SEMINAR LMGP - 18.02.2020 - Dr Matias VELAZQUEZ

Dr Matias Velazquez
Laboratoire SIMaP UMR 5266 CNRS-Grenoble INP-UGA, Saint Martin d’Hères

Abstract :

A unique probe of the new physics beyond the Standard Model will be realized in the upcoming decade by several next-generation experiments, in particular CUPID, aiming at the detection of neutrinoless double-beta decay [1]. The basic option for CUPID is to exploit the infrastructure of the recently started CUORE experiment with a tonne-scale bolometer array. The key points addressed for a bolometric technique to be applied in CUPID are reproducible crystal growth and detector technologies satisfying the project requirements on the production, purity and performance. A technology of mass production of high quality, radiopure, natural and 6Li-enriched Li2MoO4 (LMO) scintillators is being developed in the CLYMENE R&D program. Crystals with varied diameters were grown in two different Czochralski configurations. The first configuration based on inductive heating (RF coil coupled with Pt crucible) was used to grow crystals of 3 to 4 cm in diameter. We investigated the detector performances and radiopurity of a cracked 158 g crystal and an uncracked 13.5 g crystal. In the latter, a good energy resolution (2–7 keV FWHM @ 0.2–5 MeV), one of the highest light yield (0.97 keV/MeV) amongst LMO scintillating bolometers, an efficient alpha particles discrimination (10��) and a potentially low internal radioactive contamination (below 0.2–0.3 mBq/kg of U/Th) were obtained. The fractured crystal led to a shorter response time and a second time constant of several hundreds of ms, and a decrease of both the FWHM resolution and the sensitivity by a factor of 17 and 70, respectively. Numerical computations of the temperature field in the furnace, melt convection and thermal stress in the crystal reveal 30% higher thermal stresses at the bottom part of the crystal in the case of a concave shape of the tail (experiment) as compared to the case of a convex shape. This could explain why the fracture started at the bottom part of the first LMO crystal boule, and highlights the importance of the crystal shape in the last stages of growth in view of a mass production. The furnace configuration used to grow 5 cm-diameter crystals was numerically optimized in order to lower the impact of thermal stresses, by means of a susceptor. The first large mass (1 kg) LMO crystals grown in this configuration will be discussed.