The CROSS Experiment: Rejecting Surface Events by PSD Induced by Superconducting Films

Abstract

Neutrinoless double-beta ($0\nu \beta \beta$) decay is a hypothetical rare nuclear transition ($T_{1/2}>10^{25}$–$10^{26}$ year). Its observation would provide an important insight into the nature of neutrinos (Dirac or Majorana particle) demonstrating that the lepton number is not conserved. This decay can be investigated with bolometers embedding the double-beta decay isotope ($^{76}\mathrm{Ge}$, $^{82}\mathrm{Se}$, $^{100}\mathrm{Mo}$, $^{116}\mathrm{Cd}$, $^{130}\mathrm{Te}$...), which perform as low-temperature calorimeters (few tens of mK) detecting particle interactions via a small temperature rise read out by a dedicated thermometer. Cryogenic Rare-event Observatory with Surface Sensitivity (CROSS) aims at the development of bolometric detectors (based on $\hbox {Li}_{{2}}\hbox {MoO}_{{4}}$ and $\hbox {TeO}_{{2}}$ crystals) capable of discriminating surface $\alpha$ and $\beta$ interactions by exploiting superconducting properties of Al film deposited on the detector surface. We report in this paper the results of tests on prototypes performed at CSNSM (Orsay, France) that showed the capability of a-few-$\upmu \mathrm{m}$-thick superconducting Al film deposited on crystal surface to discriminate surface $\alpha$ from bulk events, thus providing the detector with the required pulse shape discrimination capability. The CROSS technology would further improve the background suppression and simplify the detector construction (no auxiliary light detector is needed to reject alpha surface events) with a view to future competitive double-beta decay searches.