Gamma-ray flux measurement and geotechnical studies at the selected site for the LABChico underground laboratory

The γ -ray flux inside La Guadalupe mine, the selected site for the construction of the underground laboratory LABChico in Mexico, is reported for energies below 3 MeV. Data were recorded with a 0.669 kg thallium-activated sodium iodide (NaI) crystal detector deployed for 3.6 hr. The detector response was calculated via Monte Carlo simulations with GEANT4 and validated against point-like calibration sources, and the γ -ray spectrum was extracted using an unfolding technique. The γ -ray flux above 250 keV and below 3 MeV is 0.1768 γ /cm ^2 /s. The two most intense γ -rays in the natural radioactive background, ^40 K and ^208 Tl, were identified. The flux measured for these isotopes is 0.0363 ± 0.0020 γ /cm ^2 /s and 0.0016 ± 0.0005 γ /cm ^2 /s, respectively. A γ -ray spectrometry analysis of rock samples showed 674.0 ± 2.0 Bq/kg, 24.0 ± 0.1 Bq/kg, and 17.7 ± 0.2 Bq/kg, of ^40 K, ^232 Th, and ^238 U, respectively. These results are compared with deep underground facilities such as SURF, SNOLAB, Boulby, Modane, and Gran Sasso, with differences observed mainly due to the rock composition. Geotechnical studies of the mine and its rock composition are also reported.