Study of the cosmogenic activation in NaI(Tl) crystals within the ANAIS experiment

Abstract

The direct detection of galactic dark matter particles requires ultra-low background conditions. NaI(Tl) crystals are applied in the search for these dark matter particles through their interactions in the detector by measuring the scintillation signal produced. The production of long-lived isotopes in materials due to the exposure to cosmic rays on Earth’s surface can be an hazard for these ultra-low background demanding experiments, typically performed underground. Therefore, production rates of cosmogenic isotopes in all the materials present in the experimental set-up, as well as the corresponding cosmic rays exposure history, must be both well-known in order to assess the relevance of this effect in the achievable sensitivity of a given experiment. Here, analysis of the cosmogenic studies developed from the ANAIS experiment NaI(Tl) detectors are presented. Installed inside a convenient shielding at the Canfranc Underground Laboratory just after finishing surface exposure to cosmic rays and thanks to the prompt data taking developed, identification and quantification of isotopes with half-lives of the order of tens of days were allowed, and thanks to the long-term operation of the detectors long-lived isotopes have been also identified and quantified. Main results for the activation yields of iodine and tellurium isotopes, [Formula: see text]Na, [Formula: see text]Sn, [Formula: see text]Cd, and tritium are presented in this work, together with the estimate of the production rates for their activation by cosmic nucleons while on Earth’s surface based on a selection of excitation functions over the entire energy range of cosmic nucleons.

Analysis of backgrounds for the ANAIS-112 dark matter experiment

“…The content of 232 Th and 238 U was determined from the measured alpha rate following Pulse Shape Analysis and the study of BiPo sequences, being the activity of the chain isotopes at the level of a few µBq/kg except for 210 Pb, which was found to be out of equilibrium with measured activity between (3.15±0.10) mBq/kg and (0.7±0.1) mBq/kg. Cosmogenic isotopes induced in the crystals when being on surface have also been carefully studied [10,11], including several short-lived Te and I isotopes which have already decayed and 109 Cd and 113 Sn, producing peaks at the binding energies of K-shell electrons (following Electron Capture); for these two isotopes, the saturation activity assumption firstly considered has been now relaxed according to the observed time evolution of the peaks. The activity of 22 Na was also quantified from the analysis of coincidences from high-energy gamma-rays, being the measured rate at 0.9 keV well reproduced by simulation using the independently quantified activity.…”

“…A procedure to quantify the initial activities of 22 Na in the rest of detectors analogue to that followed for D2 has been applied in different set-ups with three crystals and in ANAIS-112 with nine crystals, looking for coincidences with 1274.5 keV depositions in other detectors. In particular, data from August to November 2017 collected in ANAIS-112 have been considered [56]. Preliminary results from ANAIS-112 together with all the available previous estimates are summarized in table 6.…”

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

“…To understand these backgrounds, we first considered the list of cosmogenic radioactive isotopes that are produced in NaI(Tl) reported in Ref. [25,26,27,28]. In Table 1 (a), we list the contributing cosmogenic isotopes with their half lives; short-lived isotopes, for which half lives are less than a year, are 125 I, 121 Te, 121m Te, 123m Te, 125m Te, 127m Te, and 113 Sn and long-lived isotopes are 109 Cd, 22 Na, 3 H, and 129 I.…”

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

“…To consider these backgrounds, we first checked the list of cosmogenic radioactive isotopes that are produced in NaI(Tl), as reported in Ref. [24,25,26,27]. In Table 3 (a), we list the contributing cosmogenic isotopes with their half lives; short-lived isotopes, for which half lives are less than a year, are 125 I, 121 Te, 121m Te, 123m Te, 125m Te, 127m Te, and 113 Sn and long-lived isotopes are 109 Cd, 3 H, and 22 Na.…”