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Register a new userFinal results of the Aurora experiment to study 2$beta$ decay of $^{116}$Cd with enriched $^{116}$CdWO$_4$ crystal scintillators
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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.
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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.
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.
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 radiopure cadmium tungstate crystal scintillator, enriched in $^{106}$Cd to 66%, with mass of 216 g ($^{106}$CdWO$_4$), was used to search for double beta decay processes in $^{106}$Cd in coincidence with four ultra-low background high purity germanium detectors in a single cryostat. New improved limits on the double beta processes in $^{106}$Cd have been set on the level of $10^{20}- 10^{21}$ yr after 13085 h of data taking. In particular, the half-life limit on the two neutrino electron capture with positron emission, $T_{1/2}^{2 uvarepsilonbeta^+}geq 1.1times 10^{21}$ yr, has reached the region of theoretical predictions. With this half-life limit the effective nuclear matrix element for the $2 uvarepsilonbeta^+$ decay is bounded as $M^{2 uvarepsilonbeta^+}_{eff}le 1.1$. The resonant neutrinoless double electron captures to the 2718 keV, 2741 keV and 2748 keV excited states of $^{106}$Pd are restricted at the level of $T_{1/2} geq (8.5times10^{20}-1.4times10^{21}$) yr.
The unique features (super-low background and large sensitive volume) of the CTF and BOREXINO set ups are used in the CAMEO project for a high sensitivity study of 100-Mo and 116-Cd neutrinoless double beta decay. Pilot measurements with 116-Cd and Monte Carlo simulations show that the sensitivity of the CAMEO experiment (in terms of the half-life limit for neutrinoless double beta decay) is (3-5) 10^24 yr with a 1 kg source of 100-Mo (116-Cd, 82-Se, and 150-Nd) and about 10^26 yr with 65 kg of enriched 116-CdWO_4 crystals placed in the liquid scintillator of the CTF. The last value corresponds to a limit on the neutrino mass of less than 0.06 eV. Similarly with 1000 kg of 116-CdWO_4 crystals located in the BOREXINO apparatus the neutrino mass limit can be pushed down to m_nu<0.02 eV.
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