Background identification in cryogenic calorimeters through $$\alpha -\alpha $$ delayed coincidences

Abstract

Localization and modeling of radioactive contaminations is a challenge that ultra-low background experiments are constantly facing. These are fundamental steps both to extract scientific results and to further reduce the background of the detectors. Here we present an innovative technique based on the analysis of $$\alpha -\alpha $$ α - α delayed coincidences in $${}^{232}$$ 232 Th and $${}^{238}$$ 238 U decay chains, developed to investigate the contaminations of the ZnSe crystals in the CUPID-0 experiment. This method allows to disentangle surface and bulk contaminations of the detectors relying on the different probability to tag delayed coincidences as function of the $$\alpha $$ α decay position.

Bolometric Double Beta Decay Experiments: Review and Prospects

“…CUPID-0 demonstrated the capability to reject surface events through α-α delayed coincidences [79], however, not at a level sufficient to reach background indices lower than 10 −4 counts/keV/kg/y. Special technologies must be developed for that.…”

Measurement of $^{216}$Po half-life with the CUPID-0 experiment

“…From previous analyses on the CUPID-0 background [15], we know that contamination are mainly localised in the bulk of the crystals and not on their surface. As a consequence, the whole decay sequence is most likely to be contained in a single crystal, with a Monte Carlo evaluated efficiency of 65.7% [16]. The expected number of random coincidences polluting the selected events can be calculated and it is expected to be small.…”

“…The expected number of random coincidences polluting the selected events can be calculated and it is expected to be small. The probability of random -coincidences in a 278 s window is < 4 ⋅ 10 −2 , given the low count rate of CUPID-0 (< 1.7 × 10 −4 Hz/crystal) [16]. The probability for a random pile-up in the same event (4 s window) simulating a 220 Rn-216 Po sequence is < 8 ⋅ 10 −3 given the ∼ 2 mHz counting rate for each crystal, considering bot and ∕ events.…”

“…In CUPID-0, a 0 experiment based on the scintillating bolometers technique [10], we combine -particle tagging and identification of delayed coincidences between subsequent decays to identify the different background sources contributing to the measured spectrum [15,16]. This iden-tification provided us with a high-purity sample of 224 Ra-220 Rn-216 Po cascade, belonging to the 232 Th chain.…”

Improving radioactive contaminant identification through the analysis of delayed coincidences with an $$\alpha $$-spectrometer

“…This technique consists in searching for couples of parent-daughter events associated with the decay of the same precursor isotope. By exploiting the information on time correlations, it is possible to suppress the background and to bring out the distinctive signatures of different radioisotopes, especially when the complementary information provided by the energy spectrum does not allow for a clear identification of the contaminants [9].…”