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Cryogenic Rare-event Observatory with Surface Sensitivity

Periodic Reporting for period 2 - CROSS (Cryogenic Rare-event Observatory with Surface Sensitivity)

Reporting period: 2019-07-01 to 2020-12-31

Atoms are made up of charged fermions: electrons and quarks. The Dirac equation represents the most advanced description of these particles, predicting that they are accompanied by opposite-charge anti-matter partners. Neutrinos are outliers in this context: they are fermions too, but they are neutral. In the standard description, they are Dirac particles like electrons, with the role of the electric charge replaced by the so-called lepton number. There is however a fascinating alternative: neutrinos could be another type of fermions, i.e. Majorana particles, which coincide with their antimatter partners. The identification of the Majorana nature of neutrinos is one of the most important and impacting programs in particle physics and cosmolgy.
The only viable tool to ascertain that neutrinos are Majorana particles is the detection of neutrinoless double beta decay (0nu-DBD), a hypothetical nuclear process consisting in the transformation of an even-even nucleus into a lighter isobar containing two more protons and accompanied by the emission of two electrons and no other particles. Present lower bounds on the 0nu-DBD half-lifes are in the range 10^24 - 10^26 yr. None of the current experiments has the potential to reach sensitivities as high as 10^27 - 10^28 yr, for which the discovery potential would be extremely high. The CROSS technology can determine a dramatic leap forward in 0nu-DBD sensitivity, reaching that unexplored region.

The methods developed in CROSS may determine significant advancements in the techniques for the detection of nuclear radiation and in the identification of small amounts of radioactive contaminants. More specifically, detectors adopting the CROSS technology can enable the detection of very low neutron fluxes with unrivaled spectroscopic capabilities, with a remarkable impact on techniques for environmental monitoring, for the control of non-proliferation of nuclear weapons and for homeland security.

CROSS will be based on arrays of bolometers whose active medium contains two promising nuclei that can undergo 0nu-DBD: Mo-100 and Te-130, embedded in Li2MoO4 (lithium molybdate) and TeO2 (tellurium oxide) single crystals, respectively. Thanks to challenging innovations, CROSS will develop a technique capable of achieving zero background at an exposure scale of 1 tonne (isotope) x year. The main problem in the search for 0nu-DBD is indeed the control of the background, due to the long life-time of the process. CROSS is developing a technique to discriminate energy depositions close to the detector surface - currently the dominant background in bolometric experiments - from those occurring in the bulk, where the 0nu-DBD signal is expected. This discrimination capability is achieved via the core development of CROSS: the separation of surface from bulk events by pulse shape discrimination. This capability is obtained by depositing superconductive films on the crystal detector surfaces. The key concept of CROSS is tested by a large demonstrator dubbed CROSS-DEM. The program of CROSS is entangled with the bolometric CUPID project, one of the most promising next-generation experiments.
The work performed so far can be summarized in the following points:
- Surface sensitivity study. Discrimination of alpha-particle surface events was clearly demonstrated with small scale prototypes in about ten above-ground cryogenic runs, using Al superconductive films deposited both on Li2MoO4 and TeO2 crystals. However, in order to achieve sensitivities to beta surface events, we had to introduce an innovation with respect to the original CROSS program, and to replace pure Al films with Pd-Al bi-layers with a superconductive behavior due to proximity effects.
- Choice of the phonon sensor. One of the objectives of CROSS was to develop a bolometric readout based on sensors consisting of superconductive NbSi films that, thanks to their remarkable response speed (of the order or less than 1 ms), are sensitive to athermal phonons and suitable for pulse shape discrimination. However, we discovered that the other type of readout foreseen in CROSS, consisting of neutron transmutation doped (NTD) Ge thermistors and mainly sensitive to thermal phonons, is sufficient to perform efficient pulse shape discrimination of surface events. We decided then to keep only NTDs, which are much easier to manipulate.
- Li2MoO4 crystal program. We have successfully developed 32 Li2MoO4 crystals out of material enriched in Mo-100 and we have confirmed that these crystals perform well as bolometric detectors and have a very high radiopurity.
- TeO2 crystal program. This program was seriously endangered by the fact that the Chinese company (SICCAS) supposed to produce large TeO2 crystals has lost the capability to grow crystals of the requested size (cubes with a side of 50 mm), since they dismantled the setup used to produce these elements for the CUORE experiment. We found therefore a viable alternative, consisting of a collaboration with an American group (University of South Carolina) in possession of 8 kg of enriched tellurium and in contact with an American company that can produce crystals of the same size as SICCAS.
- CROSS cryostat and CROSS cryogenic facility in Canfranc underground laboratory. The CROSS underground cryostat, which will house CROSS-DEM, was installed and successfully tested in April 2019, one year in advance with respect to the CROSS schedule.
- Two relevant cryogenic runs with large-size crystals have been performed in the CROSS underground facility. These runs showed that the facility can test final-size (45 x 45 x45 mm) Li2MoO4 CROSS bolometers at the highest performance level. An agreement with the CUPID collaboration was established in order to use the facility in a joint CROSS/CUPID program, aiming at a definition of the CUPID detector configuration, which could adopt Al coating developed in CROSS.
- The development of a dedicated electronics and DAQ system for CROSS-DEM is in progress. In particular, we have successfully tested new custom DAQ boards.
The current results of bolometric experiments (in particular, CUORE) show that the dominant background source consists of energy-degraded alpha particles emitted by surface contamination. The “classical” solution to this problem, proposed for the next-generation bolometric experiment CUPID, is to develop luminescent bolometers, rejecting alpha events by the simultaneous detection of heat and light and exploiting the different light yield of alpha events with respect to beta-events. CROSS proposes a revolutionary approach that will make detectors sensitive to events occurring at their surface, rejecting not only surface alpha- but also surface beta-events, which otherwise would become the dominant background source in ton-scale experiments.
The expected results until the end of the project are:
- establishment of a protocol to achieve sensitivity to alpha and beta surface events in large mass enriched Li2MoO4 and TeO2 bolometers using thin superconductive film coating.
- start up of an experiment (CROSS-DEM) to demonstrate the effectiveness of the above mentioned protocol with a view to future searches; this experiment will be a very sensitive 0nu-DBD search by iself, competing with many of the most advanced projects.
- major contributions to the CUPID project.
The CROSS facility in Canfranc
Photograph of the set-up used for the CROSS-prototype tests.
Identification of surface alpha events in a prototype with Al film
Qualitative scheme of the mechanism of surface-event identification in CROSS.