First in-beam gamma-ray spectroscopy of (n, xn) reactions with EXOGAM at NFS

by Hemantika Sengar (3rd year PhD student, GANIL)
Supervisor: Emmanuel Clément (GANIL)

Fast-neutron-induced (n,xn) reactions occupy a region of reaction phase space never explored for nuclear structure: although widely used for cross-section measurements and data evaluation, they have never been exploited as a structure probe the way charged particles, heavy ions, and beta decay have. This work reports the first high-resolution in-beam gamma spectroscopy of (n,xn) reactions with a fast neutron beam, using the EXOGAM HPGe array at the Neutrons for Science facility (GANIL/SPIRAL2) on natural Ni and Pb targets up to incident neutron energies of ~40 MeV. Time-of-flight tagging of the white neutron beam, gamma-gamma coincidence analysis, and angular correlations were combined to study the well-characterised 56Ni and 208Pb regions, chosen because their established level schemes provide a clean testing ground for a new probe. As the first operation of EXOGAM under such an intense fast-neutron flux, the experiment also settled an open feasibility question through a full study of neutron-induced detector damage: whether HPGe arrays can sustain prolonged operation in this environment and support future campaigns at NFS.

On the structure side, the 58Ni(n,n’) scheme was extended well beyond the previous measurements, and the 58Ni(n,2n)57Ni channel was mapped for the first time, yielding new states in this otherwise well-studied nucleus, one decaying by an E1 transition with a tentative positive-parity assignment consistent with a cross-shell intruder configuration invisible to all earlier probes. The angular correlations show that fast neutrons induce measurable nuclear alignment, and the analysis delivers spin cut-off factors alongside spins, parities, energies, and cascades that feed directly into reaction codes such as TALYS and into shell-model calculations.

On the data side, relative gamma-production cross-sections for Ni and Pb isotopes were measured far beyond existing data; since Pb is the dominant shielding and structural material in reactor and accelerator environments, a shorter Pb test run at the end of the campaign extended its (n,xn) cross-sections well past the ~20 MeV ceiling of previous GELINA data while also adding newer gamma cross-sections.

Together these results establish (n,xn) reactions as a distinct structure probe and connect three usually separate goals: the new levels and spin-parities are the same inputs the cross-section codes need, so adding them sharpens predictions directly, while any residual disagreement is itself diagnostic, with the channel-by-channel analysis pinpointing where the code models the reaction wrongly, such as its overestimation of the (n,n’p) channel.
In this work, a central motivation has been to make structure measurement, data evaluation, and reaction-mechanism understanding feed into one another instead of treating them as separate goals.