector with active and passive shield. The whole system is in operation now. We had severe damage in our detector system due to the big earthquake last year. It has been recovered and we had minimum delay in our research project.It is essential to have low background system to search for above rare process. We achieved quite low background level which is comparable to other laboratories around the world. It should be noted that our detector system resides on the sea level though others are underground. Our future experiment in a underground laboratory is quite hopeful since usually 1 to 2 orders of magnitude of background level reduction is expected. The result was presented at the international symposium on "Weak and Electromagnetic Interactions in Nuclei" WEIN'95. Our achivement is summarized in the following.(1) The detector system has the lowest energy threshold (-3 keV) among similar systems with well studied efficiency and energy resolution.(2) Central CaF2 (Eu) crystal is viewed by pure CaF_2 crystal which act as active shield as well as light guide. It is a first detector which has 4pi active shield.(3) We studied radioactive contamination in the detector and established a way to reduce the background due to alpha emitters.We will be measuring the background level at the sea level until we reach the limit of the system. The we will move the detector to the underground laboratory and start full data taking.