Experimental Study And Simulation Analysis For Detectors' Performance In Dark Matter Search Experiment

Dark Matter is an important component of cosmic mass and many searching experiments have been carried out for the probable candidate, WIMP(Weakly Interactive Massive Particles). A research plan was proposed by Tsinghua University collaborated with Seoul Nation University in Korea and Academia Sinica in Taipei aiming to low energy WIMP detection. We expect to detect WIMP in mass range 1-10GeV using 1kg Ge crystal, which will give the most stringent exclusive curve at the 1-10GeV/c2 WIMP mass region.To reduce the influence of background level, low background lead and copper is used for passive shielding, a CsI(Tl) detector for active shielding and the whole detection system will be put in Y2L underground laboratory whose minimum depth is 700m. A 5g HPGe detector prototype is used for system test. After analysis for background content and its interactions with detector materials, simulation programs based on Geant4 were applied for the shielding effect. Result shows thatγrays from environment can be well blocked by lead and copper and those from shielding materials can be vetoed by CsI(Tl) detector. Muon can be easily vetoed for large signal in detectors, while shielding efficiency for neutron is not as good as others because it's neutral. Fortunately, the neutron flux in Y2L is quite low so that it can be ignored. Simulation result proves that shielding efficiency meets the satisfaction and the system can be applied for WIMP detection.This thesis also paid attention to experimental performance study of detectors. Experimental result shows that the HPGe detector has very low energy threshold. Measured value now is 300eV which is already the lowest limit and further effort still goes on for expected value, 100eV. CsI(Tl) crystal detector has good veto efficiency and PSD capacity that is very effective for depressing the background level. For the big crystal dimension, CsI detectorresponse has some dependency on particle incident position, but the veto capacity is hardly influenced. Internal background of CsI crystal is a very important background source, measurement and calculation result shows that can be well reduced by CsI detector itself to reach the level experiment requires. In conclusion, the detection system works in good condition, and now data acquisition is in progress.