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تسجيل مستخدم جديدSearch for double beta decay of Zinc and Tungsten with the help of low-background ZnWO4 crystal scintillators
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Double beta processes in 64-Zn, 70-Zn, 180-W, and 186-W have been searched for with the help of large volume (0.1-0.7 kg) low background ZnWO4 crystal scintillators at the Gran Sasso National Laboratories of the INFN. Total time of measurements exceeds 10 thousands hours. New improved half-life limits on double electron capture and electron capture with positron emission in 64-Zn have been set, in particular (all the limits are at 90% C.L.): T1/2(0nu2EC)> 1.1e20 yr, T1/2(2nuECbeta+)>7.0e20 yr, and T1/2(0nuECbeta+)>4.3e20 yr. The different modes of double beta processes in 70-Zn, 180-W, and 186-W have been restricted at the level of 1e17-1e20 yr.
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A search for the double beta decay of zinc and tungsten isotopes has been performed with the help of radiopure ZnWO4 crystal scintillators (0.1-0.7 kg) at the Gran Sasso National Laboratories of the INFN. The total exposure of the low background meas
urements is 0.529 kg yr. New improved half-life limits on the double beta decay modes of 64Zn, 70Zn, 180W, and 186W have been established at the level of 10^{18}-10^{21} yr. In particular, limits on double electron capture and electron capture with positron emission in 64Zn have been set: T_{1/2}(2 u 2K) > 1.1 10^{19} yr, T_{1/2} (0 u 2epsilon) > 3.2 10^{20} yr, T_{1/2} (2 u epsilon beta^+) > 9.4 10^{20} yr, and T_{1/2} (0 u epsilon beta^+) > 8.5 10^{20} yr, all at 90% C.L. Resonant neutrinoless double electron capture in 180W has been restricted on the level of T_{1/2} (0 u 2epsilon) > 1.3 10^{18} yr. A new half-life limit on alpha transition of 183W to the metastable excited level 1/2^- 375 keV of 179Hf has been established: T_{1/2} > 6.7 10^{20} yr.
A search for the double beta 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 improve
d half-life limits on the double beta processes in 106Cd were established at the level of 10^{19}-10^{21} yr; in particular, T_{1/2}(2 u epsilon beta^+) > 2.1 10^{20} yr, T_{1/2}(2 u 2beta^+) > 4.3 10^{20} yr, and T_{1/2}(0 u 2epsilon) > 1.0 10^{21} yr. The resonant neutrinoless double electron captures to the 2718 keV, 2741 keV and 2748 keV excited states of 106Pd are restricted to T_{1/2}(0 u 2K) > 4.3 10^{20} yr, T_{1/2}(0 u KL1) > 9.5 10^{20} yr and T_{1/2}(0 u KL3) > 4.3 10^{20} yr, respectively (all limits at 90% C.L.). A possible resonant enhancement of the 0 u 2epsilon processes is estimated in the framework of the QRPA approach. The radioactive contamination of the 106CdWO4 crystal scintillator is reported.
A cadmium tungstate crystal boule enriched in $^{116}$Cd 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 Res
olution 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 $^{116}$CdWO$_4$ 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 $^{116}$CdWO$_4$ sample (326 g) was tested with the help of ultra-low background high purity germanium $gamma$ detector. Monte Carlo simulations of double $beta$ processes in $^{116}$Cd were used to estimate the sensitivity of an experiment to search for double $beta$ decay of $^{116}$Cd.
The radioactive contamination of ZnWO4 crystal scintillators has been measured deep underground at the Gran Sasso National Laboratory (LNGS) of the INFN in Italy with a total exposure 3197 kg x h. Monte Carlo simulation, time-amplitude and pulse-shap
e analyses of the data have been applied to estimate the radioactive contamination of the ZnWO4 samples. One of the ZnWO4 crystals has also been tested by ultra-low background gamma spectrometry. The radioactive contaminations of the ZnWO4 samples do not exceed 0.002 -- 0.8 mBq/kg (depending on the radionuclide), the total alpha activity is in the range: 0.2 - 2 mBq/kg. Particular radioactivity, beta active 65Zn and alpha active 180W, has been detected. The effect of the re-crystallization on the radiopurity of the ZnWO4 crystal has been studied. The radioactive contamination of samples of the ceramic details of the set-ups used in the crystals growth has been checked by low background gamma spectrometry. A project scheme on further improvement of the radiopurity level of the ZnWO4 crystal scintillators is briefly addressed.
PbWO4 crystal scintillators are discussed as an active shield and light-guides in 116Cd double beta decay experiment with CdWO4 scintillators. Scintillation properties and radioactive contamination of PbWO4 scintillators were investigated. Energy res
olution of CdWO4 detector, coupled to PbWO4 crystal as a light-guide, was tested. Efficiency of PbWO4-based active shield to suppress background from the internal contamination of PbWO4 crystals was calculated. Using of lead tungstate crystal scintillators as high efficiency 4-pi active shield could allow to build sensitive double beta experiment with 116CdWO4 crystal scintillators.
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