Measurement of low-energy events due to Rn-222 daughter contamination on the surface of a NaI(Tl) crystal

Abstract

It has been known that decays of daughter elements of Rn-222 on the surface of a detector cause significant background at energies below 10 keV. In particular Pb-210 and Po-210 decays on the crystal surface result in significant background for dark matter search experiments with Nal(Tl) crystals. In this report, measurement of Pb-210 and Po-210 decays on surfaces are obtained by using a Rn-222 contaminated crystal. Alpha decay events of Po-210 on the surface are measured by coincidence requirements of two attached crystals. Due to recoiling of Pb-206, rapid nuclear recoil events are observed. A mean time characterization demonstrates that Pb-206 recoil events can be statistically separated from those of sodium or iodine nuclear recoil events, as well as electron recoil events. (C) 2018 Elsevier B.V. All rights reserved.