Gamma And Neutron Background Research In High Energy Region On CDEX-1 Point-contact High Purity Germanium Detector

The direct detection of dark matter and the search for neutrinoless double beta decay?0???are both important experiments in modern astro-particle physics.Due to its low activity,low energy threshold and good energy resolution,high purity germanium detector was widely used in the two kinds of experiments.The key to these experiments is to establish an extremely low background environment as far as possible and the explanation of the experiment result depends on the understanding of background components and their contributions in the experiment spectrum.The background analysis of the measured spectrum in high energy region is one of the main work in the 0??experiment.Since the intimate connection bettween the low energy region and the high energy region,the background analysis in high energy region is also useful to the background understanding in low energy region?<12 keV?for dark matter detection.Based on the measured data of CDEX-1 point-contact high purity germanium detector,this dissertation aims at the background analysis in high energy region?500³000 keV?.The data processing methods for heigh energy region were established.According to the features of signal from the detector's preamplifier,the efficiency problem induced by the inhibit period was analyzed.The energy spectrum of the two phases were abtained by pulse information extraction,signal discrimination,energy calibration and efficiency correction.According to the configuration features CDEX-1 system and the distribution characteristics of the radioactive impurities,the response spectrums of cosmogenic isotopes and natural radioactive nuclides at different location were obtained by Geant4 simulation.Comparing the simulated spectra with the experiment spectrum,the background contribution of cosmogenic isotopes were derived according to the corresponding Kx peaks or gamma peaks and the background contribution of natural radioactive were obtained by fitting the experiment spectrum with the simulated spectra.The background models for the two phases were established and analyzed respectively.For the neutron background,A Gd-loaded liquid scintillator detector has been developed based on the pulse shape discrimination and the coincidence measurement.The fast neutron background in CJPL has been measured by the neutron detector.Besides,the characteristics of the neutron response spectrum was studied by using a ²⁵²Cf neutron source.Combined these results,the neutron background contribution in the experient spectrum was evaluated.On the basis of the background analysis in high energy region,the 304 kg·day data from the second phase were used to evaluate the 0??signal strength and the half-life limit of 0??decay has been derived asT^0?_1/2?6.43×10^{2 2}yr,90%C.L.with a profile likelihood method.