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

TL;DR

This study measures low-energy background events caused by radon progeny on NaI(Tl) crystal surfaces, crucial for dark matter detection, by analyzing alpha decays and nuclear recoils with coincidence techniques.

Contribution

It provides a novel measurement method for surface radon progeny decays on NaI(Tl) crystals using coincidence detection and recoil event analysis.

Findings

Surface radon decay events can be statistically distinguished from other recoil events.

Recoil events from $^{206}$Pb can be separated from sodium, iodine, and electron recoils.

Measurement techniques improve understanding of background sources in dark matter experiments.

Abstract

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.