Speaker
Description
Nuclei masses are crucial for the understanding of nuclear forces that give to the atomic nucleus its binding energy, are responsible for its structure and drive its evolution all over the nuclide chart. They are also mandatory to evaluate the reaction rates entering the modelling of explosive nucleosynthesis processes. In this context, the so-called Isobaric Multiplet Mass Equation (IMME) derived by E.P. Wigner in 1957 within the isospin symmetry formalism plays an important role. It links the masses of degenerated isobaric analogue states of a given total isospin T with a simple formula: M(T,Tz)=a(T) + b(T) Tz + c(T) Tz² with the 3rd total isospin component Tz =(N-Z)/2. Its robustness makes it a powerful tool, e.g., to predict the masses of unbound nuclei beyond the proton drip-line, give hints about the isospin mixing of nuclear states, put constrains on the contribution of three-body terms in the nuclear Hamiltonian and, more generally, it allows addressing the fundamental question of the isospin symmetry of nuclear forces.
The (A=32,T=2) quintet brings nowadays the strongest constrain on the validity of the IMME through a fit to the masses of the 5 analogue states involved in 5 different nuclei. The present contribution deals with a precise determination of the mass of the lowest T=2 excited state in 32S, for which the currently most precise value is a few standard deviations away from the IMME prediction. The ground-state mass of 32S being known with a high precision, that of the T=2 excited state was derived at the AIFIRA platform of CENBG, France, by measuring the energy of the gamma rays involved in its de-excitation. The state was populated resonantly in the 31P(p,γ) reaction at ~3.2 MeV and its decay scheme was studied by means of γ-ray spectroscopy using high-purity germanium detectors. The data analysis procedure, consisting of on-line and off-line energy calibrations, Doppler correction evaluations and Geant4 based simulations will be presented and the obtained result discussed.
Electronic address: thomasjc@ganil.fr