Radiopurity studies of the rock samples from the probable sites for an underground laboratory

Efforts are underway in India to setup an underground laboratory for neutrino physics and rare nuclear processes. The radiation from surrounding rocks can have a significant impact on the sensitivity of these experiments and hence it is important to assess the base background. With this motivation, radiopurity measurements have been carried out on rock samples from a few probable sites for the underground laboratory using low background gamma spectroscopy. The rock samples are collected from possible tunnel sites near Jhakri dam and Atal tunnel in Himachal Pradesh, and Zojila pass in Jammu and Kashmir. Measurements have been carried out using TiLES (TIFR Low Background Experimental Setup), employing only passive shielding, to measure the trace elemental concentrations of primordial elements like ²³⁸U, ²³²Th and ⁴⁰K. All three samples show moderate ⁴⁰K content (0.5-1.3 ppm). The Atal rock sample has lowest concentrations (less than 1 ppm) of ²³²Th and ²³⁸U, although somewhat higher than the nearby Aut region. The Jhakri sample shows relatively higher levels (∼ 2 ppm) of ²³²Th and ²³⁸U. Therefore, from gamma and neutron background considerations Atal tunnel region will be preferable