# Development of Cryogenic Detectors for Neutrinoless Double Beta Decay Searches with CUORE and CUPID
## 4 Beyond the CUORE Sensitivity
### An Alternative to Cryogenic Calorimeters  
As mentioned above, the scintillation-based detectors are suitable candidates for the search of \(0\
u\beta\beta\). They can natively perform active background reduction and are easier to scale up than cryogenic calorimeters, since they can be operated at less extreme temperatures. In addition, these detectors are expected to allow versatility in the choice of isotope under study, good detection efficiencies, and possibility to include enriched materials. The limit to this technology is the energy resolution achievable by the light signal. This limit comes from the statistical fluctuation in the number of collected photons, limited by the light collection efficiency of the chosen scintillator-photodetector system. Such a limit can be overcome by defining a new design for performing scintillation detectors. Such a development is centered on the improvement in resolution, and has the target of \(\text{FWHM}\ \leq\ 0.02\cdot Q_{\beta\beta}\), to ensure at least a limited effect (\(\leq\)10%) of \(2\
u\beta\beta\) on the background. For the chosen design, suitable scintillators containing \(0\
u\beta\beta\) candidate isotopes have to be chosen. In such a context, suitable stands for the highest possible light yield and fast scintillation time. With such characteristics, the detector would not be limited by the crystal properties, allowing major improvements based on readout system upgrades. Such improvements will be related to the increase in light emission, optimization of photon extraction and optimization of readout chain. For the increase in light emission the operation at cryogenic temperatures provides an effective strategy. When the vibrational modes of a scintillating material are deactivated, in fact, the vibrational non-radiative de-excitation is damped. As a consequence, the light emission increases [90]. On photon extraction, different wrapping strategies can be implemented, surrounding the crystal with materials capable of redirecting the outgoing scintillation photons toward the photodetector. Lastly, the choice of an efficient photon detector is the most important aspect to be considered, since at this first measurement stage the actual level of the collection efficiency is determined. As a consequence, detectors with high quantum efficiency have to be preferred, in order to obtain better performances [91]. Additionally, consideration of gain fluctuations, non-uniform efficiency and readout noise have to be considered, since all of these aspects take part in spoiling the resolution. The FLARES (Flexible Light Apparatus for Rare Events Search) project tried to follow this concept. It proposed the development of a detector based on the use of Silicon Drift Detectors (SDD) to read the light emitted by large scintillation crystals, cooled at 120 K [92; 93]. SDDs are characterized by low noise and high quantum efficiency, while the scintillators containing \(0\
u\beta\beta\) candidates can be enhanced in their performances by the low temperatures. Considering a completely different approach, the resolution problem can be addressed by selecting more performing scintillating materials. In this framework, the scintillating nanocrystals play an important role. These new generation of compounds, in fact, is characterized by extremely high light emission, and they can be designed to work at a certain wavelength, thus making the optimization of their readout chain easier [94; 95]. In addition, the absence of non-radiative de-excitation channels makes the radiative recombination extremely quick, with scintillation emission time on the order of few ns. At the current status, different nanoscintillators are available, in terms of different structures and constituting elements. This wide range of choices makes them versatile, since it is possible to choose the needed materials with the needed emission spectrum to fit the demands of a certain application.