Luminescent Underground Molybdenum Investigation for NEUtrino mass and nature – LUMINEU

Bolometers are amongst the most interesting nuclear detectors for the conduction of next-generation Double Beta Decay searches. A bolometer consists of an energy absorber linked to a temperature sensor. The signal, collected at very low temperatures (typically <20 mK for large bolometers), consists of a thermal pulse registered by the sensor, which can be either a small semiconductor crystal, a thin superconductive film or a metallic magnetic thermometer. The majority of the most promising high Q-value candidates can be studied with the bolometric technique in the so-called “source=detector” approach, joining high energy resolution and large efficiency. High Q-values are preferred, as they lead to larger phase space and to lower background. The highest relevant gamma line in natural radioactivity is the 2615 keV line of 208Tl in the 232Th decay chain. Therefore, a detector based on a Double Beta emitter with a Q-value above 2615 keV represents an optimal starting point for a future experiment. However, the energy region above 2615 keV is dominated by alpha particles. Hence derives the power of scintillating bolometers, the technology proposed for LUMINEU. Since the alpha light yield is generally appreciably different from the beta/gamma light yield at equal deposited energy, while the bolometric thermal response is substantially equivalent, the simultaneous detection of scintillation (light channel) and thermal signals (heat channel) can reject fully alpha background. The isotope proposed for LUMINEU is Mo-100, with a Q-value of 3034 keV. The scintillating crystals to be operated as bolometers are based on the compound ZnMoO4.

During the preparation of this report (May 2015), we have achieved several results in line with the program of the project and definitely showing that the LUMINEU technology is suitable for a large-scale DBD experiment.
Crystals. The crystallization procedure was further optimized (e.g. we understood, with the support of a dedicated bolometric test, that W doping at 0.5% level helps crystallization). Further R&D was performed at NIIC to reduce the irrecoverable losses. One large enriched boule (1.4 kg) was produced (April 2015) and is now ready to be cut into the elements for the final LUMINEU single modules.
Sensors. Ten Ge wafers were irradiated at the Orphée reactor for the NTD sensor production specific for LUMINEU. Preliminary tests were performed. Due to high resistivity, an additional fine-tuning irradiation will be necessary. Several NbSi film superconductive thermistors have been prepared and characterized, but we failed to integrate them in a working light detector up to now.
Light detectors. We made systematic studies on different methods to hold the wafers at CSNSM. We analyzed also the effect of the thermistor size. This activity has allowed us to produce at least 10 NTD-equipped detectors adequate for the final LUMINEU single module. A fully integrated MMC-based light detector was delivered by KIP Heidelberg and was tested with direct X-ray irradiation and scintillation light from a small ZnMoO4 crystal, confirming a very fast detector rise time (20 µs).
Prototypes. Two large natural crystals (340 g) operated in LSM have reached the requested performance for a next-generation experiment: energy resolution better than 7 keV FWHM; alpha rejection power better than 99.9%; internal radiopurity better than 4 microBq/kg in Th and U chains.

The present perspectives of the LUMINEU program are the following:
- The large enriched crystal boule will be cut to produce at least 2 elements with m~400 g to be tested underground (Gran Sasso or Modane) for a long background test. This experiment is scheduled before summer and will provide the final single module foreseen in the LUMINEU programme.
- A MoU with the EDELWEISS collaboration is in force (March 2015), in order to regulate the installation and the operation of the LUMINEU detectors in the EDELWEISS setup and to fix the features of the ZNMOO4 pilot experiment that will be performed in it.
- A Monte Carlo simulation of the background expected for the ZNMOO4 detectors in the EDELWEISS cryostat was completed. A background model is under development based on this tool.
- The production of temperature sensors for LUMINEU will continue in the next months. First usable NTDs will be ready before summer, but they will be probably too resistive to be used in LUMIENU. A further irradiation to decrease resistivity is in programme. As for the light detectors based on NbSi films, the first operational prototypes are expected in few months.
- For the MMC sensors, tests on fully integrated prototypes will go on.
- An expansion of LUMINEU with an enlarged collaboration (to ITEP Moscow and INFN Rome and Gran Sasso) is under study. It will be based on a MoU, already signed, involving ITEP, IN2P3 and INFN, making available 10 kg of enriched material.

1. M. Mancuso et al., An Aboveground Pulse-Tube-Based Bolometric Test Facility for the Validation of the LUMINEU ZnMoO4 Crystals – J. Low Temp. Phys. 176(2014)571-577.
2. M. Loidl et al., Concept of Metallic Magnetic Calorimeters for Rare Event Search in the LUMINEU Project – J. Low Temp. Phys. 176(2014)624-630.
3. L. Bergé et al., Purification of molybdenum, growth and characterization of medium volume ZnMoO4 crystals for the LUMINEU program – JINST 09 (2014) P06004
4. A.S.Barabash et al.,Enriched Zn100MoO4 scintillating bolometers to search for 0?2ß decay of 100Mo with the LUMINEU experiment - Eur. Phys. J. C 74(2014)3133
arXiv:1405.6937 [physics.ins-det]
5. E. Armengaud et al., Development and underground test of radiopure ZnMoO4 scintillating bolometers for the LUMINEU 0?2ß project - JINST 10 (2015) P05007
6. M.Tenconi (for the LUMINEU collaboration), LUMINEU: a pilot scintillating bolometer experiment for neutrinoless double beta decay search - Physics Procedia 61(2015)782-786.
7. V.Ya.Degoda et al., X-ray luminescence of ZnMoO4 crystals developed in the framework of the LUMINEU program - Proc. Int. Conf. on Oxide Materials for Electronic Engineering OMEE’2014, 26-30.05.2014, Lviv, Ukraine, p. 265-266.
8. V.Ya.Degoda et al., Luminescence of ZnMoO4 crystals developed for the LUMINEU double beta decay experiment - Solid State Phenomena 230(2015)184-192.

Neutrinoless Double Beta Decay (0v-DBD) is in a central position for the study of elementary particles and fundamental interactions. It has strong implications on topics of cosmological character as well. After the discovery of neutrino flavor oscillations, crucial issues remain open, such as the absolute neutrino mass scale and the mass hierarchy, together with the quest of the neutrino nature: Dirac or Majorana fermion? The purpose of this project, named LUMINEU, is to set the bases for the realization of a next-generation 0v-DBD experiment with unprecedented sensitivity. To succeed, LUMINEU proposes to reduce the residual background due to alpha particles owing to the simultaneous measurement of the light and the heat generated in a nuclear event. This double read-out approach will allow rejecting interactions due to alpha particles with efficiency close to 1. If the scintillating bolometers contain a candidate to 0v-DBD with transition energy higher than the natural gamma radioactivity end-point (2.6 MeV), the rejection of alpha particles enables a virtually zero-background experiment for the exposures required to scrutinize the inverted hierarchy region of the neutrino mass pattern.
LUMINEU envisages the study of large ZnMoO4 scintillating crystals, containing the excellent candidate 100Mo, which is featured by a Q-value around 3 MeV. The ZnMoO4 crystals will be grown with an advanced technique which warrants excellent crystal quality, extreme purity and negligible waste of the starting material. A mass of 400 g for the single module is foreseen.
The rejection of alpha particles is correlated to the quality of the light measurement. A special effort will be dedicated to the optimization of the scintillation light detectors in terms of energy threshold, size, reproducibility and time response. Reducing this time would be the key in case of ultimate background due to the pile-up of standard DBD with neutrino, particularly relevant in the case of 100Mo. As a consequence, beside the production of standard semiconductor sensors (NTD) by nuclear transmutation, new sensors will be studied: high impedance superconductive films and metallic paramagnetic thermometers. The coupling of such sensors to massif crystals without loss in their nominal performances would be a strong innovation.
This result would have an impact on astroparticle physics beyond the 0v-DBD. This may lead to important advancements in dark matter direct detection in EDELWEISS-3 and EURECA. The progress in thermal sensors and their coupling, will lead to the improvement of the energy resolution and threshold of the heat channel and to a better sensitivity to WIMPs in particular at low masses. LUMINEU will take advantage of contributions from both communities. In the same time, both field of research will take benefit of LUMINEU’s developments.
The results on the scintillating crystals and on the light detectors will enable a 0v-DBD pilot experiment performed in an underground environment and containing a considerable amount of enriched molybdenum (about 1 kg). After the conclusion of LUMINEU, the realization of a large-scale experiment looking at the 20 meV region for the effective neutrino mass will be just a matter of political will and fund availability.