Radiation Background Investigations for Environmental and Rare Decay Studies

Abstract

Studies of low-level environmental radioactivity and experiments to investigate solar neutrino, dark matter (DM), double beta decay (DBD), and rare nuclear decays (T1/2 > 1018 years) have minimal event rates, which require stringent background conditions and thereby makes most of the experimental search challenging. Given the rare nature of the processes, it is necessary to perform a detailed study of natural background radiation to investigate the sensitivity of the measurement and plan its reduction for different targeted experiments. For exploring the background issues related to rare event searches, the devel opment of dedicated low background measurement setups is crucial to reaching adequate sensitivity levels. The present thesis investigates various aspects of radiogenic background relevant to low-level environmental radioactivity and rare decay searches employing single or combinations of low background HPGe spectrometers. A low background setup ILM (IIT Ropar Low-background Measurement setup) above ground is being set up at IIT Ropar in India for rare decay studies. As of now, ILM-0, a demonstrator of ILM, has been achieved, which consists of a carbon-loaded HPGe detector. The low background HPGe detector has been is characterized using point-like γ-ray sources in a wide energy range of 80-1408 keV at IIT Ropar. A preliminary detector model has been developed using the GEANT4 simulation package to estimate the photopeak efficiency of an HPGe detector employed in the setup. The present model shows an average relative deviation of ∼10%, and the experimental setup has shown improved measurement sensitivity for counting environmental samples and quantifying radionuclides. Investigations of specific activity and its variation in soil samples from the arable lands in the Ropar district of Punjab state in India have been carried out. As a part of this thesis, feasibility studies have been carried out to investigate the single beta and double beta decay modes in 96Zr and 112Sn isotopes, respectively. The physics simulations have been performed using a GEANT4 simulation package employing 2/4 HPGe detectors (∼33% R.E.) to optimize the source configuration for rare decay experiments. For the β decay of 96Zr, the results suggest that ∼70 g of 50% enriched source will yield mass efficiency of ∼12-20 g-% for 568-1091 keV gamma-ray pair in the 96Nb decay cascade, com parable to the Finch et al. (Nucl. Instrum. Methods A, 806, 70, 2016). The feasibility study of positron double beta decay modes involving simultaneous emission of 2/4 gamma rays of 511 keV in 112Sn (EC-β+) and 106Cd (β+β+) has been performed using a coinci dence setup of two HPGe detectors. To estimate the sensitivity to search for β+β+/ECβ+ processes in 112Sn, the ambient background has been measured with moderate Pb shielding around the coincidence setup of two low background HPGe detectors in the laboratory of TIFR, Mumbai at the sea level. The coincident detection of the 511 keV pair significantly improves the background in the region of interest. From background measurements with ∼40 g of natSn, the sensitivity for Tβ+β+ 1/2 (106Cd) and TEC−β+ 1/2 (112Sn) is estimated to be ∼1019- 1020 y for 1 y of measurement time with enriched samples (90%). Efforts are underway in India to set up an underground laboratory for planning the low background experiments. In order to support these efforts, the radiopurity studies of rock samples from the Aut tunnel have been carried out using the TiLES at TIFR, Mumbai. The f indings of the present studies were compared with the Bodi West Hills (BWH) of the Theni district in Madurai, Tamil Nadu. Compared with BWH rock, the Aut rock appears to have a lesser amount of 232Th and a somewhat higher amount of 238U. It has been found that the concentration of 40K in Aut rock was observed to be lower by a factor of ∼1000 as compared to the BWH rock samples. It is noted that the Aut rock trace impurity concentrations were considerably lower than the respective worldwide average values. Overall, the ambient gamma-ray background at Aut is expected to be lower than at the BWH site, indicating the suitability of the site for low background experiments. In order to assess the impact of the long-lived neutron-induced activities, fast neutron activation experiments have been carried out on the Aut and BWH rock samples. Irradi ations were carried out at BARC-TIFR Pelletron Linac Facility, Mumbai, at two incident proton energies, 12 and 22 MeV, to cover a broader energy range and compare the yield of the observed products on neutron energy. Neutron activation studies of Aut rock have revealed mostly short-lived activity. The fast neutron activation studies of both Aut and BWH rock samples have indicated the presence of long-lived activities like 54Mn (0.855y) and 22Na (2.60y). Still, the resultant gamma-ray energies are lower than 1500keV, and no significant long-lived contributions at E >2 MeV were observed. The low energy neutron f lux arising due to spontaneous fission and (α,n) reactions, dominated by 238U, is expected to be around 3×10−6 ncm−2s−1, which is similar to other underground laboratories. The Aut site is expected to have lower ambient gamma-ray background than the BWH, while the low-energy neutron background is expected to be similar. From the radioactive background investigations, Aut appears to be a good site for building an underground laboratory for rare decay studies.