Search

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$\nu$2$\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$\nu$2$\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$$m_\nu$$\rangle$ $\le$ $(1.1-1.6)$ eV, depending on the nuclear matrix elements used in the estimations. New improved limits on other $2\beta$ 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., Comment: 4 pages, 2 figures

An experiment to search for double beta decay in 106Cd is running in the DAMA/CRYS set-up at the Gran Sasso National Laboratory of the INFN (Italy). In this experiment a 106CdWO4 crystal scintillator (215 g) enriched in 106Cd to 66%, used in previous measurements in different configurations, is now in measurement in coincidence with two large volume low background CdWO4 crystal scintillators in close geometry. This further measurement will allow us to reach a sensitivity at the level of the most sensitive double β+ experiments (T1/2 ≃ 1020 −1022 yr) and to explore the region of the theoretical expectations for some decay modes.An experiment to search for double beta decay in 106Cd is running in the DAMA/CRYS set-up at the Gran Sasso National Laboratory of the INFN (Italy). In this experiment a 106CdWO4 crystal scintillator (215 g) enriched in 106Cd to 66%, used in previous measurements in different configurations, is now in measurement in coincidence with two large volume low background CdWO4 crystal scintillators in close geometry. This further measurement will allow us to reach a sensitivity at the level of the most sensitive double β+ experiments (T1/2 ≃ 1020 −1022 yr) and to explore the region of the theoretical expectations for some decay modes.

A radiopure cadmium tungstate crystal scintillator, enriched in 106 Cd to 66%, with mass of 216 g (106CdWO4) was used in coincidence with four ultra-low background HPGe detectors contained in a single cryostat to search for double beta (2β) decay processes in 106 Cd. New improved half-life limits on the 2β processes in 106 Cd have been set on the level of 1020 — 1021 yr after 13085 h of data taking deep underground (3600 m w.e.) at the Gran Sasso National Laboratories of INFN (Italy). In particular, the limit on the two neutrino electron capture with positron emission, Teβ+2 1/2≥ 1.1 × 1021 yr, has reached the region of theoretical predictions. The resonant neutrinoless double electron captures to the 2718, 2741 and 2748 keV excited states of 106 Pd are restricted on the level of T2e0v 1/2 ≥ (8.5 × 1020 - 1.4 × 1021) 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., Comment: 14 pages, 5 figures and 2 tables

Some recent results obtained in the search for double beta decay in vari- ous isotopes obtained in experiments carried out at LNGS with the DAMA/R&D, DAMA/Ge set-up and the STELLA facility will be summarized.

A radiopure cadmium tungstate crystal scintillator, enriched in 106Cd (106CdWO4), was used to search for double beta decay processes in 106Cd in coincidence with an ultra-low background set-up containing four high purity germanium (HPGe) detectors in a single cryostat. The experiment has been completed after 13085 h of data taking. New improved limits on most of the double beta processes in 106Cd have been set on the level of 1020−1021 yr. Tn particular, the half-life limit on the two neutrino electron capture with positron emission, T1/2 ≥ 1.8 × 1021 yr, reached the region of theoretical predictions.

An experiment to search for 2β processes in 116Cd with the help of enriched (to 82%) cadmium tungstate crystal scintillators is in progress at the Gran Sasso National Laboratory of the INFN (LNGS, Italy). After 11074 h of data taking in the last configuration, the preliminary estimate for the half-life of 116Cd relatively to 2ν2β decay is T1/2 = [2.52 ± 0.02(stat.) ± 0.14(syst.)] × 1019 yr. By using the data of previous stages of the experiment with a similar level of background (≈ 0.1 counts/(keV kg yr) in the energy interval 2.7 – 2.9 MeV; the total time of measurements is 19770 h) we have obtained a new limit on the 0ν2β decay of 116Cd to the ground state of 116Sn: T1/2 ≥ 1.9 × 1023 yr at 90% C.L. New limits on different 2β processes in 116Cd (decays with majorons, transitions to the excited levels) are obtained on the level of T1/2 ≥ 1020 – 1022 yr.

An experiment to search for double beta processes in 106Cd by using cadmium tungstate crystal scintillator enriched in 106Cd (106CdWO4) in coincidence with the four crystals HPGe detector GeMulti is in progress at the STELLA facility of the Gran Sasso underground laboratory of INFN (Italy). The 106CdWO4 scintillator is viewed by a low-background photomultiplier tube through a lead tungstate crystal light-guide produced from deeply purified archaeological lead to suppress gamma quanta from the photomultiplier tube. Here we report the first results of the experiment after 3233 hours of the data taking. A few new improved limits on double beta processes in 106Cd are obtained, in particular T1/2(2nuECb+) > 8.4e20 yr at 90% C.L.

Cerium, neodymium and gadolinium contain double beta active isotopes. The most interesting are 150 Nd and 160 Gd (promising for 02 search), 136 Ce (2 + candidate with one of the highest Q 2). The main problem of compounds containing lanthanide elements is their high radioactive contamination by uranium, radium, actinium and thorium. The new generation 2 experiments require development of methods for a deep purification of lanthanides from the radioactive elements. A combination of physical and chemical methods was applied to purify cerium, neodymium and gadolinium. Liquid-liquid extraction technique was used to remove traces of Th and U from neodymium, gadolinium and for purification of cerium from Th, U, Ra and K. Co-precipitation and recrystallization methods were utilized for further reduction of the impurities. The radioactive contamination of the samples before and after the purification was tested by using ultra-low- background HPGe gamma spectrometry. As a result of the purification procedure the radioactive contamination of gadolinium oxide (a similar purification efficiency was reached also with cerium and neodymium oxides) was decreased from 0.12 Bq/kg to 0.007 Bq/kg in 228 Th, from 0.04 Bq/kg to

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.

Cadmium tungstate crystal scintillators enriched in 106Cd and 116Cd 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 106Cd and 116Cd 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., Comment: 9 pages, 1 figure (4 eps-figs), 1 table. Accepted for publication in Radiation Measurements as a Proceedings of the 8th International Conference on Luminescent Detectors and Transformers of Ionizing Radiation (LUMDETR 2012), September 09-14, 2012, Halle (Saale), Germany

There are several potentially double beta active isotopes among the lanthanide elements. However, even high purity grade lanthanide compounds contain 238U, 226Ra and 232,228Th typically on the level of ∼ (0.1 - 1) Bq/kg. The liquid-liquid extraction technique was used to remove traces of U, Ra and Th from CeO2, Nd2O3 and Gd2O3. The radioactive contamination of the samples before and after the purification was tested by using ultra-low-background HPGe γ spectrometry at the underground Gran Sasso National Laboratories of the INFN (Italy). After the purification the radioactive contamination of gadolinium oxide by Ra and Th was decreased at least one order of magnitude. The efficiency of the approach to purify cerium oxide from Ra was on same level, while the radioactive contamination of neodymium sample before and after the purification is below the sensitivity of analytical methods. The purification method is much less efficient for chemically very similar radioactive elements like lanthanum, lutetium and ...

The Aurora experiment to investigate double beta decay of $^{116}$Cd with the help of 1.162 kg cadmium tungstate crystal scintillators enriched in $^{116}$Cd to 82\% is in progress at the Gran Sasso Underground Laboratory. The half-life of $^{116}$Cd relatively to the two neutrino double beta decay is measured with the highest up-to-date accuracy $T_{1/2}=(2.62\pm0.14)\times10^{19}$ yr. The sensitivity of the experiment to the neutrinoless double beta decay of $^{116}$Cd to the ground state of $^{116}$Sn is estimated as $T_{1/2} \geq 1.9\times10^{23}$ yr at 90\% CL, which corresponds to the effective Majorana neutrino mass limit $\langle m_{\nu}\rangle \leq (1.2-1.8)$ eV. New limits are obtained for the double beta decay of $^{116}$Cd to the excited levels of $^{116}$Sn, and for the neutrinoless double beta decay with emission of majorons., Comment: 5 pages, 3 figures, 1 table, submitted as proceedings of the TAUP 2015 conference

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 2$\varepsilon0\nu$ 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-2$\varepsilon0\nu$ decays was found, and only T_{1/2} limits were established in the range of 10^{14}-10^{21} yr., Comment: Proceedings of TAUP 2011 Conference

Feasibility studies on a search for 7 Li solar axion have been carried out. In particular, the data collected with a LiF powder of 243 g measured during 722 h with a 408 cm 3 low background HP Ge detector (GSOR) at the Gran Sasso National Laboratories (LNGS) of INFN has allowed to set limit on the 7 Li solar axion mass of 13.9 keV, better than the previously available one: 16.0 keV [A.V. Derbin, et al., JETP Lett. 81 (2005) 365]. It has also been shown that improved results would be achieved with LiF(W) crystal which—in contrast with the LiF powder—does not show a relatively high contamination by U/Th chains. The potential backgrounds and the reachable sensitivity of an experiment are discussed.

Double beta processes in 64Zn, 70Zn, 180W, and 186W 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 (Italy). The total measurement time exceeds ten thousand hours. New improved half-life limits on double electron capture and electron capture with positron emission in 64Zn have been set, in particular (all the limits are at 90% C.L.): T 1 / 2 0 ν 2 e ⩾ 1.1 × 10 20 yr , T 1 / 2 2 ν e β + ⩾ 7.0 × 10 20 yr , and T 1 / 2 0 ν e β + ⩾ 4.3 × 10 20 yr . In addition, new T 1 / 2 bounds were set for different modes of 2β processes in 70Zn, 180W, and 186W at the level of 1017–1020 yr.