Orateur
Description
The $^{57}$Ni nucleus, with $Z=28$ and $N=29$, sits one neutron above the doubly-magic $^{56}$Ni core and provides a clean testing ground for cross-shell excitations across the $N=28$ gap. Its level scheme has been mapped extensively through fusion-evaporation at high spin and through single-neutron pickup at low excitation, yet a substantial region of $(J, E_x)$ phase space, low-spin states off the yrast line above 3 MeV, has been never seen before with any probe applied to date. We report the first nuclear structure study of $^{58}$Ni$(n,2n)^{57}$Ni, performed with the EXOGAM HPGe array at the NFS facility of GANIL-SPIRAL2 using a white neutron beam spanning 10-40 MeV. Coincidence $\gamma$-ray spectroscopy reveals a previously unobserved levels. The transitions are absent from background, activation, and $^{\mathrm{nat}}$Pb control measurements, and its E1 character, established from combined angular and polarization asymmetry analysis, constrains the new level to a positive parity state. The state seems to be a strong candidate for a low-spin member of the $\nu(pf)^{-1}g_{9/2}$ intruder multiplet whose $9/2^+$ bandhead lies 400 keV higher. Cross-section measurements show discrepancies of opposite sign relative to TENDL evaluations, providing independent evidence that the new level carries significant population strength absent from current nuclear data libraries.