On the possibility to search for double beta decay of initially unstable (alpha/beta radioactive) nuclei

Possibilities to search for double beta decay of alpha/beta unstable nuclei, many of which have higher energy release than "conventional" (beta stable) double beta decay candidates, are discussed. First experimental half-life limits on double beta decay of radioactive nuclides from U and Th families (trace contaminants of the CaWO_4, CdWO_4 and Gd_2SiO_5 scintillators) were established by reanalyzing the data of low-background measurements in the Solotvina Underground Laboratory with these detectors (1734 h with CaWO_4, 13316 h with CdWO_4, and 13949 h with Gd_2SiO_5 crystals).Comment: 15 pages, 6 figure

Quest for double beta decay of 160-Gd and Ce isotopes

The double beta decay study of 160-Gd has been performed in the Solotvina Underground Laboratory with the help of Gd_2SiO_5(Ce) crystal scintillator (volume 95 cc). The background of the detector in the vicinity of the 2 beta energy of 160-Gd was reduced to 1.0 cpd/keV kg. The new improved half-life limits have been established for neutrinoless 2 beta decay of 160-Gd to the ground and first excited levels of 160-Dy: T1/2 > 2.3(1.3)E21 yr at 68%(90%) C.L. The T1/2 bounds have been also set for two neutrino mode as well as for 2 beta decay with Majorons emission. Also the limits were established for different 2 beta decay processes in 136-Ce, 138-Ce and 142-Ce.Comment: 12 pages, 6 figures, submitted to Nucl. Phys.

High sensitivity double beta decay study of 116-Cd and 100-Mo with the BOREXINO Counting Test Facility (CAMEO project)

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.Comment: 29 pages, LaTex, 9 eps figure

New limits on di-nucleons decay into invisible channels

Data of the radiochemical experiment [E.L.Fireman, 1978] with 1.7 t of KC_2H_3O_2, accumulated deep underground during ~1 yr, were reanalyzed to set limits on di-nucleons (nn and np) decays into invisible channels (disappearance, decay into neutrinos, etc.). The obtained lifetime bounds tau_np > 2.1 10^25 yr and tau_nn > 4.2 10^25 yr (at 90% C.L.) are better (or competitive) than those established in the recent experiments.Comment: 3 pages, accepted in JETP Letter

The event generator DECAY4 for simulation of double beta processes and decay of radioactive nuclei

The computer code DECAY4 is developed to generate initial energy, time and angular distributions of particles emitted in radioactive decays of nuclides and nuclear (atomic) deexcitations. Data for description of nuclear and atomic decay schemes are taken from the ENSDF and EADL database libraries. The examples of use of the DECAY4 code in several underground experiments are described.Comment: 8 pages, 1 fi

Neutrinoless Double Beta Decay within QRPA with Proton-Neutron Pairing

We have investigated the role of proton-neutron pairing in the context of the Quasiparticle Random Phase approximation formalism. This way the neutrinoless double beta decay matrix elements of the experimentally interesting A= 48, 76, 82, 96, 100, 116, 128, 130 and 136 systems have been calculated. We have found that the inclusion of proton-neutron pairing influences the neutrinoless double beta decay rates significantly, in all cases allowing for larger values of the expectation value of light neutrino masses. Using the best presently available experimental limits on the half life-time of neutrinoless double beta decay we have extracted the limits on lepton number violating parameters.Comment: 16 RevTex page

Investigation of β + β + and β + /EC decay of 106 Cd

A low background scintillation detector with a CdWO4 crystal of 1.046 kg was used to search for β+β+ and β+/EC processes in 106Cd. For the neutrinoless mode the limits T1/2(0νβ+β+) ≥ 2.2 · 1019 y and T1/2(0νβ+/EC) ≥ 5.5 · 1019 y were obtained with 90% C.L. For the possible two neutrino decay limits of T1/2(2νβ+β+) ≥ 9.2 · 1017 y and 1/2(2νβ+/EC) ≥ 2.6 · 1017 y have been determined with 99% C.L

New results of 116Cd double beta decay study with 116CdWO4 scintillators

A new phase of 116Cd double beta decay experiment is in progress in the Solotvina Underground Laboratory. Four enriched 116CdWO4 scintillators with total mass 339 g are used in a set up, whose active shield is made of 15 natural CdWO4 crystals (20.6 kg). The background rate in the energy interval 2.5-3.2 MeV is 0.03 counts/y*kg*keV. The half-life for 2-neutrino 2-beta decay of 116Cd is measured as T{1/2}(2-neutrino) = [2.6+-0.1(stat)-0.4+0.7}(syst)]*10**19 y. The T{1/2} limits for neutrinoless 2-beta decay of 116Cd are set as T{1/2} >= 0.7(2.5)*10**23 y at 90%(68%) C.L. for transition to ground state of 116Sn, while for decays to the first 2+ and second 0+ excited levels of 116Sn as T{1/2}>=1.3(4.8)*10**22 y and >=0.7(2.4)*10**22 y with 90%(68%) C.L., respectively. For 0-neutrino 2-beta decay with emission of one or two Majorons, the limits are T{1/2}(0-neutrino M1) >=3.7(5.8)*10**21 y and T{1/2}(0-neutrino M2)>=5.9(9.4)*10**20 y 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(neutrino)<=2.6(1.4) eV, eta <=3.9*10**-8, lambda <=3.4*10**-6, and g{M}<= 12(9.5)*10**-5 at 90%(68%) C.L., respectively.Comment: 28 pages, 9 figures (LaTeX). Phys. Rev. C (in press

High sensitivity GEM experiment on double beta decay of 76-Ge

The GEM project is designed for the next generation 2 beta decay experiments with 76-Ge. One ton of ''naked'' HP Ge detectors (natural at the first GEM-I phase and enriched in 76-Ge to 86% at the second GEM-II stage) are operating in super-high purity liquid nitrogen contained in the Cu vacuum cryostat (sphere with diameter 5 m). The latest is placed in the water shield. Monte Carlo simulation evidently shows that sensitivity of the experiment (in terms of the T1/2 limit for neutrinoless 2 beta decay) is 10^27 yr with natural HP Ge crystals and 10^28 yr with enriched ones. These bounds corresponds to the restrictions on the neutrino mass less than 0.05 eV and 0.015 eV with natural and enriched detectors, respectively. Besides, the GEM-I set up could advance the current best limits on the existence of neutralinos - as dark matter candidates - by three order of magnitudes, and at the same time would be able to identify unambiguously the dark matter signal by detection of its seasonal modulation.Comment: LaTeX, 20 pages, 4 figure