In-situ gamma-ray background measurements for next generation CDEX experiment in the China Jinping Underground Laboratory a a a a < a a a,b a H. Ma , Z. She , W. H. Zeng , Z. Zeng , , M. K. Jing , Q. Yue , J. P. Cheng , J. L. Li and a H. Zhang aKey Laboratory of Particle and Radiation Imaging (Ministry of Education) and Department of Engineering Physics, Tsinghua University, Beijing 100084 bCollege of Nuclear Science and Technology, Beijing Normal University, Beijing 100875 ARTICLEINFO ABSTRACT Keywords: In-situ -ray measurements were performed using a portable high purity germanium spectrometer in In-situ -ray measurements Hall-C at the second phase of the China Jinping Underground Laboratory (CJPL-II) to characterise Environmental radioactivity the environmental radioactivity background below 3 MeV and provide ambient -ray background Underground laboratory parameters for next generation of China Dark Matter Experiment (CDEX). The integral count rate Rare event physics of the spectrum was 46.8 cps in the energy range of 60 to 2700 keV. Detection efficiencies of the CJPL spectrometer corresponding to concrete walls and surrounding air were obtained from numerical calculation and Monte Carlo simulation, respectively. The radioactivity concentrations of the walls 238 232 in the Hall-C were calculated to be 6:8 , 1:5 Bq/kg for U, 5:4 , 0:6 Bq/kg for Th, 81:9 , 14:3 40 Bq/kg for K. Based on the measurement results, the expected background rates from these primordial radionuclides of future CDEX experiment were simulated in unit of counts per keV per ton per year (cpkty) for the energy ranges of 2 to 4 keV and around 2 MeV. The total background level from *2 primordial radionuclides with decay products in secular equilibrium are 0:63 and 4:6 × 10 cpkty for energy ranges of 2 keV to 4 keV and around 2 MeV, respectively. 1. Introduction In this paper, in-situ -ray HPGe spectrometry was applied to measure the environmental radioactivity of the The environmental background in the walls of the Hall-C of the CJPL-II and validate the underground laboratories mainly includes cosmic muons, results of background control measures with GeTHU. This muon induced particles, rays from rocks and concrete, and method is widely used in environmental -ray measurements neutrons from the ( , n) reaction and spontaneous fission in underground laboratories for its advantage of not of radionuclides in the surrounding materials. Since the requiring sample preparation [21–23]. This portable HPGe cosmic rays and cosmogenic radioactivity are drastically spectrometer was calibrated to obtain the angular response decreased with thick rock overburden as shielding [1], and its detection efficiency of walls of Hall-C based on Beck 222 these underground laboratories [2–6] become favorable sites formula [24]. Since Rn and its daughters in the air induce for rare event searching experiments requiring ultra-low main background for -ray measurement, we employed background environment. Moreover, the muon and neutron Monte Carlo simulation with GEANT4 [25] to eliminate background in underground laboratories have been studied their contributions accordingly. The next section will comprehensively in literatures [7–11]. describe the setup of in-situ measurements in CJPL-II and The China Jinping Underground Laboratory (CJPL) the derivation of detection efficiencies in detail. The results of measured spectrum and radioactivity concentration will locates in a traffic tunnel under Jinping Mountain, southwest be presented and discussed in the third section. The last of China [12] with about 2400 m rock overburden vertically. CJPL-I has been in normal operation and provided low section will present the expected background level induced background environment for two dark matter experiments, by the primordial radionuclide in the concrete walls for the CDEX [13–18] and PandaX [19] since Dec. 2010. The future CDEX experimental facility. second phase of CJPL (CJPL-II) was proposed to meet arXiv:2005.05498v2 [physics.ins-det] 25 Jan 2021 the growing space demands from rare event searching 2. Experiment and methods experiments. The construction of CJPL-II started in 2014 2.1. In-situ Measurements and the cavern excavation was completed in Jun. 2017. The portable coaxial HPGe detector manufactured by For better control of background radioactivity of CJPL-II, raw materials of concrete and other construction material ORTEC was used in the in-situ -ray measurements of were screened and selected by low-background -ray high CJPL-II and its location under measurements in Hall-C is purity germanium (HPGe) spectrometer named GeTHU [20] shown in Fig.1. The level of radon, emanated naturally from before used. the concrete and rocks, was measured by an AlphaGUARD < radon monitor produced by Saphymo Gmbh which was Corresponding author: [email protected] placed next to the HPGe detector. The dimensions of Hall-C ORCID(s): 0000-0003-1243-7675 ( Z. Zeng) were obtained by a laser range finder. H. Ma et al.: Preprint submitted to Elsevier Page 1 of 6 In-situ measurements in CJPL-II Auxiliary Gate No. 1 Auxiliary Gate No. 2 bounds in integration. To consider the penetrations of rays, Draining Tunnel Traffic Tunnel A the upper limits of integral related to the concrete thickness No. 1 Gate No. 2 Gate Traffic Tunnel B is set to be the actual thickness (0.2 m). The 0.2m-thick Middle Connecting Tunnel concrete will decrease gamma rays from the rock more than Service Tunnel A Service Tunnel C Service Tunnel D A1 A2 Service Tunnel B C1 C2 an order of magnitude, and the rock has less radioactivities Hall A B3 B4 Hall C Connecting Tunnel AB Connecting Tunnel CD compared with concrete as shown in Table2. In this work, B1 B2 D1 D2 Hall B Hall D we ignore the penetrating gamma rays from rock. Following similar procedures described in Ref. [23], the numerical conversion factors are derived. 2 Detector r P zmax ymax xmax 13m cal Floc = f.E; /gloc.x; y; z/dxdydz; 27m 13.5m A cal Êzmin Êymin Êxmin 129m Detector (2) F 1m Height where the loc is the numerical conversion factor for r A 14m different surfaces of the Hall-C. cal and cal are the distance Top View Sectional View between detector and calibration source, and the calibration source activity, respectively. is the concrete density while P x; y; z x; y; z Figure 1: The layout of CJPL-II (top) and the measurement is the gamma ray intensity. /min and . /max location inside Hall-C (bottom). are the lower bound and upper bound of integral, which are related to the different surfaces of the Hall-C. gloc.x; y; z/ is the coefficient representing the attenuation effect from The layout of CJPL-II tunnels and experimental halls are concrete and air. The attenuation coefficients come from the illustrated in Fig.1. The CJPL-II has four experimental halls NIST XCOM [27]. and each hall is divided into two sub-halls with numbers The angular correction factors and the effective front areas as postfix. The China Dark Matter Experiment (CDEX), are determined through angular calibration experiment with with scientific goals of searching for light DM, operates 241 137 60 a mixed calibration source containing Am, Cs, Co germanium detector array in CJPL-I currently. Since the 152 and Eu. The f.E; / surface is fitted by Eq.3. The future CDEX experiment will be located in the Hall-C relative biases between the fitting curve and the experimental [26] and the dimensions of four experimental halls and data points are treated as the systematic uncertainties of their construction materials are almost the same. In-situ factors. measurements in this work were carried out in Aug. 2019 2 Q2.ln E/ +Q1.ln E/+Q0 5 in the Hall-C of CJPL-II. f.E; / = AEF ⋅ e ⋅ .P5 4 3 2 (3) 2.2. Detector calibration + P4 + P3 + P2 + P1 + P0/; The relationship between the rates of -ray peaks and the where AEF represents the effective front area, and Q or radionuclide activities can be described as Eq.1 and the P with numerical subscripts are parameters determined by numerical calculations are applied to determine the detection fitting. efficiency for different -ray peaks [24]. Nf Nf N Substituting the f.E; / in the Eq.2, one gets the f E; 0 ; = . / = × × (1) numerical conversion factors for certain walls and certain - A N0 A ray energy. The angular response of detector fluctuates with where Nf is rate of the -ray peak and A is the concentration a relatively small range between 0° and 150° but decreases *1 of the radionuclide ( with the unit of Bq ⋅ kg for the rapidly after 160°, since most rays from the back of concrete). f.E; / is the coefficient representing angular detector are shielded by the liquid nitrogen dewar and cold response factor multiplying the effective front area and is finger. the zenith angle between the location of radionuclides and germanium detector. E is the energy of ray while is 3. Results and discussion zenith angle. _A is the -ray flux received at detector’s 3.1. Spectrum analysis location per unit concentration of nuclides. N0_ is the detector response to the incident rays at 0° (called the The energy spectra with the integral rate of 46.8 cps effective front area) while Nf _N0 is the detector angular between 60 and 2700 keV was measured in experimental response. In this work, the last two were measured in the Hall-C with the in-situ -ray spectrometer as shown in angular calibration experiment. Fig.2. The -ray peaks from primordial radionuclides are labeled. As shown in Eq.2, we integrate the detection efficiencies with the dimensions of Hall-C.