Radiopurity of NaI(Tl) crystals for PICOLON dark matter experiment

TL;DR

This paper reports on the radiopurity of NaI(Tl) crystals used in the PICOLON dark matter experiment, demonstrating impurity levels comparable to DAMA/LIBRA crystals, crucial for low-background dark matter searches.

Contribution

The study presents the radiopurity measurements of a new NaI(Tl) crystal produced with established purification techniques, achieving impurity levels similar to DAMA/LIBRA crystals, supporting dark matter detection efforts.

Findings

Impurity levels of $^{232}$Th, $^{226}$Ra, and $^{210}$Po are comparable to DAMA/LIBRA crystals.

Background rate in 2-6 keV energy region is 2-5 events/d/kg/keV.

The new crystal's radiopurity supports its use in sensitive dark matter experiments.

Abstract

The dark matter observation claim by the DAMA/LIBRA collaboration has been a long-standing puzzle within the particle physics community. Efforts of other research groups to verify the claim have been insufficient by significant radioactivity of present NaI(Tl) crystals. PICOLON (Pure Inorganic Crystal Observatory for LOw-energy Neut(ra)lino) experiment conducts independent search for Weakly Interacting Massive Particles (WIMPs) using NaI(Tl) crystals. Our NaI(Tl) crystal manufactured in 2020 (Ingot #85) reached the same purity level as DAMA/LIBRA crystals. In this report, we describe the radiopurity of the new Ingot #94 crystal produced using the same purification technique as Ingot #85. The $\alpha$-ray events were selected by pulse-shape discrimination method. The impurities in the Ingot #94, $^{232}$Th, $^{226}$Ra and $^{210}$Po radioactivity were $4.6\pm 1.2~\mathrm{\mu Bq/kg}$, $7.9\pm 4.4~\mathrm{\mu Bq/kg}$, and $19\pm 6~\mathrm{\mu Bq/kg}$, which are equivalent to those of the DAMA/LIBRA crystals. The background rate in the energy region of 2-6 keV , was 2-5 events/d/kg/keV without applying a veto trigger.