Pulse shape analysis of scintillation signals from pure and xenon-doped liquid argon for radioactive background identification

Abstract

The response of a 19 kg active mass liquid argon scintillation detector to gamma, alpha and neutron radiation and its characteristic scintillation photon time distribution has been studied in the framework of the GERDA double beta decay research and development program, The achieved photo-electron yield of 1.24 pe/keV was stable during the two years of operation. The detector exhibits excellent energy resolution of 8.7/17.2 keV (a) for gamma energies of 60/239 keV. Radon was loaded into the liquid argon to study alpha energy quenching and decay time correlation of its progenies. A robust pulse shape analysis method was used to identify and discriminate amongst the different radiation types, 60 keV gamma signals could be discriminated against neutron recoils of the same visible energy with a miss-identification probability of < 5.10(-4) limited by statistics and ambient backgrounds. Xenon doping of liquid argon increased the photo-electron yield and improved the spectroscopic performance of the detector leading to an energy resolution of 7.2/15.4 keV (sigma) for 60/239 keV. The discrimination power improved slightly with the addition of xenon up to concentrations of 300 ppm. Applications for background identification and discrimination in double beta decay search with (76)Ge crystals, as well as for Dark Matter search with liquid argon are discussed.