The research work I propose will be performed in the framework of the CUPID project, a next generation experiment aiming at the search of neutrinoless double beta decay of 100-Mo with a ton-scale array of Li2MoO4 crystals operated as scintillating bolometers. An appropriate choice of the materials is crucial to accomplish the CUPID background level goal of >10^-4 cts/keV/kg/yr. This is true for any other experiment searching for rare events.
The aim of this research is to characterize the background of two batches of four 45x45x45 mm Li2MoO4 crystals produced with newly synthesized molybdenum powders at different purity levels, never tested before, with the final outcome of choosing the most suitable for the realization of the CUPID detectors. A detailed identification and quantification of the radioactive contamination of these scintillating bolometers paves the way for any other future experiment willing to adopt molybdenum powders. Another important outcome of the research I will perform concerns the high sensitivity measurement of the surface contamination by exploiting the coincidences among different detectors, profiting from an experimental setup where the crystals will be arranged in a compact array of 4 crystals. The bolometers will be instrumented with light detectors, to take advantage from the different light signal produced by alpha and beta/gamma particles to perform a background rejection via particle identification.
The LUMINEU and CROSS projects and the CUPID demonstrator CUPID-Mo proved that, on the basis of their costs and performances, the Li2MoO4 crystals represent the most appropriate scintillating bolometers for the realization of the CUPID experiment. Nevertheless, the ~97% enriched molybdenum powders used for the crystal growth of such projects belong to a batch that was previously used in the NEMO-3 experiment [1]: their quantity was limited and are not available anymore. For the ~1600 CUPID crystals production, such powders had to be re-synthesized, and new measurements are necessary to determine the quality of the resulting crystals.
My research is intended to characterize, for the first time, new Li2MoO4 crystals produced with different molybdenum powders, with the aim of evaluating their suitability for the investigation of neutrinoless double beta decay. The appropriate choice of the materials for the detector realization is indeed a crucial ingredient to accomplish the zero-background requirement in the region of interest, and therefore to push new generation experiments like CUPID towards sensitivities never reached before. The outcome of this work aims at leading the decisions of CUPID concerning the crystals production. In this framework, a significant reduction of the foreseen budget would be realized in case the standard purity level powders resulted to match the low contamination requirements. More in general, a detailed characterization of the radioactivity background of these scintillating bolometers paves the way for any other future experiment willing to adopt molybdenum powders.
Besides the molybdenum powders characterization, a second innovative aspect of the research I will perform concerns the measurement of the surface contamination by exploiting the coincidences among different detectors. Events from surface contaminants are typically characterized by simultaneous energy releases in neighboring crystals, when these are exposed one to each other. CUPID-Mo recently produced an estimation of this contribution with a delayed coincidences study: this was the only possible approach to be adopted with that experimental setup, where the cylindrical crystals are arranged in 1-crystal-per-floor towers, each tower contained into a copper case. A more precise evaluation of the surface contamination is instead possible in the research I will perform. Each set of 4 cubic crystals will be arranged in a compact array structure: with this configuration it will be possible to benefit from the larger proximity among bolometers, that will be exposed one to each other, and an increased efficiency in the coincidences between neighboring detectors will be obtained. The contribution of surface contaminants will be then disentangled from the bulk one, as in the latter case single crystal events are expected, while in the first multiple crystals are usually involved. The sensitivity to surface contaminations will be further increased with the use of multiple light detectors exposed to the free crystals' sides, so that the coincidences between bolometric and light signals will be also exploited, with the additional feature of particle identification. The outcome of this study aims thus at establishing the CUPID crystals surface contribution, and including it in the CUPID background budget. To accomplish this goal, I will take advantage from my past experience in CUORE, where I contributed to the development of the background model with a specific study on the TeO2 crystals¿ bulk and surface alpha contamination. This research proposal is the result of such experience acquired during my PhD in this field, with the step-forward of applying it to scintillating bolometers.