Background model for the NaI(Tl) crystals in COSINE-100

Abstract

The COSINE-100 dark matter search experiment is an array of NaI(Tl) crystal detectors located in the Yangyang Underground Laboratory (Y2L). To understand measured backgrounds in the NaI(Tl) crystals we have performed Monte Carlo simulations using the Geant4 toolkit and developed background models for each crystal that consider contributions from both internal and external sources, including cosmogenic nuclides. The background models are based on comparisons of measurement data with Monte Carlo simulations that are guided by a campaign of material assays and are used to evaluate backgrounds and identify their sources. The average background level for the six crystals (70 kg total mass) that are studied is 3.5 counts/day/keV/kg in the (2-6) keV energy interval. The dominant contributors in this energy region are found to be 210 Pb and 3 H.

Study of cosmogenic radionuclides in the COSINE-100 NaI(Tl) detectors

“…This is especially the case for 3 H, which has no distinguishing γ/X-ray peak that can be exploited. Therefore, we simulated background spectra from 3 H in the six NaI(Tl) crystals and used the extracted spectral shapes in the data fitting, while floating their unknown fractions [21]. In this way, we determine the initial activity A 0 of 3 H from the global background fitting model [21], with results shown in Table 7.…”

“…The search for an annual modulation signal requires a complete understanding of background sources and their time dependence. To accomplish this, a complete simulation that accurately models the background energy spectra measured in the detector is required [21]. In addition to backgrounds from long-lived radioactive contaminations in the crystal bulk and surfaces, we have to deal with time-dependent backgrounds from short-lived cosmogenically activated radionuclides.…”

Monte Carlo simulation of the SABRE PoP background

“…The DAMA [26] experiment finds that a background contribution from internal 210 Pb is visible only in some of the crystals and quotes a range of activity levels of 0.005 -0.03 mBq/kg. On the other hand, the ANAIS and COSINE experiments, who both have published their background models and compared with data [27,28], quote a much larger contamination, around 0.7 mBq/kg in their best crystals and up to 3.2 mBq/kg. In this work, we have used 0.03 mBq/kg, the highest level among the 210 Pb contaminations found in DAMA crystals, as we are confident that the SABRE procedure for crystal growth and handling will minimize the 210 Pb contamina-tion.…”

A search for solar axion induced signals with COSINE-100

“…These backgrounds are modeled using the Geant4 [33] based detector simulation described in Ref. [26]. Several sources of uncertainties in the background model are included in this analysis.…”