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

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