title: Probing Nuclear Structure Relevant for Neutrinoless Double-Beta Decay with Nuclear Resonance Fluorescence
Creators: name: Friman-Gayer, Udo
Publisher: name: UNSPECIFIED
Year: 2020

description: Neutrinoless double-beta (0νββ) decay is a hypothetical second-order process of the weak interaction, which, if observed, would reveal neutrinos as the first example of so-called Majorana particles which are their own antiparticles. Furthermore, since the decay rate for 0νββ decay is directly related to the effective mass of the electron neutrino, it would allow for a direct determination of the neutrino mass. However, an obstacle for the planning of future 0νββ-decay searches and for a quantitative extraction of the neutrino mass are currently the poorly constrained nuclear matrix elements which mediate the decay process. These matrix elements have the be supplied by nuclear theory, which is challenged with the phenomena of nuclear shape evolution and shape coexistence that prevail in regions of the nuclear chart where most 0νββ-decay candidates are located. A major problem is the lack of sensitive experimental data, which are required to fix the parameters of effective theories. Based on a previous successful study, the nuclear structure of the candidate pairs 82Se/82Kr and 150Nd/150Sm was investigated in this work using the method of nuclear resonance fluorescence. The observables of interest were the decay channels of a low-lying collective nuclear excitation, the scissors mode, which are expected to be highly sensitive to the location of the candidate pairs in the phase diagram of nuclear shapes. The scissors mode can be studied selectively and with a high degree of model indepedence with the chosen method. The experiments were performed at the High-Intensity Gamma-Ray Source which currently provides the most intense, linearly polarized, quasi-monochromatic photon beam at the energies of interest. Using the high sensitivity of the polarized beam, magnetic dipole excitations, which are the manifestations of the scissors mode in even-even nuclei, were identified and their decay behavior was characterized. A known drawback of experiments with monoenergetic photon beams, namely the lack of a photon-flux calibration, was solved in the present work without any additional instrumentation by calibrating the flux on the nonresonant scattering of photons on the targets. For this purpose, a detailed Monte-Carlo particle simulation application was developed. For all nuclei of interest, decay branches on the order of few percent could either be observed, or constrained to such small values. Two effective nuclear models, the shell model and the interacting boson model, which are also frequently used to predict 0νββ decay matrix elements, were used for a preliminary interpretation of the data. For the nucleus 82Se, the shell model gave a good description of the energies of excited 1+ states and the total observed strength. The good agreement allowed for an interpretation of the structure of the wave functions of the scissors mode candidates, which advised against a simple relation between the measured quantities and the shape coexistence in that nucleus. For the higher-mass isotopes, a careful parameter adjustment in the framework of the interacting boson model was able to reproduce the entire low-energy structure of 150Nd and, with minor exceptions also of 150Sm. The new parameter sets in this model were used together with our collaborator to update previous predictions of nuclear matrix elements for 0νββ decay. Note that the analysis of the data on the A=150 nuclei was done by Jörn Kleemann. This work presents only his main results.