Orateur
Description
Fast-neutron-induced nxn reactions occupy a region of reaction phase space that has never been explored from the perspective of nuclear structure studies. Compared to fusion-evaporation they transfer modest angular momentum and populate states off the yrast line; compared to transfer reactions they impose no single-particle selectivity and reach residual nuclei through statistical population governed by level density. Until recently the $\gamma$-ray spectroscopy of $(n,xn)$ reactions has been limited by the lack of high-flux fast-neutron beams coupled to large HPGe arrays. The Neutrons for Science (NFS) facility at GANIL-SPIRAL2 changes this: with a white neutron beam extending to 44 MeV and a flux several orders of magnitude above competing facilities, it enables high-resolution multi-coincidence $\gamma$-ray spectroscopy of fast-neutron reactions for the first time.
I present results from the first experiment coupling the EXOGAM HPGe array to NFS, using $^{\mathrm{nat}}$Ni and $^{\mathrm{nat}}$Pb targets over fourteen days of effective beam time. The $(n,2n)$ channel on $^{58}$Ni provides the first nuclear-structure study of $^{57}$Ni via neutron removal and reveals a previously unobserved levels. One of the states observed is a strong candidate for a low-spin member of the $\nu(pf)^{-1}g_{9/2}$ intruder multiplet across the $N=28$ gap, invisible to fusion-evaporation and to pick-up reactions before. Cross-section measurements show discrepancies with TENDL evaluations in the $^{57}$Ni that provide independent evidence for the new state and an entry point for updating evaluated nuclear data. A short test measurement at the end of the campaign on $^{\mathrm{nat}}$Pb allowed extending cross-section measurements of nxn reactions to higher neutron energies beyond ~20 MeV.
Beyond the physics, the experiment also addressed an open question of feasibility: whether HPGe detectors can sustain prolonged operation in intense fast-neutron environments. We find resolution degradation consistent with cumulative dose, fully recovered through standard thermal annealing, with no permanent damage. The combination of NFS neutron flux, EXOGAM resolution, and tractable detector damage establishes a new spectroscopic platform for nuclear structure studies at NFS.