The next generation of ultra-low background physics experiments will reach energy regions and detector sensitivities beyond those previously used to solve many relevant problems of science. For instance, exploring the nature of dark matter, and answering the question of charge-parity (CP) violation of neutrinos in the lepton sector, require ultra-low background rates in the region of interest of detectors.;This thesis studies two aspects related to rare event physics. First, a model of ionization efficiency was developed for low energy nuclear recoils in germanium, a common dark matter target. The fundamental physics processes of stopping power below 100 keV were investigated; it was observed that a component of nuclear stopping power contributes to ionization efficiency. To correctly interpret the experimental threshold, a reliable model for ionization efficiency is necessary.;Experimental verification of this model was completed using a neutron source incident on a germanium detector. A Monte Carlo simulation was carried out in parallel by another member of the research group. We used shape analysis to compare the experimental data with the proposed Barker-Mei model and an established model for ionization efficiency, Lindhard et al. with k = 0.159. We found agreement between the experimental data and the Monte Carlo simulations to within 4% for both models. Thus, we conclude that the models are valid for the range of 1 keV to 100 keV.;The second component was the evaluation of cosmogenic background events from muons and muon-induced neutrons in liquid argon for a long baseline neutrino oscillation experiment. Analytical models were developed to calculate the background event rates of cosmogenically produced nuclei, particularly 40Cl, with rock overburdens of 0.712 km water equivalent (km.w.e.) and 4.3 km.w.e. The predicted rates were compared to a Monte Carlo simulation of a liquid argon target at similar overburden depths performed by another member of the research group.;In this thesis, several important rare event physics processes related to new physics beyond the Standard Model are reviewed. The critical signatures and the required interpretation methods are discussed.
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