The CDEX-1 1 kg Point-Contact Germanium Detector for Low Mass Dark Matter Searches
Ke-Jun Kang, Qian Yue, Yu-Cheng Wu, Jian-Ping Cheng, Yuan-Jing Li, Yang Bai, Yong Bi, Jian-Ping Chang, Nan Chen, Ning Chen, Qing-Hao Chen, Yun-Hua Chen, You-Chun Chuang, Zhi Dend, Qiang Du, Hui Gong, Xi-Qing Hao, Qing-Ju He, Xin-Hui Hu, Han-Xiong Huang, Teng-Rui Huang, Hao Jiang, Hau-Bin Li, Jian-Min Li, Jin Li, Jun Li, Xia Li, Xin-Ying Li, Xue-Qian Li, Yu-Lan Li, Heng-Ye Liao, Fong-Kay Lin, Shin-Ted Lin, Shu-Kui Liu, Lan-Chun Lv, Hao Ma, Shao-Ji Mao, Jian-Qiang Qin, Jie Ren, Jing Ren, Xi-Chao Ruan, Man-Bin Shen, Lakhwinder Singh, Manoj Kumar Singh, Arun Kumar Soma, Jian Su, Chang-Jian Tang, Chao-Hsiung Tseng, Ji-Min Wang, Li Wang, Qing Wang, Tsz-King Henry Wong, Shi-Yong Wu, Wei Wu, Yu-Cheng Wu, Hao-Yang Xing, Yin Xu, Tao Xue, Li-Tao Yang, Song-Wei Yang, Nan Yi, Chun-Xu Yu, Hao Yu, Xun-Zhen Yu, Xiong-Hui Zeng, Zhi Zeng, Lan Zhang, Yun-Hua Zhang, Ming-Gang Zhao, Wei Zhao, Su-Ning Zhong, Zu-Ying Zhou, Jing-Jun Zhu, Wei-Bin Zhu, Xue-Zhou Zhu, Zhong-Hua Zhu
aa r X i v : . [ phy s i c s . i n s - d e t ] M a y Submitted to Chinese Physics C
The CDEX-1 1 kg Point-Contact Germanium Detector for Low MassDark Matter Searches
KANG Ke-Jun ‡ , YUE Qian ‡† , WU Yu-Cheng ‡ , CHENG Jian-Ping ‡ , LI Yuan-Jing ‡ , BAI Yang ‡ , BI Yong ‡ ,CHANG Jian-Ping ‡ , CHEN Nan ‡ , CHEN Ning ‡ , CHEN Qing-Hao ‡ , CHEN Yun-Hua ‡ , CHUANG You-Chun ,DENG Zhi ‡ , DU Qiang ‡ , GONG Hui ‡ , HAO Xi-Qing ‡ , HE Qing-Ju ‡ , HU Xin-Hui ‡ , HUANG Han-Xiong ‡ ,HUANG Teng-Rui , JIANG Hao ‡ , LI Hau-Bin , LI Jian-Min ‡ , LI Jin ‡ , LI Jun ‡ , LI Xia ‡ , LI Xin-Ying ‡ , LIXue-Qian ‡ , LI Yu-Lan ‡ , LIAO Heng-Ye , LIN Fong-Kay , LIN Shin-Ted , LIU Shu-Kui ‡ , LV Lan-Chun ‡ , MAHao ‡ , MAO Shao-Ji ‡ , QIN Jian-Qiang ‡ , REN Jie ‡ , REN Jing ‡ , RUAN Xi-Chao ‡ , SHEN Man-Bin ‡ ,Lakhwinder SINGH , Manoj Kumar SINGH , Arun Kumar SOMA , SU Jian ‡ , TANG Chang-Jian ‡ , TSENGChao-Hsiung , WANG Ji-Min ‡ , WANG Li ‡ , WANG Qing ‡ , WONG Tsz-King Henry , WU Shi-Yong ‡ , WUWei ‡ , WU Yu-Cheng ‡ , XING Hao-Yang ‡ , XU Yin ‡ , XUE Tao ‡ , YANG Li-Tao ‡ , YANG Song-Wei , YI Nan ‡ ,YU Chun-Xu ‡ , YU Hao ‡ , YU Xun-Zhen ‡ , ZENG Xiong-Hui ‡ , ZENG Zhi ‡ , ZHANG Lan ‡ , ZHANGYun-Hua ‡ , ZHAO Ming-Gang ‡ , ZHAO Wei ‡ , ZHONG Su-Ning ‡ , ZHOU Zu-Ying ‡ , ZHU Jing-Jun ‡ , ZHUWei-Bin ‡ , ZHU Xue-Zhou ‡ , and ZHU Zhong-Hua ‡ Department of Engineering Physics, Tsinghua University, Beijing, 100084 Institute of Nuclear Physics, China Institute of Atomic Energy, Beijing, 102413 School of Physics, Nankai University, Tianjin, 300071 NUCTECH Company, Beijing, 100084 School of Physical Science and Technology, Sichuan University, Chengdu, 610065 YaLong River Hydropower Development Company, Chengdu, 610051 Institute of Physics, Academia Sinica, Taipei, 11529 Department of Physics, Banaras Hindu University, Varanasi, 221005
Abstract:
The CDEX Collaboration has been established for direct detection of light dark matter particles, usingultra-low energy threshold p-type point-contact germanium detectors, in China JinPing underground Laboratory(CJPL). The first 1 kg point-contact germanium detector with a sub-keV energy threshold has been tested in apassive shielding system located in CJPL. The outputs from both the point-contact p + electrode and the outside n + electrode make it possible to scan the lower energy range of less than 1 keV and at the same time to detect the higherenergy range up to 3 MeV. The outputs from both p + and n + electrode may also provide a more powerful methodfor signal discrimination for dark matter experiment. Some key parameters, including energy resolution, dead time,decay times of internal X-rays, and system stability, have been tested and measured. The results show that the 1 kgpoint-contact germanium detector, together with its shielding system and electronics, can run smoothly with goodperformances. This detector system will be deployed for dark matter search experiments. Key words:
CDEX, point-contact germanium detector, dark matter, CJPL
PACS:
Light dark matter particles with masses of less than10 GeV have been come a new target for direct detectionexperiments. In order to search for dark matter WeaklyInteracting Massive Particles (WIMP) in the low massregion, it is necessary to develop a detector system withan ultra-low energy threshold, as well as keeping its back-ground level ultra-low. High Purity Germanium (HPGe)has been chosen as the target and detector for dark mat-ter searches due to its very low radioactivity, very good energy resolution, ultra-low energy threshold and mod-ular structure, which makes it easy to scale up to largerand larger masses of detector array while keeping almostthe same performances as that of a small mass detectormodule. The China Dark matter Experiment (CDEX)Collaboration was established in 2009 to start a new pro-gram for searching for light dark matter, using ultra-low energy threshold germanium array detector systems.The physical goals and technical feasibility of the CDEXexperiment were explored some time ago before the col- † Corresponding author. E-mail: [email protected] ‡ The member of CDEX Collaboration ubmitted to Chinese Physics C laboration itself [1]. The first physics results for a darkmatter search with an ultra-low energy threshold HPGedetector in a surface laboratory were published by theTEXONO collaboration [2], and are also partially basedon such endeavors.Other experiments, such as CoGeNT [3], XENON [4],CDMS [5], CRESST [6], DAMA [7] and so on, scan thelow mass region for dark matter with different targetsbased on different technologies. The most stringent ex-clusive curve so far has been given by the XENON ex-periment in 2012. The WIMP-nucleon spin-independentcross-section is about 10 − cm at a WIMP mass of50 GeV, but the sensitivity is still not good in the lowmass region of less than 10 GeV [6], even though theresults from XENON have excluded the regions claimedby CoGeNT, DAMA and CRESST. The results from Co-GeNT, DAMA and CRESST are also inconsistent witheach other. All of these new results show us that thesearching for WIMP in the low mass region has becometopic of keen debate in recent years.The CDEX collaboration will directly detect lightdark matter particle WIMPs with masses of about 10GeV using a tonne-scale germanium detector array com-posed of many 1 kg-scale PCGe (point-contact germa-nium) detectors. The energy threshold level and otherperformances of the tonne-scale detector should be al-most the same, therefore, as that of a 1 kg-scale detec-tor. A 1kg-scale PCGe detector which can achieve anultra-low energy threshold of less than 500 eV makes itpowerful enough to scan the low mass region of dark mat-ter. As a first step, the CDEX collaboration has studieda 1 kg PPCGe detector (CDEX-1). China JinPing underground Laboratory (CJPL) is lo-cated in the central part of a 17.5 km-long traffic tun-nel which was built for the construction of hydropowerplants on both sides of JinPing Mountain in Sichuanprovince, southwest China. The rock overburden in thecentral part of the traffic tunnel is about 2400 m (6720m water equivalent depth). The construction of CJPLstarted in 2009 and the laboratory has been formallyrunning since Dec. 2010. The current volume of CJPLis about 4000 m [8]. The cosmic-ray flux has beenmeasured by two triple-coincident scintillation countertelescopes and the Muon flux measured to be about 60muons y − m − [9]. This low flux is highly beneficial fordark matter searches and other rare event experimentsin situ. Both the deep rock overburden to shield fromcosmic-ray and the ambient rock with very low radioac-tivity make CJPL the best underground laboratory inthe world for ultra-low background experiments such asdark matter, double beta decay and so on. It is also planned to further enlarge the space available at CJPLto host more experiments in the future. The CDEX-1 detector system has been set up in apolyethylene room, which has 1 meter-thick wall insidethe CJPL. The whole system consists of three parts: a1 kg point-contact germanium detector; electronics andread out system; and a shielding system. This paper willdescribe the structure and performance of CDEX-1 insitu at CJPL.
The point-contact technology of a HPGe detectorwas developed several decades ago based on the generalcoaxial germanium detector technology [10]. In order toachieve an ultra-low energy threshold, the area of thegermanium detector electrode should be as small as pos-sible. We know that beside electronics noise the noiseof a detector depends mainly on the capacitance of thedetector. The capacitance of a detector is mainly relatedto the size of the electrode on the germanium detector.The electrode size of the contact point on a germaniumdetector can reach mm-scale and the corresponding ca-pacitance can be ∼ + type contact on the outer surface and a tiny p + type contact as the central electrode. The small diameterof the central electrode with the order of 1 mm reducesthe capacitance of the detector to the order of 1 pF andgreatly improves the intrinsic noise characteristics.2 ubmitted to Chinese Physics C Fig. 1. The structure of CDEX-1 1 kg PPCGe detector.
The pre-amplifier outputs of the CDEX-1 1 kgPPCGe detector include the S1 signal from the point-contact p + electrode and S2 signal from the n + electrodewhich also served as the HV electrode. The p + pointcontact signal is readout by a pulsed optical feedbackpreamplifier with an ultra-low noise JFET nearby, andthe signal from the n + type electrode is also readout by aresistive feedback preamplifier. Each pre-amplifier has 4 outputs: three identical OUT_E for energy measurementand one
OUT_T for timing measurement which was notused for this experiment. The multiple outputs providemore choices to connect more main amplifiers with differ-ent shaping times and gains. All outputs should be wellconnected to the high impedance inputs of downstreammodules. The detector is recommended to be operatedunder +3500 V high voltage.The electronics and data acquisition system of theCDEX-1 1 kg PPCGe is illustrated simply in Fig.2. Allthe NIM/VME modules and crates remain commercialproducts from Canberra [11] and CAEN [12] companies.In order to distinguish different pulse shapes, the sig-nals from one
OUT_E of each preamplifier are amplifiedby a fast timing amplifier (Canberra 2111) and then fedinto the flash analog-to-digital converter (FADC, CAENV1724, 100MHz sampling frequency) for fast pulse dig-ital processing. The other preamplifier outputs are di-rectly connected into a conventional spectroscopy am-plifier (Canberra 2026) and then fed into the FADC fordigitization. The signals from n + electrode are also fedinto the FADC for digitization. One signal from the p + point contact electrode is discriminated after the spec-troscopy amplifier and served as one of the trigger forthe detector system. The random trigger signals at rateof 0.05 Hz from a pulse generator are used to measurethe dead time of the electronics and read out system. Allthe data is transferred to a PC through a duplex opticalfiber. The total trigger rate of the CDEX-1 1 kg PPCGedetector system is kept less than 10 Hz for long termdata taking. Fig. 2. The schematic electronic diagram of CDEX-1 1 kg PPCGe detector ubmitted to Chinese Physics C The CDEX-1 1 kg PPCGe detector was installed intoCJPL in order to avoid background from cosmic-rays.A passive shielding system has been setup for shieldingfrom gamma ray or neutron backgrounds from ambientrock and materials. The structure of the shielding systemis shown in Fig.3 and the materials from outside to in-side are: 20 cm thick lead to shield from external gammaradiation from rock and other materials; and 20 cm thickboron-loaded polyethylene for neutron deceleration andthermal neutron absorption. The whole shielding sys-tem is located inside a 1 m thick layer of polyethylenefor neutron shielding, which is not shown in Fig.3. TheCDEX-1 1 kg PPCGe detector was housed in the shield-ing system along with LN2 Dewar. A 20 cm thick layerof OFHC copper surrounds the cryostat of the PPCGedetector to further decrease the residual gamma back-ground from outside. The internal space between the 20cm OFHC copper shielding and the cryostat flushed withpure nitrogen gas to eliminate radioactive radon gas.
Fig. 3. The shielding system of CDEX-1
The CDEX-1 1 kg PPCGe detector has been in-stalled and thoroughly tested to achieve its optimal per-formances. Due to the 1.5 mm OFHC copper window,the 1 kg PPCGe detector could not be calibrated withexternal low-energy gamma or X-rays, as they cannotpass through the OFHC copper window. The studiesof the detector performances have to be done using itsown internal characteristic X-ray lines in the low energyrange.The measured spectrum includes the 10.37 keV K-shell X-ray from Ge and Ge atoms, the 8.98 keV X-ray of Zn atom and even 1.29 keV L-shell X-ray from Ge and Ge atoms. These internal characteristic X-ray lines can be used to calibrate the detector, monitorthe stability of the detector system and study the energyresolution at different energy ranges. Many of the char-acteristic X-ray lines at the energy range of less than 12keV are summarized in Table 1.
Table 1. The energies and lifetimes of K-shell andL -shell X-rays for different atoms.
Atomic E (K-shell) E (L-shell) Lifetime species (keV) (keV) (day) As 11.10 1.414 80.30 Ge 10.37 1.298 11.43 Ge 10.37 1.298 270.8 Ga 9.66 1.194 0.047 Zn 8.98 1.096 244.3 Ni 7.71 0.926 6.077 Co 271.9 Co 7.11 0.846 77.28 Co 70.87 Fe 6.54 0.769 997.1 Mn 5.99 0.695 312.3 Cr 5.46 0.628 27.70 V 4.97 0.564 330
As illustrated in Fig.2, different gains and shapingtimes are chosen to process the pre-amplifier signals. Thechannels from S1 have been set to only cover the low en-ergy range below 12 keV and they can only be calibratedby the intrinsic characteristic X-ray lines of the naturallong-life cosmogenic radioactive nuclei. Meanwhile, thechannels from S2 are used to trace the backgrounds inrelatively higher energy regions. So the channels fromS2 are calibrated by some radiation source samples, e.g.Europium in our cases. Many peaks are fitted to do theenergy calibration and energy linearity study. The 10.37keV peak and its fit result are shown in Fig.4 as a sample.The calibration results of different channels are displayedin Fig.5, showing one channel from S1 and three chan-nels from S2. The calibration information can be seen inTab.1. At the same time, the zero energy point can bedefined with random trigger events.
Fig. 4. The 10.37 keV K-shell X-ray peak of , Ge and the energy resolution fitting result. ubmitted to Chinese Physics C Fig. 5. Energy calibrations of the CDEX-1 PPCGedetector including both S1 channel (a) and threechannels from S2 with different gains (b) (c) (d). Fig. 6. Spectra associated with S1 channel (a) andthe channels from S2 (b) (c) (d) with differentgains.Table 2. Selection of X-ray and gamma lines for calibration
S1 OUT E2 S2 OUT E3 S2 OUT E2 S2 OUT E1Background Source
EuIsotope E (keV) FWHM (keV) E (keV) FWHM (keV) E (keV) FWHM (keV) E (keV) FWHM (keV) , Ge (X) 1.299 0.251 121.8 4.44 121.8 5.82 121.8 7.47 Zn (X) 8.979 0.212 244.7 4.81 244.7 7.02 344.3 6.39 , Ge (X) 10.37 0.222 344.3 4.19 344.3 5.34 778.9 8.04778.9 6.39 867.4 9.60867.8 7.52 964.1 7.651408 7.00
The measured energy spectra for calibration areshown in Fig.6. Various characteristic X-ray and gammalines can be clearly seen. The resolutions of different en-ergy peaks are calculated and given in Tab.2. One cansee from the spectra that there are many other charac-teristic X-ray peaks in the S1
OUT_E2 spectrum and thatthe energy threshold can be brought down to less than1 keV without any electronic noise suppression. The X-ray peak from Zn L-shell can also be identified. So,we can see from the background spectrum of S1
OUT_E2 that the detector has an ultra-low energy threshold andgood energy resolution.
Fig. 7. Decays of characteristic X-rays associatedwith Ge ( T / = 11 . The characteristic X-rays observed in the low-energyspectrum from the point-contact electrode of the 1 kgPPCGe detector originate from the cosmogenic activa-tion of the germanium crystal. After a long time ofexposure to cosmic rays at ground level the intensitiesof these X-rays will achieve a balanced status. After thegermanium detector was moved into CJPL, the balancedstatus was broken and the number of radioactive nucleidecreased due to the lower production rate, which is re-lated to the much lower Muon flux inside CJPL. Onecan then measure the decays of some short life-time ra-dioactive nuclei. Fig.7 shows the decays of the 10.37keV K-shell EC X-ray (KX-ray) peak and the 1.29 keVL-shell EC X-ray peak (LX-ray) from Ge and Ge.Due to the relatively long half-life of Ge, the decay ismainly induced by the Ge isotope. The rate data is fit-ted with exponential decay plus a constant background.The decay times of the 10.37 keV peak (11 . ± . . ± . Ge ( T / = 11 . To calculate the real event rate, it is necessary toknow the dead time of the data acquisition (DAQ) sys-5 ubmitted to Chinese Physics C tem of CDEX-1 1 kg PPCGe detector. One can see thatin Fig.2, there is a signal generator which contributesabout 0.05 Hz to the total trigger rate of the CDEX-1DAQ system. This periodic trigger can be consideredas independent from the physical triggers from the 1 kgPPCGe detector and so be served as random triggers.The dead time of CDEX-1 DAQ system can then becalculated as the ratio of the unrecorded random trig-ger number and the generated random trigger number.Based on the data already collected the dead time of theCDEX-1 DAQ system is less than 1%.
Fig. 8. Stability check of the CDEX-1 PPCGe detector.
To verify the validity of the data, several preliminaryoffline analysis were carried out, including trigger rate,random trigger efficiency and the ratio of real time tolive time. In Fig.8, one can see that the trigger rate andrandom trigger efficiency of the CDEX-1 DAQ are bothrelatively stable, showing that we can expect the wholeexperiment system to run smoothly and stably.
The CDEX collaboration has been established tosearch for dark matter particles with a tonne-scale massgermanium detector array system. As the first stage ex-periment, the CDEX collaboration has set up a point-contact germanium detector with a mass of 1 kg scale(CDEX-1) and studied the performance of the whole sys-tem in CJPL. The results show that the CDEX-1 detec-tor system can run smoothly with good energy resolutionand an ultra-low energy threshold. For the next step thisdetector system will be used to directly search for darkmatter particle WIMPs and we look forward to physicsresults being available soon.This work is Supported by National Natural ScienceFoundation of China (10935005, 10945002, 11275107,11175099) and National Basic Research program ofChina(973 Program) (2010CB833006).
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