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Studies of the 2β decay of 116Cd, 180W and 186W have been performed in the Solotvina Underground Laboratory with the help of the super-low background 116CdWO4 crystal scintillators. The half-life value of the two-neutrino 2β decay of 116Cd has been revised as T 1/2 2ν =2.9±0.1( stat ) −0.3 +0.4 ( syst )×10 19 yr . Besides, the new half-live limits on different modes of this process have been established for two natural tungsten isotopes, in particular, T1/2⩾1.0 (1.6)×1021 yr at 90% (68%) C.L. for the neutrinoless 2β decay of 186W to the ground (0+) and first excited (2+) level of 186Os.

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 2\(\beta \) 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 \( 0\nu 2\beta \) decay) is \(\approx 4\times 10^{24}\) yr with a 1 kg source of \(^{100}\)Mo and \(\approx 10^{26}\) yr with \(\approx \)100 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 \(m_\nu \leq 0.06\) eV. Similarly with 1000 kg of \(^{116}\)CdWO\(_4\) crystals located in the BOREXINO apparatus the neutrino mass limit can be pushed down to \(m_\nu \leq 0.02\) eV.

The unique features of the CTF and BOREXINO set ups are used for a high sensitivity study of 100 Mo and 116 Cd neutrinoless 2 β decay. Pilot measurements with 116 Cd and Monte Carlo simulation show that the sensitivity of the CAMEO experiment (in terms of the T 1/2 limit for 0 ν 2 β decay) is (3–5) ×10 24 yr with a 1 kg source of 100 Mo ( 116 Cd, 82 Se, 150 Nd) and ≈10 26 yr with 65 kg of 116 CdWO 4 crystals placed in the CTF. The last value corresponds to a limit on the neutrino mass of m ν ⩽0.06 eV. Moreover, with 1000 kg of 116 CdWO 4 crystals located in the BOREXINO apparatus the neutrino mass limit can be pushed down to m ν ⩽0.02 eV.

Final results of the 2 β decay experiment, performed in the Solotvina Underground Laboratory with the help of low-background 116 CdWO 4 crystal scintillators, are presented. The revised half-life value of the two-neutrino 2 β decay of 116 Cd has been measured as T 1 / 2 2 ν = 2.9 −0.3 +0.4 × 10 19 yr, and the new half-life limit on the neutrinoless 2 β decay of 116 Cd has been established as T 1 / 2 0 ν ⩾ 1.7 (2.6)×10 23 yr at 90% (68%) C.L. The latter corresponds to a restriction on the Majorana neutrino mass: m ν ⩽ 1.7 (1.4) eV at 90% (68%) C.L. The CAMEO project could further advance sensitivity of the 116 Cd experiment to m ν ⩽ 0.05 eV.

The measurements with 0.33 kg low background 116CdWO4 scintillators are in progress in the Solotvina Underground Laboratory. After 13254 h of data taking the new half-life limits for different modes of neutrinoless 2β decay of 116Cd are presented, in particular for 0ν2β decay of 116Cd to the ground state of 116Sn: T1/2;(0ν2β) ≥ 1.3(1.8) × 1023 yr at 90%(68%) C.L.

Radioactive contaminations of CaWO4, ZnWO4, CdWO4, and Gd2SiO5:Ce crystal scintillators were measured in the Solotvina Underground Laboratory. The Monte Carlo simulation of contributing radioactive sources, the time‐amplitude and pulse‐shape analyses of the experimental data were applied. CaWO4 and GSO scintillators are considerably polluted by uranium and thorium, while the total α activity in ZnWO4 and CdWO4 scintillators does not exceed the level of a few mBq/kg. Particular radioactivity was observed in CaWO4 (α active 180W), CdWO4 (β active 113Cd, 2v2β decay of 116Cd, 180W), and GSO (α active 152Gd) detectors. The radioactive contamination of the studied scintillation crystals is compared with that of commonly used detectors.

Final results of the double $\ensuremath{\beta}$ decay experiment, performed with the help of low background ${}^{116}{\mathrm{CdWO}}_{4}$ crystal scintillators in the Solotvina Underground Laboratory, are presented. In particular, the revised half-life value of the two-neutrino 2\ensuremath{\beta} decay of ${}^{116}\mathrm{Cd}$ has been measured as ${T}_{1/2}^{2\ensuremath{\nu}}{=2.9}_{\ensuremath{-}0.3}^{+0.4}\ifmmode\times\else\texttimes\fi{}{10}^{19}\mathrm{yr},$ and the new half-life limit on the neutrinoless $2\ensuremath{\beta}$ decay of ${}^{116}\mathrm{Cd}$ has been established as ${T}_{1/2}^{0\ensuremath{\nu}}g~1.7(2.6)\ifmmode\times\else\texttimes\fi{}{10}^{23}\mathrm{yr}$ at 90% (68%) C.L. The latter corresponds to a restriction on the neutrino mass: ${m}_{\ensuremath{\nu}}l~1.7(1.4)\mathrm{eV}$ at 90% (68%) C.L. Besides, new ${T}_{1/2}$ bounds (at the level of ${10}^{17}\char21{}{10}^{21}\mathrm{yr})$ were set for various $2\ensuremath{\beta}$ processes in ${}^{106}\mathrm{Cd},$ ${}^{108}\mathrm{Cd},$ ${}^{114}\mathrm{Cd},$ ${}^{180}\mathrm{W},$ and ${}^{186}\mathrm{W}$ nuclei.

A new phase of a ${}^{116}\mathrm{Cd}$ double \ensuremath{\beta} decay experiment is in progress in the Solotvina Underground Laboratory. Four enriched ${}^{116}{\mathrm{CdWO}}_{4}$ scintillators with total mass of 339 g are used in a setup, whose active shield is made of 15 natural ${\mathrm{CdWO}}_{4}$ crystals (20.6 kg). The background rate in the energy interval 2.5--3.2 MeV is 0.03 counts/yr kg keV. The half-life for 2\ensuremath{\nu}2\ensuremath{\beta} decay of ${}^{116}\mathrm{Cd}$ is measured as ${T}_{1/2}(2\ensuremath{\nu})=2.6\ifmmode\pm\else\textpm\fi{}0.1(\mathrm{stat}{)}_{\ensuremath{-}0.4}^{+0.7}(\mathrm{syst})\ifmmode\times\else\texttimes\fi{}{10}^{19}$ yr. The ${T}_{1/2}$ limits for neutrinoless 2\ensuremath{\beta} decay of ${}^{116}\mathrm{Cd}$ are set at ${T}_{1/2}g~0.7(2.5)\ifmmode\times\else\texttimes\fi{}{10}^{23}$ yr at 90%(68%) C.L. for transition to ground state of ${}^{116}\mathrm{Sn},$ while for decays to the first ${2}_{1}^{+}$ and second ${0}_{1}^{+}$ excited levels of ${}^{116}\mathrm{Sn}$ at ${T}_{1/2}g~1.3(4.8)\ifmmode\times\else\texttimes\fi{}{10}^{22}$ yr and $g~0.7(2.4)\ifmmode\times\else\texttimes\fi{}{10}^{22}$ yr with 90%(68%) C.L., respectively. For 0\ensuremath{\nu}2\ensuremath{\beta} decay with emission of one or two Majorons, the limits are ${T}_{1/2}(0\ensuremath{\nu}M1)g~3.7(5.8)\ifmmode\times\else\texttimes\fi{}{10}^{21}$ yr and ${T}_{1/2}(0\ensuremath{\nu}M2)g~5.9(9.4)\ifmmode\times\else\texttimes\fi{}{10}^{20}$ yr at 90%(68%) C.L. Restrictions on the value of the neutrino mass, right-handed admixtures in the weak interaction, and the neutrino-Majoron coupling constant are derived as ${m}_{\ensuremath{\nu}}l~2.6(1.4)$ eV, $\ensuremath{\eta}l~3.9\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8},$ $\ensuremath{\lambda}l~3.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6},$ and ${g}_{M}l~12(9.5)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$ at 90%(68%) C.L., respectively.

The indication for the alpha decay of 180-W with a half-life T1/2=1.1+0.8-0.4(stat)+-0.3(syst)x10^18 yr has been observed for the first time with the help of the super-low background 116-CdWO_4 crystal scintillators. In conservative approach the lower limit on half-life of 180-W has been established as T1/2>0.7x10^18 yr at 90% C.L. Besides, new T1/2 bounds were set for alpha decay of 182-W, 183-W, 184-W and 186-W at the level of 10^20 yr.