The neutrinoless double beta decay is a hypothetical nuclear process whose detection could significantly alter our understanding of the universe. If observed, this decay would provide crucial insights into the masses of neutrinos, representing a major step towards a theoretical framework that could eventually replace the Standard Model of particle physics. Furthermore, since neutrinoless double beta decay violates lepton number conservation, its discovery could help explain the matter-antimatter asymmetry in the universe through leptogenesis.
The COBRA experiment employs CdZnTe room temperature semiconductor detectors to search for neutrinoless double beta decays. Between 2011 and 2019, the collaboration operated a demonstrator setup consisting of 64 CdZnTe crystals, each with a volume of 1 cm3 , at the underground laboratory LNGS. The primary aim of this Demonstrator was to evaluate the detector technique’s capabilities regarding a search for the neutrinoless double beta decay.
This thesis presents a comprehensive final analysis of the COBRA Demonstrator data, outlining the conclusions drawn from its operation. Two major investigations are described: the development of a background model and the calculation of half-life limits for the neutrinoless double beta decay.
In preparation for the background model, the thesis compiles all available information from prior studies on the Demonstrator’s background. A detailed examination of background features and their properties is provided. Additionally, a coincidence analysis is performed to gain further insights into specific background components. Ultimately, the collected data is used to construct a background model. This model identifies contaminations of the Rn-222 decay chain on a holding structure in the experiment as the primary source of background events. It also emphasises the
substantial influence of the varnish covering the CdZnTe crystals on alpha decay background. The findings suggest that a thicker and more uniform varnish layer could significantly aid in both characterising and reducing the background. The background model offers a thorough overview of the strengths and challenges involved in using CdZnTe crystals in a low-background environment.
The regions of interest for five double beta isotopes present in CdZnTe are investigated. Using a Bayesian analysis, the half-life limits for the corresponding neutrinoless double beta decays are calculated with 90 % credible intervals: T_1/2^114Cd
> 4.6 · 10²¹ yr, T_1/2^116Cd > 2.7 · 10²¹ yr, T_1/2^70Zn > 0.026 · 10²¹ yr, T_1/2^128Te > 8.6 · 10²¹ yr, and T_1/2^130Te > 13.4 · 10²¹ yr. These values represent a significant improvement over previous analyses based on smaller subsets of the COBRA Demonstrator data. The results demonstrate the potential of CdZnTe crystals in the search for neutrinoless double beta decays.