E.N. Galashov

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.

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.

Search for double beta decay processes in 106Cd with the help of 106CdWO4 crystal scintillator

A search for the double processes in 106Cd was carried out at the Gran Sasso National Laboratories of the INFN (Italy) with the help of a 106CdWO4 crystal scintillator (215 g) enriched in 106Cd up to 66%. After 6590 h of data taking, new improved half-life limits on the double beta decay processes in 106Cd were established at the level of 10^{19} - 10^{21} yr. A possible resonant enhancement of the neutrinoless2EC processes is estimated in the framework of the QRPA approach. The radioactive contamination of the 106CdWO4 crystal scintillator is reported.

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.

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.

Properties of CdWO4 and ZnWO4 scintillators at liquid nitrogen temperature

Two samples of 10 mm diameter and 10 mm height CdWO4 and ZnWO4 scintillators were studied at room and liquid nitrogen temperatures. At room temperature the crystals were coupled to a Photonis XP3212 photomultiplier. During experiments at low temperatures a Large Area Avalanche Photodiode was used as a photodetector. Measurements of light output, non-proportionality of the light yield and intrinsic resolution as a function of gamma-ray energies were performed at both temperatures. The non-proportionality for the two crystals reveals temperature dependence, showing a more proportional behavior at liquid nitrogen temperature. Intrinsic energy resolution values for both crystals also show temperature dependence.

Development of radiopure cadmium tungstate crystal scintillators from enriched 106Cd anD 116Cd to search for double beta decay

Cadmium tungstate crystal scintillators enriched in 106Cd up to 66% (106CdWO4) and in 116Cd up to 82% (116CdWO4) have been developed. The low radioactive contamination of the crystals measured on the level of ≤ 1.5 mBq/kg (40K), ≤ 0.005 - 0.012 mBq/kg (226Ra), 0.04 - 0.07 mBq/kg (228Th) allows to carry out high sensitivity experiments to search for double beta processes in 106Cd and 116Cd.

Searches for neutrinoless resonant double electron captures at LNGS

Several experiments were performed during last years at underground (3600 m w.e.) Laboratori Nazionali del Gran Sasso (LNGS) of the INFN, Italy; to search for resonant 2e0ν captures in 96Ru, 106Cd, 136Ce, 156Dy, 158Dy, 180W, 184Os, 190Pt with the help of HP Ge semiconductor detectors, and ZnWO4 and 106CdWO4 crystal scintillators. No evidence for r-2e0ν decays was found, and only T1/2 limits were established in the range of 1014 - 1021 yr.

Double β experiments with the help of scintillation and HPGe detectors at Gran Sasso

A search for double beta decay of 64,70Zn, 180,186W was carried out by using low background ZnWO4 crystal scintillators, while a CeCl3 scintillation detector was applied to investigate 2β processes in 136,138,142Ce. A search for 2β decay of 96,104Ru, 156,158Dy, 190,198Pt and study of 2ν2β decay of 100Mo to the first excited 0+ level of 100Ru were realized by ultra‐low background HPGe γ spectrometry. Moreover, CdWO4 crystal scintillators from enriched 106Cd and 116Cd isotopes were developed to search for 2β decay of 106Cd and 116Cd. Finally, experiments aimed to investigate 96,104Ru and 116Cd are in progress and a new phase of the experiment to search for 2β processes in 106Cd is in preparation.

First Results of the Experiment to Search for 2β Decay of 106Cd with the Help of 106CdWO4 Crystal Scintillators

An experiment to search for 2b processes in {sup 106}Cd with the help of {sup 106}CdWO{sub 4} crystal scintillator (mass of 215 g), enriched in {sup 106}Cd up to 66%, is in progress at the Gran Sasso National Laboratories of the INFN (Italy). After 1320 h of data taking, limits on double beta processes in {sup 106}Cd have been established on the level of 10{sup 19}-10{sup 20} yr, in particular (all the results at 90% C.L.): T{sub 1/2}(0{nu}2{epsilon})>3.6x10{sup 20} yr, T{sub 1/2}(2{nu}{epsilon}{beta}{sup +})>7.2x10{sup 19} yr, and T{sub 1/2}(2{nu}2{beta}{sup +})>2.5x10{sup 20} yr. Resonant 0{nu}2{epsilon} processes have been restricted as T{sub 1/2}(0{nu}2K)>1.4x10{sup 20} yr and T{sub 1/2}(0{nu}LK)>3.2x10{sup 20} yr. A possible resonant enhancement of the 0{nu}2{epsilon} processes is estimated in the framework of the QRPA approach.