Measurement of low-energy background events due to ${}^{222}$Rn contamination on the surface of a NaI(Tl) crystal

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