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Register a new userImprovement of radiopurity level of enriched $^{116}$CdWO$_4$ and ZnWO$_4$ crystal scintillators by recrystallization
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As low as possible radioactive contamination of a detector plays a crucial role to improve sensitivity of a double beta decay experiment. The radioactive contamination of a sample of $^{116}$CdWO$_4$ crystal scintillator by thorium was reduced by a factor $approx 10$, down to the level 0.01 mBq/kg ($^{228}$Th), by exploiting the recrystallization procedure. The total alpha activity of uranium and thorium daughters was reduced by a factor $approx 3$, down to 1.6 mBq/kg. No change in the specific activity (the total $alpha$ activity and $^{228}$Th) was observed in a sample of ZnWO$_4$ crystal produced by recrystallization after removing $approx 0.4$ mm surface layer of the crystal.
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Double beta decay of $^{116}$Cd has been investigated with the help of radiopure enriched $^{116}$CdWO$_4$ crystal scintillators in the experiment Aurora. The half-life of $^{116}$Cd relatively to the 2$ u$2$beta$ decay of $^{116}$Cd to the ground level of $^{116}$Sn is measured with the highest up-to-date accuracy as $T_{1/2}$ = [2.69 $pm$ 0.02 (stat.) $pm$ 0.14 (syst.)] $times$ 10$^{19}$ yr. A new improved limit on the 0$ u$2$beta$ decay of $^{116}$Cd to the ground state of $^{116}$Sn is set as $T_{1/2}geq 2.4 times 10^{23}$ yr at 90% C.L., that corresponds to the effective Majorana neutrino mass limit in the range $langle$$m_ u$$rangle$ $le$ $(1.1-1.6)$ eV, depending on the nuclear matrix elements used in the estimations. New improved limits on other $2beta$ processes in $^{116}$Cd (decays with majoron emission, transitions to excited levels of $^{116}$Sn) were set at the level of $T_{1/2}geq 10^{21}-10^{22}$ yr.
The double-beta decay of $^{116}$Cd has been investigated with the help of radiopure enriched $^{116}$CdWO$_4$ crystal scintillators (mass of 1.162 kg) at the Gran Sasso underground laboratory. The half-life of $^{116}$Cd relatively to the $2 u2beta$ decay to the ground state of $^{116}$Sn was measured with the highest up-to-date accuracy as $T_{1/2}=(2.63^{+0.11}_{-0.12})times10^{19}$ yr. A new improved limit on the 0$ u$2$beta$ decay of $^{116}$Cd to the ground state of $^{116}$Sn was set as $T_{1/2}geq 2.2 times 10^{23}$ yr at 90% C.L., which is the most stringent known restriction for this isotope. It corresponds to the effective Majorana neutrino mass limit in the range $langle m_ uranglele(1.0-1.7)$ eV, depending on the nuclear matrix elements used in the estimations. New improved half-life limits for the 0$ u$2$beta$ decay with majoron(s) emission, Lorentz-violating $2 u2beta$ decay and $2beta$ transitions to excited states of $^{116}$Sn were set at the level of $T_{1/2}geq 10^{20}-10^{22}$ yr. New limits for the hypothetical lepton-number violating parameters (right-handed currents admixtures in weak interaction, the effective majoron-neutrino coupling constants, R-parity violating parameter, Lorentz-violating parameter, heavy neutrino mass) were set.
The ZnWO$_4$ is an anisotropic crystal scintillator; for its peculiar characteristics, it is a very promising detector to exploit the so-called directionality approach in the investigation of those Dark Matter (DM) candidates inducing nuclear recoils. Recently, in the framework of the ADAMO project, an R&D to develop high quality and ultra-radiopure ZnWO$_4$ crystal scintillators has been carried out. In the present paper the measurements to study the anisotropic response of a ZnWO$_4$ to $alpha$ particles and to nuclear recoils induced by neutron scattering are reported. Monochromatic neutrons have been produced by a neutron generator at ENEA-CASACCIA. The quenching factor values for nuclear recoils along different crystallographic axes have been determined for three different nuclear recoils energies. These results open the possibility to realize a pioneer experiment to investigate the above mentioned DM candidates by means of the directionality.
Studies on double beta decay processes in $^{106}$Cd were performed by using a cadmium tungstate scintillator enriched in $^{106}$Cd at 66% ($^{106}$CdWO$_4$) with two CdWO$_4$ scintillation counters (with natural Cd composition). No effect was observed in the data accumulated over 26033 h. New improved half-life limits were set on the different channels and modes of the $^{106}$Cd double beta decay at level of $lim T_{1/2}sim 10^{20}-10^{22}$ yr. The limit for the two neutrino electron capture with positron emission in $^{106}$Cd to the ground state of $^{106}$Pd, $T^{2 umathrm{EC}beta^+}_{1/2}geq2.1times 10^{21}$ yr, was set by the analysis of the $^{106}$CdWO$_4$ data in coincidence with the energy release 511 keV in both CdWO$_4$ counters. The sensitivity approaches the theoretical predictions for the decay half-life that are in the range $T_{1/2}sim10^{21}-10^{22}$ yr. The resonant neutrinoless double-electron capture to the 2718 keV excited state of $^{106}$Pd is restricted at the level of $T^{0 umathrm{2K}}_{1/2}geq2.9times10^{21}$ yr
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 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 $^{116}$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.
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