Background simulation of a low background gamma ray spectrometer with an array of five germanium detectors

A new gamma ray spectrometer with an array of five germanium detectors, called ARGUS, is designed to screen and select materials used in rare-event search experiments. In this paper, we investigate the background characteristics of ARGUS using Monte Carlo simulation. The expected background count rate was found to reach the level of 10 counts per day per kg of germanium crystal (cpkd) in the energy region of 60 keV to 2700 keV. The minimum detectable activity (MDA) for germanium array was estimated via the maximum likelihood method. With an 100-day measurement of a 118.3 kg high purity copper sample, the MDA were calculated to be 28.8 μBq/kg for ²¹²Pb (238 keV) and 32.8 μBq/kg for ²¹⁴Bi (609 keV). The ARGUS is planned to be built in the China Jinping Underground Laboratory in 2025.