V.I. Tretyak

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.

Performance of ZnMoO4 crystal as cryogenic scintillating bolometer to search for double beta decay of molybdenum

Zinc molybdate (ZnMoO4) single crystals were grown for the first time by the Czochralski method and their luminescence was measured under X-ray excitation in the temperature range 85–400 K. Properties of ZnMoO4 crystal as cryogenic low temperature scintillator were checked for the first time. Radioactive contamination of the ZnMoO4 crystal was estimated as ≤ 0.3 mBq/kg (228Th) and 8 mBq/kg (226Ra). Thanks to the simultaneous measurement of the scintillation light and the phonon signal, the α particles can be discriminated from the γ/β interactions, making this compound extremely promising for the search of neutrinoless Double Beta Decay of 100Mo. We also report on the ability to discriminate the α-induced background without the light measurement, thanks to a different shape of the thermal signal that characterizes γ/β and α particle interactions.

Performances of a CeF3 crystal scintillator and its application to the search for rare processes

The performances of a CeF3 crystal scintillator with a mass of 49.3 g have been investigated. In particular, the alpha/beta light ratio and the possibility of a pulse-shape discrimination between alpha particles and gamma quanta have been studied. The radioactive contamination of the crystal has been investigated. The application of the obtained results in the search for rare processes such as the two neutrino double electron capture in Ce-136 and Ce-138 and the alpha activity in Ce-142 has been investigated. As a result, new T-1/2 limits for these processes have been deter-mined (90% C.L.): T-1/2(2v2) (K)(Ce-136)greater than or equal to 2 7 x 10(16) yr, T-1/2(2v2) (K)(Ce-138)greater than or equal to 3.7 x 10(16) yr and T-1/2(alpha) (Ce-142)greater than or equal to 2.9 x 10(18) yr

New limits on spin-dependent coupled WIMPs and on 2 beta processes in Ca-40 and Ca-46 by using low radioactive CaF2(Eu) crystal scintillators

The development of highly radio pure CaF2(Eu) crystal scintillators has been performed aiming at a substantial sensitivity enhancement of the 2 beta decay investigation and of the search for dark matter particles with spin-dependent (SD) interaction. The results of CaF2(Eu) background measurements and simulation are presented. New and highly improved T-1/2 limits on the 2 beta decay of Ca-46 and the double electron capture of Ca-40 are obtained as well as further results on SD coupled WIMPs

New experimental limit on the electron stability and non-paulian transitions in Iodine atoms

A new limit on the mean life of the electron in the disappearance approach has been established: tau(e) > 4.2(2.4).10(24) yr at 68% (90%) C.L., by using the similar or equal to 100 kg DAMA Nal(Tl) set-up at the Gran Sasso National Laboratory of INFN

Search for the nucleon and di-nucleon decay into invisible channels

Data collected deep underground at the Gran Sasso National Laboratory of INFN by using the similar or equal to 6.5 kg DAMA liquid Xenon scintillator have been analysed to search for nucleon and di-nucleon decays into invisible channels (decay to nu (i) )over bar>(i) or to nu (i) )over bar>(i)nu (i) or to 5 nu (i), etc., with i = e, mu, tau) or disappearance. The considered statistics has been 2257.7 kg x day. The new obtained limits are: tau (p) = 1.9 x 10(24) yr, tau (nn), = 1.2 x 10(25) yr and tau (pp) = 5.5 X 10(23) yr at 90% C.L. The latter values are the same or better than those previously established by the Frejus Collaboration; in particular, the limits for the NN decay into nu (tau) )over bar>(tau) are set for the first time

Radioactive contamination of ZnWO4 crystal scintillators

Abstract The radioactive contamination of ZnWO 4 crystal scintillators has been measured deep underground at the Gran Sasso National Laboratory (LNGS) of the INFN in Italy with a total exposure 3197xa0kgxa0h. Monte Carlo simulation, time–amplitude and pulse–shape analyses of the data have been applied to estimate the radioactive contamination of the ZnWO 4 samples. One of the ZnWO 4 crystals has also been tested by ultra-low background γ spectrometry. The radioactive contaminations of the best ZnWO 4 samples are estimated to be less than 0.002xa0mBq/kg ( 228 Th and 226 Ra), the total α activity is 0.18xa0mBq/kg. The β active 65 Zn and the α active 180 W have been detected in ZnWO 4 crystals. The effect of the re-crystallization on the radiopurity of the ZnWO 4 crystal has been studied. The radioactive contamination of the ceramic components of the set-ups used in the crystals growth has been checked by low background γ spectrometry. Some ideas for future improvement of the radiopurity level of ZnWO 4 crystal scintillators are briefly 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.

Further study of CdWO4 crystal scintillators as detectors for high sensitivity 2β experiments: Scintillation properties and pulse-shape discrimination

Abstract Energy resolution, non-proportionality in the scintillation response, α / β ratio, pulse shape for γ rays and α particles were studied with CdWO 4 crystal scintillators. Some indication for a difference in the emission spectra for γ rays and α particles was observed. No dependence of CdWO 4 pulse shape on emission spectrum wavelengths under laser, α particles and γ ray excitation was observed. Dependence of scintillation pulse shape for γ quanta and α particles and pulse-shape discrimination ability on temperature was measured in the range of 0–24xa0°C.