Abstract
We present a background model for dark matter searches using an array of NaI(Tl) crystals in the COSINE-100 experiment that is located in the Yangyang underground laboratory. The model includes background contributions from both internal and external sources, including cosmogenic radionuclides and surface ^{210}Pb contamination. To build the model in the low energy region, with a threshold of 1 keV, we used a depth profile of ^{210}Pb contamination in the surface of the NaI(Tl) crystals determined in a comparison between measured and simulated spectra. We also considered the effect of the energy scale errors propagated from the statistical uncertainties and the nonlinear detector response at low energies. The 1.7 years COSINE-100 data taken between October 21, 2016 and July 18, 2018 were used for this analysis. Our Monte Carlo simulation provides a non-Gaussian peak around 50 keV originating from beta decays of bulk ^{210}Pb in a good agreement with the measured background. This model estimates that the activities of bulk ^{210}Pb and ^{3}H are dominating the background rate that amounts to an average level of 2.85pm 0.15 counts/day/keV/kg in the energy region of (1–6) keV, using COSINE-100 data with a total exposure of 97.7 kgcdot years.
Highlights
COSINE-100 is an NaI-based experiment for the direct detection of dark matter particles, with an array of 106 kg NaI(Tl) crystals
The encapsulated NaI(Tl) crystal assembly is equipped with two 3-inch Hamamatsu R12669SEL photomultiplier tubes (PMTs) that are each configured to generate two readouts, a high-gain signal from the anode and a lowgain signal from the 5th-stage dynode that are acquired in independent channels
The newer version better describes the non-Gaussian peak around 50 keV from the beta decay of 210Pb; it is attributed to 46.5 keV emissions of conversion/Auger electrons and γ /X-ray together with about 4 keV mean energy of beta electrons from the decay to the excited state of 210Bi, which results in a non-Gaussian peak
Summary
COSINE-100 is an NaI-based experiment for the direct detection of dark matter particles, with an array of 106 kg NaI(Tl) crystals. It has been operating at the Yangyang underground laboratory (Y2L) since September 2016 [1,2,3]. To verify the DAMA/LIBRA modulation signal, a complete understanding of the background energy spectrum is required. We used 1.7 years of COSINE-100 data taken from October 21, 2016 to July 18, 2018 with a 106 kg array of low background NaI(Tl) crystals. We present here the background modeling to represent 1.7 years of COSINE-100 data with a total effective mass of 61.3 kg from crystals C2, C3, C4, C6 and C7, excluding three crystals due to a high noise rate and low light yields. Events for which none of the two occurred are classified as single-hit
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