Soutenance publique
THESE Lorenzo Domenichetti
Lundi 27 octobre 2025, 10h00
CIBB, seminar room, 2nd floor
Lien zoom: ill.zoom.us/j/98037567457
Meeting ID: 980 3756 7457
Passcode: 461361
Titre : New spectroscopic information and theoretical interpretations of the As-76 nucleus
Nouvelles informations spectroscopiques et interprétations théoriques du noyau As-76
Langue : La soutenance aura lieu en anglais.
Abstract :
In this PhD thesis, an investigation of the structure of the 76As nucleus will be presented. The odd-odd nature of this nucleus led previous studies of its low-energy structure to investigate residual proton-neutron interactions. In recent years, this nucleus received increasing attention due to its role in the double-beta decay of 76Ge to 76Se. 76As is the intermediate nucleus in this decay. So far, no measurement of the neutrinoless double-beta (0νββ) decay has been achieved, and only lower limits for the lifetime of this process could be estimated for a few candidates. Theoretical predictions show large discrepancies, and one of the largest sources of uncertainty comes from the nuclear structure contributions to the decay matrix elements. A systematic theoretical investigation of the impact of the intermediate nuclei on the decay matrix elements has been recently carried out. Following such work, the main goals of the experiments performed on 76As focused on the identification of 1+ states involved in the two-neutrino double-beta (2νββ) process, and on a general improvement in the knowledge on the 76As level scheme. Experimental results, such as the confirmation of 1+ spin assignment, and additional information on the transitions from excited states in 76As, can indeed provide input for models used for matrix element calculations. These objectives provide the primary motivation of the present thesis project. To study the excited states in 76As, a thermal-neutron capture experiment was conducted at the Institut Laue-Langevin (ILL) using a 75As target. The experiment was performed at FIPPS (Fission Product Prompt gamma-ray Spectrometer), ILL’s dedicated instrument for high-resolution gamma-ray spectroscopy. FIPPS is an array of eight High-Purity Germanium (HPGe) Compton-suppressed clover detectors, and during the experiment it was combined with the IFIN-HH array, consisting of eight additional HPGe Compton-suppressed clover detectors. The detection system covered approximately 70% of the solid angle, enabling efficient multi-gamma-ray coincidence measurements. This experiment represents the first use of a germanium detector array to study the radiation following radiative neutron capture in 75As. A similar experiment was previously conducted at ILL in the 1980s. The present work, benefiting from significant advancements in experimental techniques, provides complementary information and significantly extends the findings of the earlier study. Over the course of 11 days, a high-resolution dataset with high statistics was collected, resulting in the identification of approximately 210 previously unobserved gamma-ray transitions and six new excited states below 1 MeV excitation energy. The relative intensity of each transition was measured with respect to the most intense de-excitation line in 76As. When transitions were not clearly resolved in the singles spectra, double coincidences were employed to extract their intensities. Furthermore, this setup enabled the construction of triple-gamma coincidence cubes. Thanks to the high statistics, these cubes proved to be crucial for the reliable identification of gamma-ray decays with low branching ratios. Angular correlation measurements allowed spin assignments of several excited states. Following the construction of the level scheme and spin assignments, a comparison between nuclear structure calculations and the experimental data was achieved. These calculations were performed using the Nuclear Shell Model and the proton-neutron Quasiparticle Random-Phase Approximation (pnQRPA) approach. The limitations of these theoretical models in describing such a complex nucleus are discussed in detail in the manuscript.
While the comparison between predicted and observed excited states is feasible at lower excitation energies (up to a few hundred keV), the complexity of the structure of 76As prevents state-by-state comparisons at higher energies. To extract some valuable information from the higher-lying excited states, a statistical analysis was carried out. The statistical description of nuclear decays relies on Photon Strength Functions (PSF) and Nuclear Level Densities (NLD), which respectively describe the average probability for the emission of a gamma ray of a given energy, and the number of excited levels per unit energy. This description is particularly suited for describing the gamma-ray spectra following the de-excitation of a compound nucleus: when individual transitions cannot be resolved, statistical models can be used to describe the average behavior of the nuclear decay. For the statistical analysis at FIPPS, the de-excitation of the nucleus was simulated using the DICEBOX code, which includes different PSF and NLD models. The DICEBOX output was used in Geant4 simulations to account for experimental effects. The simulation outputs from different models were finally compared to experimental data. The comparison between simulations and experimental data was carried out through the Two-Step Cascade (TSC) method. Since no previous analysis of this kind was ever performed at FIPPS, an experiment on a previously studied isotope (96Mo) was first performed, and the results of this experiment were successfully compared to published data. The second step of the study consisted of applying the method to 76As data. Various comparisons were carried out, including tests of PSF models with different E1/M1 ratios. The properties of the low-energy excited states in 76As were also used as parameters in the simulations, and their impact on the simulated TSC intensities was evaluated. The successful statistical analysis of 95Mo(n,γ) data obtained at FIPPS demonstrated the potential of future experiments using the FIPPS+IFIN-HH array to test PSF and NLD models.
Future perspectives include the analysis of a (p,n) experiment conducted at IKP in Cologne, as well as a planned experiment at DANCE at LANSCE. These two experiments will make available data able to confirm and to provide new experimental input for the spectroscopic and statistical analyses, respectively. Moreover, additional (n,γ) experiments on other intermediate double-beta decay nuclei have already been carried out at FIPPS (128I, 130I), while others are currently planned (136Cs).
Direction de thèse:
C. Michelagnoli
Jury:
A. Algora
K. Wimmer
A. Larsen
A. Lopez-Martens
S. Roccia Salad-Damaz
S. M. Lenzi
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