Analysis of alpha-emitting isotopes in an inorganic scintillator

Large scintillating crystals are used in searches for dark matter and other elusive phenomena where the presence of trace radioisotopes is a major source of background. We describe a method for determining alpha-emitters present in concentrations very much less than 10−12. The optimal filter for pulse shape discrimination, described by Gatti nearly 40 years ago, is implemented using modern digital techniques. Results are presented for a large NaI(Tl) scintillator; we give a detailed quantitative analysis of its radio-impurities, showing that the 235U series is essentially absent and that both the 232Th and 238U series are far from equilibrium, the main contaminants being the radium isotopes and their short-lived daughters.