Depth profile study of 210Pb in the surface of an NaI(Tl) crystal

Abstract The surface 210Pb is one of the main background sources for dark-matter-search experiments using NaI(Tl) crystals, and its spectral features associated with the beta-decay events for energies less than 60 keV depends on the depth distribution of 210Pb in the surface of an NaI(Tl) crystal. Therefore, we must understand the profile of surface 210Pb to precisely model the background measurement in the low-energy region for the low-background experiment using NaI(Tl) crystals. We estimate the depth profile of the surface 210Pb contamination by modeling the measured spectrum of the alpha emission from the decay of 210Po at the decay sequence of the surface 210Pb contamination that is obtained using an 222Rn-contaminated crystal. In order to describe the energy spectra of the surface contamination we perform a log-likelihood fit of the measured data to a sum of Geant4 Mote Carlo simulations, weighted by an exponential curve as a function of the surface depth. The low- and high-energy events from the beta decay of surface 210Pb are also modeled to improve the depth profile for shallow depths. We simulate the energy spectra from beta decays of 210Pb that are exponentially distributed in the surface by following two exponential functions where the mean-depth coefficients are free parameters in the data fitting; we observed that the energy spectra are in good agreement with the measured data.