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

Abstract

COSINE-100 is a direct detection dark matter search experiment that uses a 106 kg array of eight NaI(Tl) crystals that are kept underground at the Yangyang Underground Laboratory to avoid cosmogenic activation of radioisotopes by cosmic rays. Even though the cosmogenic activity is declining with time, there are still significant background rates from the remnant nuclides. In this paper, we report measurements of cosmogenic isotope contaminations with less than one year half-lives that are based on extrapolations of the time dependent activities of their characteristic energy peaks to activity rates at the time the crystals were deployed underground. For longer-lived 109 Cd (T 1/2 = 1.6 y) and 22 Na (T 1/2 = 2.6 y), we investigate time correlations of characteristic γ/X-ray peaks. The inferred sea-level production rates are compared with caluclations based on the ACTIVIA and MENDL-2 model calculations and experimental data. For 3 H, which has a long, 12.3 year half-life, we evaluated the activity levels from the exposure times and determined a cosmogenic activation rate that is consistent with other measurements.

Development of ultra-pure NaI(Tl) detectors for the COSINE-200 experiment

“…[54], 1-month exposure time near sea level can produce 0.004 mBq/kg of 3 H and 0.05 mBq/kg of 22 Na. These activities are an order of magnitude lower than measured activities in COSINE-100 C6, which are 0.11 mBq/kg and 0.73 mBq/kg [54] of 3 H and 22 Na, respectively. Thus, we expect that the background contributions from these long-lived cosmogenic radioisotopes will be sufficiently low for the COSINE-200 experiment.…”

“…The cosmogenic production of radioactive isotopes in the NaI(Tl) crystal contributes a non-negligible amount of background in ROI of the COSINE-100 experiment [35,54]. This is mainly due to long-lived nuclides such as 3 H and 22 Na.…”

Annual modulations from secular variations: relaxing DAMA?

“…The upgraded DAMA/LIBRA detector confirmed the earlier result of DAMA/NaI, collecting more data and reaching a significance of about 13 σ (loosely speaking) for the cumulative exposure [1][2][3][4][5][6]. The ANAIS [7][8][9] and COSINE [10][11][12] experiments employ NaI crystals like DAMA, and are also looking for an annual modulation in the counting rate. So far they have not reported any modulation, but their sensitivity is not yet sufficient to probe the DAMA signal [12,13].…”

“…A decrease of the total background rate could arise from different radioactive contaminants present in NaI crystals at the beginning of the data-taking as short and medium term cosmogenic activations or intrinsic out-of-equilibrium contaminants [7,8,11,43,44]. The most critical ones are those with a half-life T 1/2 of a few tens of years, comparable to the total time of data-taking, and an energy spectrum in the RoI of the detector.…”

Cosmogenic Activation in Double Beta Decay Experiments

“…• Cosmogenic isotopes have been identified in NaI(Tl) detectors used in DAMA/LIBRA [221], COSINE [220,222] and ANAIS [223,224] experiments. Specific studies have been made to quantify production rates of several nuclides like some iodine and tellurium isotopes from data taken in ANAIS [225,226] and COSINE [227] and 3 H and 22 Na could have a relevant impact in the very low energy region. • Cosmogenic activation was very relevant in the scintillating bolometers made of CaWO 4 used in the CRESST-II experiment [228], as distinct gamma lines were observed from activation of W isotopes.…”