# Development of Cryogenic Detectors for Neutrinoless Double Beta Decay Searches with CUORE and CUPID
## 3 The CUORE Development  
The CUORE experiment history, starting from its first prototype MiBETA, is an excellent summary of the efforts needed to optimize the cryogenic calorimeter approach to the search for \(0\
u\beta\beta\)[25]. Initially, the main difficulty to be addressed was increasing the reliability of these detectors in terms of both operation time and number of simultaneously operated channels. To minimize the efforts needed, \({}^{130}\)Te was chosen as the isotope under study, given its high natural isotopic abundance (34.2% [26]). The MiBETA R&D allowed to operate initially a single crystal of TeO\({}_{2}\) for one year [27], proving the feasibility of this technique. After this initial proof of concept, a more complex array was designed and put to test, combining 20 crystals with different dimensions and mass of about 340 g [28]. These steps paved the way to the MiBETA experiment [29], which solidified the previous results and promoted the possibility to operate a large array of cryogenic detectors. At this stage, background became an issue to be dealt with in the experimental optimization. The \(2\
u\beta\beta\)-induced background was strongly limited by the good energy resolution of these detectors, while the spurious background had to be addressed. To reduce cosmogenic background, these experiments were placed underground in the Gran Sasso National Laboratories (LNGS), an infrastructure providing a 3600 m.w.e. shielding against cosmic rays [30; 31; 32; 33]. From the experimental results, the main spurious background contribution was identified in the degraded alphas. The signature of these events is a continuous spectrum below 9 MeV, unavoidable in experiments sensitive only to the energy deposition. In order to reduce this contribution as well as the general level of radioactivity, the materials building the detector and those directly facing it were selected trough strict screening procedures. The goal was to identify the least contaminated materials, which were then chosen to build the future experiments. In particular, oxygen-free copper (OFC) was selected for the detector holder, and polytetrafluoroethylene (PTFE) was used to hold the crystals in place. To directly address the issue of degraded alphas, the surface of all materials was treated to remove the external layers, which are responsible for the majority of these events. Additionally, the detector shields have been optimized and chosen as a dual copper/lead layer. To reduce the bremsstrahlung emission from \(\beta\) rays, characteristics of standard lead shieldings, an ancient roman lead shield was built, characterized by low \({}^{210}\)Pb content [34]. Once these improvements were defined, the first step toward the final CUORE design was the CUORICINO array [35], simultaneously operating 62 crystals with different geometries. The results obtained with this experiment [36; 37] allowed to select a final shape and configuration for the detector. The chosen design was based on \(5\times 5\)\(\times\) 5 cm\({}^{3}\) cubical TeO\({}_{2}\) crystals arranged in towers composed of 13 planes combining four crystals each. The design was finalized and operated as the CUORE-0 experiment [38], which served as a general test for the CUORE assembly line. The CUORE-0 detector is, in fact, one of the 19 towers now operating in the CUORE cryostat. Its operation allowed to test and finalize both the hardware and software tools needed for the ongoing CUORE operation and analysis [39; 40]. Parallel to the optimization of a multi-crystal array of cryogenic calorimeters, significant steps have been made towards the development of a cryostat. The target device had to keep \(\sim\)1 ton of crystals cooled around 10 mK, while keeping at a minimum the amount of radioactive contaminants in its building materials. The design process led to the realization of the CUORE cryostat, a one-of-its-kind machine built to satisfy these impressive goals [41].