CORDIS - EU research results

Low-background Underground Cryogenic Installation For Elusive Rates

Final Report Summary - LUCIFER (Low-background Underground Cryogenic Installation For Elusive Rates)

The LUCIFER project was conceived as a possible experiment aiming to the observation of the neutrinoless double beta decay (0ν2β). Such experiment, in case of success, would demonstrate the Majorana nature of the neutrino mass and would allow to set the neutrino mass scale. Even the non observation of the process would give a valuable information since it would set the best ever limit for the neutrino mass, having ruled out the entire, so-called, 'degenerate' mass region and having significantly penetrated the 'inverted hierarchy' one. At the same time the experiment would have demonstrated the winning technology for the exploration of the entire region. The keystones of the project were the capability of producing Zn82Se crystals with the required bolometric and scintillating quality and the design and realization of an experimental apparatus guaranteeing a background reduction to less than 10-3 c/kg/y, a factor 100 better than anything seen so far in the field.
The production of Zn82Se crystals is an important achievement of LUCIFER project, in spite of the many technological difficulties raised by such a complex task: the enrichment of natural selenium and its purification for crystal growth use, the synthesis of Zn82Se powder compatible with crystal growth and finally the growth of Zn82Se crystals and their mechanical processing. All production steps were supposed to be made with a special care for the material loss and possible recycling and performed in such conditions to avoid the radio-contamination of intermediary products and of final crystal samples used as scintillating bolometers. Fig. 1 gives two optically polished Zn82Se crystals ready to use in cryogenic experiments.

Fig. 1 Two final shaped and optically polished Zn82Se crystals completely machined at LNGS (see attachment Stand alone description of the project and its outcomes)

The cryogenic measurement made on three ZnSe (non-enriched) and three Zn82Se crystals of routine production proved that the production technology was correct and the mechanical/holder configuration used can be applied also for the final LUCIFER detector experiment. In spite of the improper operation conditions (21 mK, instead of nominal 10 mK) the crystals showed good performances with a mean energy resolution of the order of 30 keV FWHM for all the measured channels. The light/heat ratio for alpha and beta particle interaction has shown the expected discrimination capability indicating that the background rejection power capability will be reached. The cryogenic test also showed a good intrinsic background component for the Zn82Se crystals and as general conclusion, it could be considered a true preliminary demonstrator for a large scale NDBD experiment based on Zn82Se crystals.
Related to the larger scale cryogenic test to be made using all the crystals produced in the frame of LUCIFER project, the experimental apparatus is currently under construction based on CUORE-0 cryostat (decommissioned at the end of November 2015). In view of this operation, a general maintenance and refurbishing process was made for the cryostat and its dedicated experimental area in the Hall A at LNGS.

Fig. 2 Schematic view of the single module and of the whole tower for the Lucifer experiment (see attachment Stand alone description of the project and its outcomes)

Though the main goal of this experiment will be the complete operation of Zn82Se scintillating bolometers tower of Lucifer experiment (figure 2), the research activities will include:
- Installation and measurement of the ZnSe tower of scintillating bolometers (LUCIFER legacy)
- Possible implementation of a second scintillating bolometer tower based on XMoO4 crystals
- Development on new type of thermal sensors based on KIDs technology (CALDER ERC project)
- Optimization of light sensors based on Neganov Luke effect
- Test of scintillating films for active rejection of surface background events