For the first time an extensive search for various double beta decay modes of Sn-124 and Sn-112 has been performed. A total exposure of 43.29 kg days has been accumulated. New half-life limits of Sn-124 into excited states of Te-124 have been obtained; the lower half-life limit for the first excited 2(+) state at 602.7 keV is T-1/2 > 3.1 x 10(18) yr (90% CL) and for the first excited 0(+) state T-1/2 > 7.7 x 10(18) yr (90% CL). For the very first time, ground state and excited state transitions of Sn-112 have been experimentally explored. The obtained half-life limits for EC/EC and beta(+)/EC into the first excited 2(+) state of Cd-112 are both T-1/2 > 1.4 x 10(18) yr (90% CL). A resonance enhancement in the decay rate for O nu EC/EC might be expected for the 0(+)-state at 1870.9 keV due to degeneracy with the Sn-112 ground state. No signal was found resulting in a lower half-life limit of T-1/2 > 1.6 x 10(18) yr (90% CL) for this decay. As all the excited state searches are based on gamma-lines, all half-life limits apply for both neutrino and neutrino-less modes. Neutrinoless ground state transitions were searched for in the EC/EC and beta(+)/EC mode and a limit of T-1/2 > 1.5 x 10(18) yr (90% CL) was obtained for EC/EC decays of Sn-112, whilst the beta(+)/EC mode results are inconclusive. (c) 2007 Elsevier B.V. All rights reserved.

A search for the various double-beta decay modes of Sn-124 and Sn-112 has been performed on 75 kg days of data. New half-life limits for excited states in Sn-124 have been obtained including a lower limit for decay into the first excited 2(+) state of Te-124 of T-1/2 > 0.87 x 10(20) yr (90% C.L.) and into the first excited 0(+) state of T-1/2 > 1.08 x 10(20) yr (90% C.L.). Ground state and excited state transitions of Sn-112 have also been experimentally explored. A limit for the 2 nu EC/EC of T-1/2 > 1.8 x 10(19) yr (90% C. L.) is obtained. The nonobservation of deexcitation gamma from the 0(+) at 1888.5 keV results in a lower half-life limit on the 0 nu EC/EC decay of Sn-112 of T-1/2 > 0.8 x 10(19)yr (90% C.L.), despite a possible resonant enhancement of the decay rate due to degenerated states.

Four 1-cm(3) CdZnTe semiconductor detectors were operated in the Gran Sasso National Laboratory to explore the feasibility of such devices for double beta-decay searches as proposed for the COBRA experiment. The research involved background studies accompanied by measurements of energy resolution performed at the surface. Energy resolutions sufficient to reduce the contribution of two-neutrino double beta-decay events to a negligible level for a large-scale experiment have already been achieved and further improvements are expected. Using activity measurements of contaminants in all construction materials a background model was developed with the help of Monte Carlo simulations and major background sources were identified. A total exposure of 4.34 kg center dot days of underground data have been accumulated allowing a search for neutrinoless double beta-decay modes of seven isotopes found in CdZnTe. Half-life limits (90% C.L.) are presented for decays to ground and excited states. Four improved lower limits have been obtained, including zero neutrino double electron capture transitions of Zn-64 and Te-120 to the ground state, which are 1.19x10(17) years and 2.68x10(15) years, respectively.