Ya.V. Vasiliev

A next-generation neutrinoless double beta decay experiment based on ZnMoO4 scintillating bolometers

Article history: The search for neutrinoless double β decay probes lepton number conservation with high sensitivity and investigates the neutrino nature and mass scale. Experiments presently in preparation will cover the quasi-degeneracy region of the neutrino mass pattern. Probing the inverted hierarchy region requires improved sensitivities and next-generation experiments, based either on large expansions of the present searches or on new ideas. We examine here a novel technology relying on ZnMoO4 scintillating bolometers, which can provide an experiment with background close to zero in the ton × year exposure scale. The promising performance of a pilot detector is presented, both in terms of energy resolution and background control. A preliminary study of the sensitivities of future experiments shows that the inverted hierarchy region is within the reach of the technique here proposed. A realistic phased approach program towards a next-generation search is presented and briefly discussed.

Variations of solid-liquid interface in the BGO low thermal gradients Cz growth for diffuse and specular crystal side surface

Abstract A global analysis of heat transfer in growing BGO (Bi4Ge3O12) crystals was carried out to explain the significant variation of the solid–liquid interface shape observed in practice during crystal growth. Special attention was given to the accurate treatment of radiative heat transfer in crystal and the gap between crystal and crucible wall. Crystal side surface was assumed to be transparent and diffuse or specular (Fresnel) reflective while melt was opaque. Spectral absorptivity of crystal was approximated by a three-band model. It is shown that rotationally driven vortex dominates under the solid–liquid interface during the whole growth process and no reconstruction of flow pattern, which could lead to interface inversion, takes place. As a result, in the case of diffuse crystal surfaces, the calculated deflection of the crystallization front does not exceed several millimeters and that completely disagrees with the observations. In contrast, for the specular crystal surface the solid–liquid interface turns out to be deeply convex toward the melt at the initial stage of the growth, and then, as the crystal is pulled, its convexity strongly diminishes. Such behavior is in good agreement with the experiment and mainly determined by specular reflection at the conical part of the crystal (its shoulder) while the effect of specular reflectivity of the cylindrical part and melt-free surface is significantly less. It is also shown that radiation transfer in the wavelength band of crystal opacity in conjunction with the specular reflection of radiation in the band of crystal transparency result in the appearance of “convex–concave” isotherms in the upper part of the crystal.

BGO crystals grown by a low thermal gradient Czochralski technique

Abstract A low thermal gradient version of the Czochralski technique was successfully adopted for growing BGO crystals. Now the technique allows us to grow water transparent, radiation hard crystals of up to 150 mm in diameter and up to 400 mm in length. Results of the application of these crystals in the endcap calorimeter of the CMD-2 detector which consists of 680 BGO crystals with a total weight of about 450 kg are discussed.

Purification of molybdenum, growth and characterization of medium volume ZnMoO4 crystals for the LUMINEU program

The LUMINEU program aims at performing a pilot experiment on neutrinoless double beta decay of 100Mo using radiopure ZnMoO4 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 ZnMoO4 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.

Low background detector with enriched 116CdWO4 crystal scintillators to search for double beta decay of 116Cd

A cadmium tungstate crystal boule enriched in 116Cd to 82% with mass of 1868 g was grown by the low-thermal-gradient Czochralski technique. The isotopic composition of cadmium and the trace contamination of the crystal were estimated by High Resolution Inductively Coupled Plasma Mass-Spectrometry. The crystal scintillators produced from the boule were subjected to characterization that included measurements of transmittance and energy resolution. A low background scintillation detector with two 116CdWO4 crystal scintillators (586 g and 589 g) was developed. The detector was running over 1727 h deep underground at the Gran Sasso National Laboratories of the INFN (Italy), which allowed to estimate the radioactive contamination of the enriched crystal scintillators. The radiopurity of a third 116CdWO4 sample (326 g) was tested with the help of ultra-low background high purity germanium γ detector. Monte Carlo simulations of double β processes in 116Cd were used to estimate the sensitivity of an experiment to search for double β decay of 116Cd.

The growth of ZnWO4 and CdWO4 single crystals from melt by the low thermal gradient Czochralski technique

The features of growth of single crystals of cadmium and zinc tungstates by the low thermal gradient Czochralski technique have been investigated. The effect of change in the growth conditions on the crystal morphology and the capture of macroinclusions is studied. Crystals 45 mm in diameter and up to 150 mm long (cylindrical part) are obtained. Their optical and scintillation characteristics are measured.

Development and underground test of radiopure ZnMoO4 scintillating bolometers for the LUMINEU 0 nu 2 beta project

The LUMINEU (Luminescent Underground Molybdenum Investigation for NEUtrino mass and nature) project envisages a high-sensitivity search for neutrinoless double beta (0 nu 2 beta) decay of Mo-100 with the help of scintillating bolometers based on zinc molybdate (ZnMoO4) crystals. One of the crucial points for the successful performance of this experiment is the development of a protocol for producing high quality large mass ZnMoO4 crystal scintillators with extremely high internal radiopurity. Here we report a significant progress in the development of large volume ZnMoO4 crystalline boules (with mass up to 1 kg) from deeply purified materials. We present and discuss the results achieved with two ZnMoO4 samples (with mass of about 0.3 kg each): one is a precursor of the LUMINEU project, while the other one was produced in the framework of LUMINEU with an improved purification / crystallization procedure. The two crystals were measured deep underground as scintillating bolometers in the EDELWEISS dilution refrigerator at the Laboratoire Souterrain de Modane (France) protected by a rock overburden corresponding to 4800 m w.e. The results indicate that both tested crystals are highly radiopure. However, the advanced LUMINEU sample shows a clear improvement with respect to the precursor, exhibiting only a trace internal contamination related with Po-210 at the level of 1 mBq/kg, while the activity of Ra-226 and Th-228 is below 0.005 mBq/kg. This demonstrates that the LUMINEU purification and crystal-growth procedures are very efficient and leads to radiopurity levels which exceedingly satisfy not only the LUMINEU goals but also the requirements of a next-generation 0 nu 2 beta experiment.

CdWO4 crystal scintillators from enriched isotopes for double beta decay experiments

Abstract Cadmium tungstate crystal scintillators enriched in 106 Cd and 116 Cd were developed. The produced scintillators exhibit good optical and scintillation properties, and a low level of radioactive contamination. Experiments to search for double beta decay of 106 Cd and 116 Cd are in progress at the Gran Sasso National Laboratories of the INFN (Italy). Prospects to further improve the radiopurity of the detectors by recrystallization are discussed.

Global analysis of heat transfer in growing BGO crystals (Bi4Ge3O12) by low-gradient Czochralski method

A numerical analysis of heat transfer in a set-up for growing BGO crystals is undertaken. Consideration is given to the correct treatment of radiative heat transport inside and outside the crystal. Effect of internal radiation is demonstrated. Conjugate heat transfer in crystal, melt and crucible was studied for different crystal lengths. Violation of the fully faceted growth at the initial stage of pulling is discussed.

Development of crystal scintillators for calorimetry in high energy and astroparticle physics

Here we describe application of the low-thermal gradient Chozchralski method to grow high quality radiopure cadmium tungstate crystal scintillators from enriched isotopes 106Cd and 116Cd, development of zinc molybdate from enriched 100Mo, highly radiopure zinc tungstate for double beta and dark matter experiments, production of bismuth germinate for frontier experiments in high energy physics.