Internal Calibration of the Pandax-II Detector with Radon Gaseous Sources
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Prepared for submission to JINST Internal Calibration of the PandaX-II Detector with Radon Gaseous Sources PandaX-II Collaboration Wenbo Ma,a Abdusalam Abdukerim,a Zihao Bo,a Wei Chen,a Xun Chen,a;b Yunhua Chen,c Chen Cheng,d Xiangyi Cui,e Yingjie Fan,f Deqing Fang,g Changbo Fu,g Mengting Fu,h Lisheng Geng,i;j Karl Giboni,a Linhui Gu,a Xuyuan Guo,c Ke Han,a;1 Changda He,a Shengming He,c Di Huang,a Yan Huang,c Yanlin Huang,k Zhou Huang,a Xiangdong Ji,l Yonglin Ju,m Shuaijie Li,e Huaxuan Liu,m Jianglai Liu,a;b;e;2 Yugang Ma,g;n Yajun Mao,h Yue Meng,a;b Kaixiang Ni,a Jinhua Ning,c Xuyang Ning,a Xiangxiang Ren,o Changsong Shang,c Lin Si,a Guofang Shen,i Andi Tan,l Anqing Wang,o Hongwei Wang,n;p Meng Wang,o Qiuhong Wang,n;q Siguang Wang,h Wei Wang,d Xiuli Wang,m Zhou Wang,a;b Mengmeng Wu,d Shiyong Wu,c Weihao Wu,a Jingkai Xia,a Mengjiao Xiao,l;r Pengwei Xie,e Binbin Yan,a Jijun Yang,a Yong Yang,a Chunxu Yu,f Jumin Yuan,o Ying Yuan,a Jianfeng Yue,c Xinning Zeng,a Dan Zhang,l Tao Zhang,a;b Li Zhao,a;b Qibin Zheng,k Jifang Zhou,c Ning Zhou,a and Xiaopeng Zhoui;1 aINPAC and School of Physics and Astronomy, Shanghai Jiao Tong University, MOE Key Lab for Particle Physics, Astrophysics and Cosmology, Shanghai Key Laboratory for Particle Physics and Cosmology, Shanghai 200240, China bShanghai Jiao Tong University Sichuan Research Institute, Chengdu 610213, China cYalong River Hydropower Development Company, Ltd., 288 Shuanglin Road, Chengdu 610051, China dSchool of Physics, Sun Yat-Sen University, Guangzhou 510275, China eTsung-Dao Lee Institute, Shanghai 200240, China f School of Physics, Nankai University, Tianjin 300071, China gKey Laboratory of Nuclear Physics and Ion-beam Application (MOE), Institute of Modern Physics, Fudan University, Shanghai 200433, China hSchool of Physics, Peking University, Beijing 100871, China iSchool of Physics, Beihang University, Beijing 100191, China jInternational Research Center for Nuclei and Particles in the Cosmos & Beijing Key Laboratory of Advanced Nuclear Materials and Physics, Beihang University, Beijing 100191, China kSchool of Medical Instrument and Food Engineering, University of Shanghai for Science and arXiv:2006.09311v3 [physics.ins-det] 4 Jan 2021 Technology, Shanghai 200093, China lDepartment of Physics, University of Maryland, College Park, Maryland 20742, USA mSchool of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China 1Corresponding author 2Spokesperson nShanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China oSchool of Physics and Key Laboratory of Particle Physics and Particle Irradiation (MOE), Shan- dong University, Jinan 250100, China pShanghai Advanced Research Institute, Chinese Academy of Sciences, Shanghai 201210, China qUniversity of Chinese Academy of Sciences, Beijing 100049, China rCenter for High Energy Physics, Peking University, Beijing 100871, China E-mail: [email protected]; zhou [email protected] Abstract: We have developed a low-energy electron recoil (ER) calibration method with 220Rn for the PandaX-II detector. 220Rn, emanated from natural thorium compounds, was fed into the detector through the xenon purification system. From 2017 to 2019, we performed three dedicated calibration campaigns with different radon sources. We studied the detector response to α, β, and γ particles with focus on low energy ER events. During the runs in 2017 and 2018, the amount of radioactivity of 222Rn were on the order of 1% of that of 220Rn and thorium particulate contamination was negligible, especially in 2018. We also measured the background contribution from 214Pb for the first time in PandaX-II with the help from a 222Rn injection. Calibration strategy with 220Rn and 222Rn will be implemented in the upcoming PandaX-4T experiment and can be useful for other xenon- based detectors as well. Keywords: Dark Matter detectors (WIMPs, axions, etc.); Noble liquid detectors (scin- tillation, ionization, double-phase); Time projection Chambers (TPC); Large detector sys- tems for particle and astroparticle physics Contents 1 Introduction1 2 Radon Calibration Sources2 3 Gaseous Source Delivery System4 4 Performance in the TPC5 4.1 Event Rate Evolution and Calibration-Induced Background6 4.2 High Energy α Spectrum8 4.3 Low Energy β Spectrum9 4.4 Delayed Coincidence Events 10 4.5 214Pb measurement from 222Rn Injection 11 5 Conclusion 12 1 Introduction While the existence of dark matter in the Universe is firmly established by cosmological observations [1], the possible particle nature of dark matter is still unknown and widely studied [2{4]. Direct detection experiments examine the possible scattering of galactic dark matter particles on ordinary matter in detectors. So far the most sensitive direct detection results, specifically for Weakly Interacting Massive Particles (WIMPs) at mass range around 100 GeV=c2, are given by dual-phase xenon Time Projection Chambers (TPC) [5{7]. A dual-phase xenon TPC excels in discriminating possible WIMP signals from back- ground events with detector response. A scattering event gives a prompt scintillation signal (usually called S1) and a delayed electroluminescence signal (S2) when drifted ionization electron enters a stronger electric field in the gas phase. WIMP would most likely scatter with xenon nuclei and trigger a nuclear recoil (NR) signal. On the contrary, electron recoil (ER) events are predominantly from β and γ particles interacting with electrons in xenon atoms. NR and ER events differ from the relative amplitude of their corresponding S2 and S1 signals (S2=S1) and therefore ER backgrounds can be effectively rejected. The PandaX-II detector was operated from 2015 to 2019 in China Jin-Ping under- ground Laboratory (CJPL-I) [8] to search for WIMPs as well as other dark matter can- didates and neutrinoless double β decay [9{14]. The PandaX-II detector had an active mass of 580 kg of xenon in a cylindrical field cage with a diameter of 646 mm and height of 600 mm, which corresponding to a maximum drift time of 360 µs [15]. The top and bottom ends of the TPC were instrumented with 110 Hamamatsu-R11410 3-inch photomultiplier { 1 { tubes (PMTs). A possible WIMP signal or background events with energy larger than a few keV would trigger the data acquisition (DAQ) to record 1 ms-long waveform. During physics data taking, the trigger rate was around 3 Hz. Offline analysis identified S1 and S2 signals from the waveforms and the charge of S1 and S2 signals were used to reconstruct the position and energy of each event with the help of different calibration sources. We have calibrated the detector with external gamma and neutron sources as well as internal radioactive sources, which is the main topic of this paper. Calibration with an internal β source provides a uniformly distributed low-energy ER events to help determine a detector-specific ER band in the (S1, S2=S1) phase space. Mul- 228 220 37 127 tiple types of sources, including CH3T[16], Th-based Rn [17], Ar [18], Xe [19], and 83mKr [20], have been developed by several collaborations for this purpose. A few multi-ton scale dual-phase xenon TPCs are currently under construction [21{23] to keep searching for WIMP signals. As the dimensions of those detectors get larger, it becomes more difficult for external sources to provide enough statistics at the center of the detectors in a reasonable time scale. Internal calibrations with gaseous sources are becoming even more essential. In this paper, we report the results from three internal calibration campaigns with radon sources at PandaX-II. Three different sources were inserted into the xenon online purification loop and emanated 220Rn reached the TPC under the influence of gas flow. β decay of the daughter nucleus 212Pb gave low energy ER events that were uniformly distributed in the detector volume. A large sample of α events from 220Rn and 216Po offered calibration at MeV scale, which is particularly important for future large scale multi-purpose TPCs. We collected about ten thousand ER events in the dark matter search region of interest (ROI) from 0 to 20 keVee. A data-driven ER distribution model in the (S1, S2=S1) phase space was constructed based on those events and used for dark matter analysis [24, 25] . The temporal event rate evolution and contaminations of radon injection were studied with high energy α events, low energy β events, and β−α coincidence events. We also presented a new method to precisely measure the 214Pb activity, which is the major background source for PandaX-II and other large scale xenon-based dark matter direct search detectors. 2 Radon Calibration Sources 220Rn sources made from natural thorium compounds were used in PandaX-II. 220Rn is a noble gas isotope with a half-life of 55 s and part of 232Th decay chain. 232Th has a natural isotopic abundance of 99.98% with a long half-life of 1.4 billion years. Possible chemical processing would break the secular equilibrium of the 232Th decay chain but keep its fast-decaying daughter 228Th, which decays to 224Ra and then 220Rn. 220Rn gas emanates from the solid and may be injected into the detector for calibration. 220Rn and its daughters decay to insignificant level within a couple of days after injection and thus a good choice for internal calibration. However, the main concerns for natural thorium sources are contaminations of 222Rn and particulate thorium compounds. 222Rn originates from 230Th { 2 { Isotope 220Rn 216Po 212Pb 212Bi 212Po Half-life 55 s 0.14 s 10.6 h 61 m 299 ns Decay mode α α β β (64.1%) α E(α) or Q-value [MeV] 6.288 6.778 0.574 2.254 8.784 Isotope 222Rn 218Po 214Pb 214Bi 214Po Half-life 3.8 d 3.1 m 26.8 m 19.9 m 164 µs Decay mode α α β β (99.98%) α E(α) or Q-value [MeV] 5.490 6.002 1.024 3.272 7.687 Table 1: Decay data of 220Rn, 222Rn, and relevant progenies.