Experimental Study For The Performance Of Low-Mass Dark Matter Particle WIMP Search System

Dark matter research is one of the hottest topics in the field of physics nowadays. What kind of particles is the dark matter composed of? How much is the ratio of dark matter to the total mass of our universe? What is the structure of our universe? These questions have made people confused who are interested in"the starry heavens"since scores of years ago. Among the many models of dark matter particles, Weakly Interacting Massive Particle (WIMP) is one of the most attractive candidates. Tsinghua University, Seoul National University in Korea and Academia Sinica in Taiwan carry out together this direct dark matter search for WIMP with Ultra-Low Energy Threshold High Purity Germanium (ULE-HPGe) detector.The experiment site is located in a 700-meter deep tunnel at Yangyang Gun, Kangwon Do, in the northeast of Korea. That tunnel is used for the construction of Yangyang Water Power Plant. In this underground lab, the background radiation level is quite low and its convenient transportation is good for the development of the experiment. Now the kernel of the detection system is a 5g ULE-HPGe detector, with the Pb and Cu layers and CsI (Tl) crystal anti-Compton detector outside as the shielding which rejects the influence ofγfrom the environment. The DAQ system, based on 64MHz FADC, under VME criterion, got installed and adjusted in the September of 2005, which is followed by the calibration of HPGe and CsI (Tl) detectors, and the results are satisfactory. The background data has been taken for more than one year, since the October of 2005, with about 180 days'data effectively accumulated. The current measured energy threshold is 300eV and the average background level in the energy region from the threshold to 100keV is about 40 cpd (counts/keV·day·kg). Meanwhile, we tried the neutron exposure experiment with Cf-252 source andγexposure experiment with the strongγsources of U-238, Th-234 and Eu-152, to measure the different characteristics of veto efficiency distributions of neutron andγ, and then to check the remaining background counts'origin that is now preliminarily considered from the environmental neutron, which accords with the simulation results via Geant4. During the exposure experiments of neutron andγ, the time response relation between the kernel HPGe detector and CsI (Tl) anti-Compton detector was surveyed, too, which removed the effect of randomly coincident vetoed events from CsI (Tl) crystal; the article also shows the observation record of the internal background of ULE-HPGe detector from the neutron exposure experiment in the November of 2005, which is beneficial to the design of the experiment to measure the quenching factor of the recoil Ge nuclei after the collision with neutron.At the end of this thesis, the exclusion curve with the current threshold and background level is given which is still far from our goal of 100eV threshold and 1 cpd level. A 4×5g detector array will be shipped to Korea in the July of 2007, on the basis of our schedule, and we are looking forward to a better result in the future.