Determination of the Half Lives of Rare Decays of Cd, Te and Zn Isotopes for the COBRA Experiment

Abstract

A search for rare decays of Cd, Te and Zn isotopes, especially double beta decays, within the COBRA experiment is presented. The measurements were made with a 2x2 array of 1 cm^3 CdZnTe semiconductor detectors which has been built in the R&D phase for a 4x4x4 prototype detector array.

For this prototype detector array a data acquisition system using the VME bus has been set up and programmed including a custom built VME-ADC which has been designed and tested. A detector simulation program based on GEANT4 has been developed which is used to calculate detection efficiencies and simulate various detector setups and signal as well as background events.

With a collected statistics of 2.9 kg*days for the two neutrino double beta decay of Cd116 a current sensitivity on the half life of about 10^17 years has been achieved. This value is only about two orders of magnitude away from the experimental half life value of 2.9*10^19 years for Cd116. The best sensitivities on the half life for neutrino-less double beta decay reach a lower half life limit of 10^20 years which corresponds to a sensitivity of 77.6 eV for the majorana neutrino mass each with 90% C.L. For some decay channels world best limits on the half life were set.

In addition to double beta decays the 4-fold forbidden non-unique beta decay of Cd113 has been investigated. The half life of this decay has been determined as (8.2 +- 0.2(stat.)+0.2/-1.0(syst.))*10^15 years (90% C.L.). Furthermore, the spectral shape as recommended by the Table of Isotopes has been proven wrong. For the radiative electron decay a modest limit on the half life of >1.3*10^20 years could be set. The charge non-conserving decay of Cd113 to In113 and only neutrals has been investigated as well resulting in a lower limit on the half life 1.47*10^19 years which surpasses the best previous measurements on Cd113 by one order of magnitude.

As can be seen with this R&D setup already good results could be achieved. Performing extensive simulations with the Monte Carlo program developed in this thesis the currently dominating background source for double beta decay searches could be identified in form of the passivation paint on the detectors.