The inference of massive neutrino states through the observation of flavor oscillations boosted the importance of direct and indirect mass searches, including the search for the hypothesized neutrinoless double beta decay (0νββ-decay). Nowadays, the search for this ultra-rare nuclear transition is one of the most active research fields at the intersection of nuclear, particle and astroparticle physics. Its main and evident feature is the explicit violation of the total lepton number, which is an accidentally conserved quantity in the Standard Model of particle physics, and would prove the Majorana nature of neutrinos.
This, in turn, would support the theoretical explanation of the origin of the observed baryon asymmetry in the universe through the process of leptogenisis and could shed light on the role of neutrinos in the early universe’s structure formation.
For the theoretical description of the 0νββ-decay, nuclear structure effects play an important role as they may affect considerably the decay rate. These nuclear effects are summarized as the nuclear matrix elements (NMEs), containing information about the initial and final states of the involved atomic nuclei and the decay mechanism. Under the assumption of light Majorana neutrino exchange, the inverse half-life is proportional to the effective Majorana neutrino mass, a kinematic phase-space factor, the involved NMEs and the fourth power of the weak axial-vector coupling gA. The search for the 0νββ-decay is driven by experiments and an accurate description of the nuclear structure effects is essential to estimate the required sensitivity to cover a certain mass range. In order to match theoretical calculations and the results of β-decay and ββ-decay studies, there is a scientific discussion regarding quenching effects of gA in nuclear media.
Different methods are being investigated to determine an effective gA at the energy scale of nuclear transitions. One of those recently proposed methods exploits the dependency of the spectrum-shape of highly forbidden β-decays on gA. An ideal candidate for such an investigation is the fourfold forbidden non-unique β-decay of Cd-113, which is the most prominent signal in the current stage of the COBRA experiment searching for 0νββ-decays with cadmium zinc telluride (CZT) solid state detectors. The detector material CZT acts as a semiconductor at room temperature and contains intrinsically several candidates for rare nuclear transitions. The experiment is located at the LNGS underground facility in Italy, which is shielded against cosmic rays by a mean rock coverage of about 1400 m. In the present demonstrator phase, it consists of 64 coplanar grid (CPG) detectors that are arranged in four layers of 4 x 4 crystals.
In the scope of this thesis, conventional and novel prototype CPG-CZT detectors, which are the basis for an anticipated large-scale experiment, are characterized by evaluating homogeneous and localized γ-ray irradiation measurements. Moreover, a novel pulse-shape discrimination (PSD) technique is established, optimized and applied in the analysis of the physics data obtained with the demonstrator array. The PSD optimization is complemented by dedicated laboratory measurements with the aim to create a pulseshape library of signal-like single-site events and high-energy cosmic muon interactions for which an analytic reconstruction model has been developed.
For the first time, the COBRA demonstrator’s full exposure from Sept.’11 to Nov.’19 is analyzed, including a detailed background characterization as well as an automatized data partitioning to identify periods with increased backgrounds. The main subject is the study of the Cd-113 β-decay’s spectrum-shape to address the quenching of gA in lowmomentum exchange nuclear processes. The analysis of the experimental data is carried out in the context of three nuclear frameworks and confirms the idea of a significantly quenched gA. Furthermore, the data are analyzed with respect to the 2νββ-decay of Cd-116 and the long-lived α-decay of Pt-190 as a localized contaminant in the CZT detectors’ electrode metalization. Finally, the prospects of a search for excited state transitions of the ββ-nuclides Cd-116 and Te-130 are studied with elaborate Monte-Carlo simulations.
The analysis section is concluded with an estimate of the achievable 0νββ-decay half-life sensitivity for multiple ββ-nuclides given the full exposure of the COBRA demonstrator and the recently achieved upgrade to the COBRA eXtended DEMonstrator (XDEM).
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:72435 |
| Date | 09 October 2020 |
| Creators | Zatschler, Stefan |
| Contributors | Zuber, Kai, Kröninger, Kevin, TU Dresden |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | info:eu-repo/semantics/publishedVersion, doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
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