Quasicontinuum $γ$-decay of $^{91,92}$Zr: benchmarking indirect ($n,γ$) cross section measurements for the $s$-process

Abstract

Nuclear level densities (NLDs) and $γ$-ray strength functions ($γ$SFs) have been extracted from particle-$γ$ coincidences of the $^{92}$Zr($p,p' γ$)$^{92}$Zr and $^{92}$Zr($p,d γ$)$^{91}$Zr reactions using the Oslo method. The new $^{91,92}$Zr $γ$SF data, combined with photonuclear cross sections, cover the whole energy range from $E_γ \approx 1.5$~MeV up to the giant dipole resonance at $E_γ \approx 17$~MeV. The wide-range $γ$SF data display structures at $E_γ \approx 9.5$~MeV, compatible with a superposition of the spin-flip $M1$ resonance and a pygmy $E1$ resonance. Furthermore, the $γ$SF shows a minimum at $E_γ \approx 2-3$~MeV and an increase at lower $γ$-ray energies. The experimentally constrained NLDs and $γ$SFs are shown to reproduce known ($n, γ$) and Maxwellian-averaged cross sections for $^{91,92}$Zr using the {\sf TALYS} reaction code, thus serving as a benchmark for this indirect method of estimating ($n, γ$) cross sections for Zr isotopes.

10 pages and 9 figures