Photoneutron cross section measurements on $^{65}$Cu: Toward understanding (n, $\gamma$) cross sections relevant to weak $s$-process nucleosynthesis

Abstract A new measurement of the $^{65}$Cu($\gamma$, n)$^{64}$Cu photoneutron cross section is performed using a quasi-monoenergetic, tunable $\gamma$-ray beams produced at the Shanghai Laser Electron Gamma Source (SLEGS). The energy spectrum of the SLEGS $\gamma$-ray beams incident on the isotopically enriched $^{65}$Cu target was monitored using a BGO detector, while the photoneutron yields are determined with a moderated $^3$He detection array with high and flat efficiency. Within the energy range of $10.1 \le E_\gamma \le 17.6$~MeV, the measured $\sigma(E_\gamma)$ data have an uncertainty of $\lesssim 4\%$, and a pronounced giant-dipole peak is observed at $E_\gamma \simeq 16.65$~MeV with a maximal cross section of $\sigma_{\text{max}} \simeq 137$~mb. These photoneutron data are compared with previous experimental results, and are employed to extract the $\gamma$-ray strength function of $^{65}$Cu above the neutron threshold. Furthermore, we calculate the radiative neutron capture cross sections and the astrophysical reaction rates for $^{64}$Cu, which a short-lived intermediate nucleus whose reaction rate controls the local abundance distribution in the weak $s$ process. It is found that the calculated $^{64}$Cu(n, $\gamma$)$^{65}$Cu data have an overall agreement with ENDF/B-VIII.0, JEFF-3.3, and TENDL-2023 evaluations and the corresponding astrophysical reaction rates are consistent with those reported in the JINA REACLIB database.