Study On The Performance Of Liquid Argon Veto-Coincidence Detector For China Dark Matter Experiment

The detection of dark matter is one of the leading edge topics in particle physics,astrophysics and cosmology. Around the world, the general method to search darkmatter is direct detection. In China Dark matter Experiment, high purity germaniumcrystal is used as a main detector for WIMP, and liquid argon is used as aveto-coincidence detector. Radiation deposits energy in argon, and excited state argonreleases scintillation light whose wavelength is about128nm. CDEX liquid argondetector collects scintillation light as signal. Numerous factors have influence on theperformance of liquid argon detector, and then will decrease the veto-coincidenceefficiency. The characteristic parameters of liquid argon detector are significant forCDEX.The thesis sets up a liquid/gas argon detection property test platform, and gets thecharacteristic parameters by experiment research. In single photoelectron responseexperiment of PMT, we get the gain of9357KB with high voltage variation,respectively at77K and297K. It is verified that the evaporation of TPB on the surfaceof PTFE sleeve achieves the function of wavelength shifter. As for shifter efficiency,the counts at peaks decrease. Three types of gas argon scintillation signals arecompared, whose purity is99.999%,99.9999%and99.99999%, respectively. Themore pure argon is tested, the more signals are collected. We use two-component andthree-component function to fit the average signal waveform, the result is: intwo-component function, decay time of fast component is11.75-15.62ns, decay time ofslow component is239.34-973.54ns, the ratio of fast/slow component is0.30-0.11; inthree-component function, decay time of fast component is10.09-10.92ns, decay timeof slow component is564.72-1852.27ns, decay time of “intermediate component” is75.37-95.09ns, the ratio of fast/slow component is0.22-0.07. The decay time of slowcomponent increases with the purity of gas argon. It is possible to monitor the purity ofargon by decay time of slow component. We use two-component function to fit theaverage signal waveform in99.999%liquid argon: decay time of fast component is19.33ns, decay time of slow component is211.40ns, and the ratio of fast/slowcomponent is0.26.Because of the Paschen law, the PMT base plate strikes a light, we take action to prevent this phenomenon.