Cosmogenic Backgrounds in Liquid Argon

Observational evidence gathered from astrophysical and cosmological sources strongly suggests the existence of dark matter. Constituting approximately 27% of the total mass-energy of the Universe, dark matter plays a critical role in the evolution of the Universe. Weakly interacting massive particles (WIMPs) are an important class of potential dark matter candidates. One of the leading technology in WIMP detection is the use of liquefied noble gases as the sensitive medium. DarkSide-20k (DS-20k) is a planned detector based on liquid argon (LAr) technology. The DS-50 experiment demonstrated the success of the technology, and now DS-20k is being developed to perform highly sensitive dark matter searches at high masses. The detection of rare events requires an ultra-low background because the background can limit its sensitivity. My thesis work is focused on the cosmogenic backgrounds in LAr target. The cosmic-ray interactions activate trace radioactive contaminants while detector materials are being stored or transported above ground. The estimation of the cosmogenic activation of detector materials before detector installation is important. I developed an activation calculation software tool named COSAC to estimate the induced activity of radioactive isotopes produced during UAr transportation. The software helps us to determine the number of radioisotopes activated while transporting the argon. Radioactive isotope production depends on cosmic-ray flux and reaction cross-section. COSAC is designed based on a compiled selection of cross-section measurement and models, flux models, as well as the user-specified transportation history and initial composition of the target argon. The code is highly flexible and easy to extend. The future LAr detectors require chemically purified and 39Ar depleted UAr. Aria is a cryogenic distillation column aiming to fulfil the target needs of the future Dark- Side detectors and it is the tallest ever built distillation column of 350m height. The second phase of my thesis work involves the preliminary studies of the Aria distillation column. I found the production rate of 40Ar isotope in Aria distillation column, and the optimal values of the operating parameters of distillation column. Cosmic rays can produce backgrounds in the detector while it is underground. The high-energy cosmic-ray muons can reach underground and produce high-energy neutrons or neutron sources by interacting with the detector or the laboratory materials. These generated neutrons can penetrate the detector’s shielding and make a WIMPlike background. To model this background, I simulated cosmogenic showers using FLUKA, a FORTRAN-based software package that excels at modelling these classes of events. A series of physics selection cuts and veto cuts are used to reject the background. After the selection cuts, a correlation study of events in TPC (inner detector) and outer veto detector is performed. This study motivates the collaboration to optimise the outer veto selection cut of DS-20k.