Ultra-Low background germanium spectrometers at the China Jinping Underground Laboratory

TL;DR

The paper addresses the need for ultra-low background gamma-ray spectroscopy to screen materials for dark matter and neutrinoless double-beta decay experiments. It details the CJPL facility and shielding environment, including CJPL-I GeTHU instruments and the CJPL-II ARGUS array plus fifteen GeTHU-like units, designed to minimize backgrounds via lead and copper shielding and RN-free air. Background modeling combines Monte Carlo simulations and Bayesian inference to identify sources such as radon and cosmogenic nuclides, with demonstrated sensitivity through copper sample measurements achieving MDA on the order of 100 microbecquerels per kilogram. These results validate CJPL-II as a capable platform for material screening, enabling more reliable pre-selection of low-radioactivity components for rare-event detectors and reducing backgrounds in future experiments.

Abstract

Four ultra-low background germanium spectrometers, called GeTHU, have been installed at the first phase of China Jinping Underground Laboratory (CJPL-I), and served for material screening of dark matter and neutrino experiments. Recently, a new multi-detector spectrometer with five germanium detectors has been developed and installed at the second phase of CJPL (CJPL-II) with a minimum detectable activity (MDA) of about 10 μBq/kg. In addition, another fifteen GeTHU-like spectrometers have been installed at CJPL-II with an MDA of about 1 mBq/kg. This paper will introduce the ultra-low background germanium spectrometers including shielding design, background characteristics and application to material screening.

Figures (4)

• Figure 1: Detectors and shielding of the GeTHU spectrometers.
• Figure 2: Background decomposition of GeTHU-V. The measured background spectrum is labeled in gray. The colored soiled lines are its decompositions. In the three lower panels displaying the normalized residual distributions the central 1$\sigma$-, 2$\sigma$- and 3$\sigma$-bands are marked in green, yellow, and red, respectively.
• Figure 3: The layout of ARGUS spectrometer.
• Figure 4: Background decomposition of ARGUS-D0 (central detector). The measured background spectrum is labeled in gray. The colored soiled lines are its decompositions. In the three lower panels displaying the normalized residual distributions the central 1$\sigma$-, 2$\sigma$- and 3$\sigma$-bands are marked in green, yellow, and red, respectively.