Design And Performance Of Time Projection Chambers For The PandaX Dark Matter Detector

Since proposed in the 1930 s,the existence of dark matter has been confirmed by a series of astronomical observations and cosmological researches.In the ?CDM model,dark matter constitutes about 26% of the current universe,although its natural character still remains unknown.Among many theoretic candidates,the Weakly Interacting Massive Particle(WIMP) is most popular,for its relic abundance after freezing out from thermal equilibrium in early universe happens to fit with that of dark matter today,and also it exists naturally in the supersymmetry model.Technologies for direct detection on WIMP-nucleon scattering have been developed for nearly two decades,among them the dual phase liquid xenon time projection chamber(LXeTPC) technology achieved the best upper limit in recent years,and therefor is thought to be one of the most promising technologies to discover solid signals from WIMP.The greatest advantage of LXeTPC is that it can simultaneously detect light and charge signals from energy deposited in liquid xenon(LXe),and distinguish between electronic recoil and nuclear recoil according to the ratio of two types of signals.Also,this technology can reconstruct the 3-dimensional position of event happened inside the detector,based on the arrival time difference between light and charge signals,as well as the signal distribution on the array of photomultiplier tubes(PMTs).As one of the earliest WIMP direct detection experiments in China,PandaX experiment employs LXeTPC technology,and runs the detector in the China Jin Ping underground Laboratory(CJPL) located in Sichuan Province.Since established in 2009,PandaX has been adopting an experimental plan of staged-upgrading.Except the detector itself,other main subsystems as the shield,the cryogenics,the electronics and data acquisition,etc.are designed and constructed towards a ton-scale LXe experiment.As the first stage,PandaX-? started design and construction in 2010 with 120 kg LXe as detection medium,and in order to improve the photon detection efficiency(PDE),the time projection chamber(TPC) is specially designed as a pancake-shape,hence has a better sensitivity to low mass dark matter signal as reported by DAMA/LIBRA,Co Ge NT,etc.As the physical operation began in March,2014,we pre-defined all signal selection cuts and also understood well the performances of detector based on the calibration data,and then pre-defined the WIMP detection window.The full physical data taking was completed in October,2014,and with a total exposure of 80.1 day × 54.0 kg,7 events were found in the WIMP detection window,consistent with our background prediction of 6.9events.Our published cross-section upper limit excluded all positive results of other experiments interpreted as WIMP-nucleon spin-independent elastic scattering.The conceptual design of PandaX-?,the second stage of PandaX,began in January,2014,and the major upgrading from PandaX-? is the detector itself.We measured the reflectivity of Teflon for xenon scintillation light with dedicated set-ups,and selected the most reflective Teflon sample to build the PandaX-? TPC.Furthermore,the improved design of TPC structure,the higher optical transparency of electrodes and the optimized design of PMT array will all help to improve the PDE of PandaXII,and the value is expected to be even higher than PandaX-?.We also improved and optimized other aspects of the detector as well as other subsystems of the experiment.With 580 kg LXe as sensitive target,the construction of PandaX-? detector has been completed.The detector is currently under the second commissioning in CJPL,and the physical data taking is scheduled to start in the end of this year.After running for a year,PandaX-? is projected to achieve an upper limit of WIMP-nucleon spin-independent elastic scattering cross-section deep into the core area of the minimal supersymmetric standard model(MSSM).The author of this thesis joined PandaX at the very beginning when the collaboration was just founded.During the research period as a graduate student,the author was involved in the design,construction,debugging and operation of the whole experiment,and in charge of works for simulation,design,fabrication and assembling of PandaX-? and PandaX-? TPCs under the guidance of supervisor.