Radioactive Background Control And Monte Carlo Analysis For PandaX Experiment

The existence of dark matter is one of the most important questions in fundamental physics today.The PandaX experiment is on mission of searching for WIMPs?weakly interactive mas-sive particles?by adopting dual phase xenon detection method.Because of the extremely low cross section between WIMPs and hadronic matter?normal matter?,it is very important to pay special attentions on the background control for PandaX,as well as the corresponding data anal-ysis.For the radiation background control purpose,two counting stations have been designed and built,which are described in detail in this thesis.Based on the HPGe?high purity germa-nium?counting technology,the counting stations have provided material screening and assaying for PandaX-I/II experiments.With the help of CJPL?China Jinping underground laboratory?,the passive gamma shielding,and Rn purging systems,the CJPL-CS?CJPL counting station?can dramatically suppress the radiation background level.And because of the 3.69 kg sensitive germanium crystal mass,and corresponding 175%relative detection efficiency,the CJPL-CS has the detection sensitivity as low as sub-mBq/kg in routine radioactive material counting non-destructively.Such method provides the PandaX experiment high efficiency and accuracy on material radioactivity counting,leading to a more strict background control and better sensi-tivity for WIMP detection.Consequently,the deeper understanding of the HPGe facility and detailed research on the data analysis are necessary for the collaboration,which is the most important topic on background control for PandaX experiment.The radiation background analysis for PandaX experiment using MC?Monte Carlo?sim-ulation method is another critical issue.This thesis will also describe the detailed physical procedures of the PandaX-I/II experiments with MC simulation,based on the full-scale un-derstanding of the structures,material properties,and background levels of the corresponding detectors.The background results predicted by the simulation matched well with experimental data.It is possible to make deeper research on the background analysis for PandaX experiment,with the comparison between the MC simulation and experimental data.Such analysis provided clues for the WIMP signal selection from the data,and decide the outcome sensitivity for WIMP detection furthermore.Partially based on MC simulation shown in this thesis,a constraint on WIMP-hadron cross section has set to be 2.5×10^-46cm²at 40 GeV,which is the best limit at the time when it was published.In summary,this thesis illustrates the details of background control and corresponding data analysis for PandaX-I/II experiments.In this work,two HPGe counting stations were designed and built,and a sub-mBq/kg sensitivity has been achieved,which represents one of the highest sensitive gamma counting stations around the world.Meanwhile,the MC background analysis for PandaX experiment was also carried out,which has a good agreement with the experimental data.The works shown in this thesis,the high sensitivities of the HPGe counting stations and MC radiation background analysis,have boosted the recorded WIMP detection sensitivity which the PandaX experiment has achieved,and will continuously provide further supports in the future.