Experimental Studies of Nuclear Matrix Elements for Neutrinoless Double Beta Decay by γ-ray Spectroscopy

Neutrinoless double beta decay (0νββ) is a hypothetical nuclear decay predicted by several extensions of the Standard Model (SM). Unlike two-neutrino double beta decay (2νββ), it proceeds without neutrino emission, violating lepton number conservation. Its observation would therefore provide evidence of physics beyond the SM and important insight into the nature of neutrinos. Precise estimations of the Nuclear Matrix Element (NME) associated with this decay are essential for current experimental searches; unfortunately, existing estimations tend to differ significantly from one another, and thus their reliability remains uncertain. In some descriptions of 0νββ decay, the process can be interpreted as a sequence of two consecutive β decays, with the two neutrinos effectively annihilating each other, suggesting that relevant information could be extracted from the study of these transitions and from the level structure of the intermediate nucleus. However, the intermediate β decays are usually not energetically favoured and are therefore difficult to observe directly. An alternative approach relies on γ spectroscopy and studies the decay from the Isobaric Analogue State (IAS) of the mother nucleus to the intermediate nucleus, or from the IAS of the intermediate nucleus to the ground state of the daughter one. The corresponding β-decay amplitude can then be inferred from the γ-decay amplitude, which can be determined experimentally by measuring the intrinsic IAS width Γ together with the number N of observed γ decays. In this thesis we studied the feasibility of such an experiment using the AGATA γ array at Laboratori Nazionali di Legnaro (LNL), together with the silicon detector SAURON. A Geant4 simulation of the setup was developed to investigate the factors affecting the determination of the intrinsic IAS width Γ from the observed γ-ray spectrum. In addition, a Monte Carlo simulation was performed to assess the achievable precision on Γ and N under realistic experimental conditions. In this kind of experiment, in order to be sensitive to these small decay branches, a high coincidence rate is expected in the silicon detector, which may result in a degradation of its performance. For this reason, we also investigated the effects of radiation damage on SAURON and tested some experimental procedures aimed at restoring its original capabilities.