PandaX-Ⅱ Dark Matter Experiment And The PandaX-4T Cryogenics,Circulation And Distillation System

The first pictures of the black hole,released on April 10.2019,have sparked a worldwide discussion about the black hole and astronomical obser-vations,but there are still many astronomical observations beyond existing theoretical models.Dark matter,which have no electromagnetic interactions,is one of the key mysteries proposed in the last century.And the weakly interacting massive particles（WIMPs）are the most favored dark matter can-didate particles.In recent years,direct dark matter detection experiments using two-phase time projection chamber technology as the detection method have continuously improved the sensitivity of dark matter detection.And the liquid xenon with high atomic weight and no long-lived isotopes has become an excellent medium for direct dark matter detection experiments.Led by Shanghai jiaotong university,the PandaX experiment is lo-cated in the China Jinping Underground Laboratory（CJPL）and also based on the two-phase liquid xenon detector for the dark matter direct detection.The PandaX-I experiment finished data taking in October 2014.Then the PandaX-Ⅱ experiment start the construction began in the same year and up-graded the liquid xenon quality from 120 kilograms to 1.1 tons.By improving the time projection chamber structure,optimization of photomultiplier tubes system and reducing the background level,PandaX-Ⅱ experiment published two important physical result in July 2016 and October 2017,respectively.Both final event number after all quality cuts are less than the expected back-ground that no dark matter signal was observed.After the collection of the different operation stages Run9 and Run10 data,the lowest spin-independent WIMP-nucleon cross sections at 40 GeV/c²of 8.6×10^-47cm²,is the world one of the most strict limits of the dark matter cross section.The author has been participate in the detector construction,operation and data analysis of the PandaX-Ⅱ dark matter detection experiment.The detector related work includes the construction and debugging of the detec-tor,the detector maintenance in CJPL,and the operation of the PandaX-Ⅱ distillation system.The data analysis section includes the uniformity correc-tion and determination of the energy reconstruction parameters during the PandaX-Ⅱ Run8 operation,and the krypton and radon background analysis during the Run9 operation.The PandaX-4T experiment with total 6 tons liquid xenon is constructed in CJPL-Ⅱ,which covers an area of about 900 m².The optimized detector structure,lower the radioactivity background requirements and increase of the detection medium quality will further improve the possibility of finding dark matter.Expected in the 6 ton×year exposure,the PandaX-4T spin-independent WIMP-nucleon cross sections limit could reach 10^-47cm².While the cryogenics system of the PandaX-Ⅱ experiment with single refrigeration can no longer suitablel for the heat leakage demand and the longer electron drift distance requires higher purification flow rate.In addi-tion,the main background in liquid xenon which the krypton and radon in liquid xenon also require a new system to lower the concentration.Based on the basic experimental requirements of the PandaX-4T,the author de-signed and calculated the cryogenics,circulation system and the distillation system of the PandaX-4T,and verified the reliability of the system through test operations.Based on the designed result,the PandaX-4T cryogenics system could provide 700 W cooling power for the stable operation of the detector,and meet 1 ton per day flow rate for the xenon filling and recovery.The purifi-cation flow rate could achieve 200 SLPM through two parallel circulation loops of the new circulation system.In addition,the PandaX-4T distillation system can attenuate the krypton concentration by 8 orders of magnitude at a flow rate of 17.7 kg/h,and the on-line radon removal flow rate of 60.8 kg/h can attenuate the radon content in the detector by 2.33 times.Through the test operation,the multi-head cooperative refrigeration scheme of the cryogenics system is feasible,and the pressure and temperature stability of the system can be maintained by the temperature regulation.The distillation system can satisfy the designed flow rate of removing krypton and radon,and no flooding was happened.The circulating flow rate and impurity attenuation coefficient of distillation system need to be further measured in other auxiliary systems.