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 2–6-keV energy interval, which produce 2.4 counts/day/keV/kg (dru) and 0.5 dru, respectively.
Highlights
To date, no other experiments, except for the DAMA experiment [7–9], have found an annual modulation signal interpreted as Weakly interacting massive particles (WIMPs) interactions with a significance of 9.2σ
It needs to specify all long-lived parts of 238U and 232Th daughters to consider broken decay chains when they are fitted to the measured data to quantify their unknown background fractions and, we grouped daughter isotopes from the full decay chains of 238U and 232Th according to their half-lives
photomultiplier tube (PMT) and materials – such as the copper shield, the copper structure to which the PMT is affixed to the crystal, the PMT base, connectors, and bunches of cables – would contain radioactive sources that can contribute to the background as an external background source inside the shielding
Summary
No other experiments, except for the DAMA experiment [7–9], have found an annual modulation signal interpreted as WIMP interactions with a significance of 9.2σ. There are several groups, such as DM-Ice [15,16], ANAIS [17,18], and SABRE [19], developing ultra-lowbackground NaI(Tl) crystals with the goal of reproducing the DAMA/LIBRA results and currently, KIMS and DM-Ice have agreed to operate a single experiment, COSINE, at Y2L using NaI(Tl) crystals and a total mass of 106 kg is being used in the first-stage experiment, COSINE-100. As part of this program we have developed ultra-low-background NaI(Tl) crystals and studied their properties in a variety of test setups with the ultimate goal of achieving purity levels that are comparable to or better than those of the DAMA/LIBRA crystals.
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