No abstract

The LUMINEU program aims at performing a pilot experiment on neutrinoless double beta decay of 100 Mo using radiopure ZnMoO 4 crystals operated as cryogenic scintillating bolometers. Growth of high quality radiopure crystals is a complex task, since there are no commercial molybdenum compounds available with the required level of purity and radioactive contamination. This paper discusses approaches to purify molybdenum and synthesize compounds for high quality radiopure ZnMoO 4 crystal growth. A combination of a double sublimation (with addition of zinc molybdate) with subsequent recrystallization in aqueous solutions (using zinc molybdate as a collector) was used. Zinc molybdate crystals up to 1.5 kg were grown by the low-thermal-gradient Czochralski technique; their optical, luminescent, diamagnetic, thermal and bolometric properties were tested.

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The LUMINEU project is a demonstrator experiment that will search for the neutrinoless double beta decay of the isotope 100 Mo embedded in zinc molybdate (ZnMoO 4 ) scintillating bolometers. In this context, a zinc molybdate crystal boule enriched in 100 Mo to 99.5 % with a mass of 171 g was grown for the first time by the low-thermalgradient Czochralski technique. The production cycle provided a high yield (the crystal boule mass was 84 % of the initial charge) and an acceptable level-around 4 %-of irrecoverable losses of the costly enriched material. Two crystals of 59 and 63 g, obtained from the enriched boule, were tested above ground at millikelvin temperatures as scintillating bolometers. They showed a good detection performance, equivalent to that of previously developed natural ZnMoO 4 detectors. These results pave the way to future sensitive searches based on the LUMINEU technology, capable of approaching and exploring the inverted hierarchy region of the neutrino mass pattern.