# Understanding NaI(Tl) crystal background for dark matter searches

## TL;DR

This study develops ultra-low-background NaI(Tl) crystals for dark matter detection, analyzing background sources through simulations to understand and quantify their contributions, focusing on surface $^{210}$Pb and internal $^{40}$K.

## Contribution

The paper presents a detailed background modeling approach for NaI(Tl) crystals using Geant4 simulations, aiding future dark matter search experiments.

## Key findings

- Background sources are dominated by surface $^{210}$Pb and internal $^{40}$K.
- Simulated background spectrum matches measured data well.
- Quantitative contributions of background components are established.

## Abstract

We have developed ultra-low-background NaI(Tl) crystals to reproduce the DAMA results with the ultimate goal of achieving purity levels that are comparable to or better than those of the DAMA/LIBRA crystals. Even though the achieved background level does not approach that of DAMA/LIBRA, it is crucial to have a quantitative understanding of the backgrounds. We have studied background simulations toward a deeper understanding of the backgrounds and developed background models for a 9.16-kg NaI(Tl) crystal used in the test arrangement. In this paper we describe the contributions of background sources quantitatively by performing Geant4 Monte Carlo simulations that are fitted to the measured data to quantify the unknown fractions of the background compositions. In the fitted results, the overall simulated background spectrum well describes the measured data with a 9.16-kg NaI(Tl) crystal and shows that the background sources are dominated by surface $^{210}$Pb and internal $^{40}$K in the 2 to 6-keV energy interval, which produce 2.4 counts/day/keV/kg (dru) and 0.5 dru, respectively.