Environmental gamma background measurement in the phase II of China Jinping Underground Laboratory

Abstract

<p indent="0mm">As one of the National Major Science and Technology Infrastructure Construction Projects of China, the Deep Underground and ultra-low Radiation Background Facility for frontier physics experiments (DURF) was built based on the phase II of China Jinping Underground Laboratory (CJPL). During its construction, various techniques, including low-radioactivity material screening and multi-layer waterproof radon-resistant layers, were implemented to control the radioactivity background in the laboratory. To evaluate the environmental gamma background, <italic>in-situ</italic> gamma-ray measurements were performed using a portable high-purity germanium spectrometer in Hall-A1, Hall-C2 (inside and outside the large polyethylene shielding chamber), and the connecting tunnel CD at CJPL-II. The integral count rate of the spectrum in the energy range of <sc>60–2700 keV</sc> in Hall-A1, Hall-C2, and the connecting tunnel CD were <sc>70–77 cps.</sc> The conversion factors for the building materials were calculated with a Monte Carlo based integration method. The radon concentrations were measured to assess the radon progeny’s contributions to the gamma background spectra. Then, the corresponding radioactivity concentrations of ²³⁸U, ²³²Th, and ⁴⁰K were calculated to be 4.69–5.81, 5.57–6.66, and <sc>127.00–149.44 Bq/kg</sc> respectively. The radioactivity concentrations of ²³⁸U and ²³²Th are close to the results measured in 2019 at the early stages of the construction of DURF, which shows that the following construction such as the multi-layer waterproof radon-resistant layers did not increase the ²³⁸U and ²³²Th concentrations. The integral count rate of the spectrum inside the polyethylene shielding chamber of Hall-C2 was around <sc>2.2 cps,</sc> which was 31 times lower than that measured outside the polyethylene shielding chamber. Artificial radionuclides such as ⁶⁰Co and ¹³⁷Cs were not detected in the spectrum. Results suggest that the radioactive background control of various building materials has achieved a good effect during the construction of DURF and the environmental gamma background reaches the international advanced level among deep underground laboratories worldwide.</p>

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