Photoneutron cross sections for Ni isotopes: Toward understanding $(n,\gamma )$ cross sections relevant to weak $s$-process nucleosynthesis

Photoneutron cross sections were measured for ${}^{58}\mathrm{Ni},{}^{60}\mathrm{Ni},{}^{61}\mathrm{Ni}$, and ${}^{64}\mathrm{Ni}$ at energies between the one-neutron and two-neutron thresholds using quasimonochromatic $\gamma$-ray beams produced in laser Compton scattering at the NewSUBARU synchrotron radiation facility. These photoneutron data are used to extract the $\gamma$-ray strength function above the neutron threshold, complementing the information obtained by the Oslo method below the threshold. We discuss radiative neutron-capture cross sections and the Maxwellian-averaged cross sections for Ni isotopes including ${}^{63}\mathrm{Ni}$, a branching point nucleus along the weak $s$-process path. The cross sections are calculated with the experimentally constrained $\gamma$-ray strength functions from the Hartree-Fock-Bogolyubov plus quasiparticle–random-phase approximation based on the Gogny D1M interaction for both $E1$ and $M1$ components and supplemented with the $M1$ upbend.

Figure 1

An excerpt of the chart of nuclei depicting Ni isotopes of the present research objectives in relation to the weak $s$-process path.

Figure 2

(a,b) the $(\gamma ,n)$ cross sections for ${}^{58,60}\mathrm{Ni}$ in comparison with the preceding data [23, 24]. (c,d) the $(\gamma ,n)$ cross sections for ${}^{61,64}\mathrm{Ni}$ measured for the first time. The error bars include both systematic and statistical uncertainties.

Figure 3

Comparison between the experimental ${}^{58}\mathrm{Ni}(\gamma ,n){}^{57}\mathrm{Ni}$ cross section (red squares) and the talys predictions for both the $(\gamma ,n)$ (solid red line) and $(\gamma ,p)$ (dotted blue line) cross sections. Experimental data from Ref. [23] (black open squares) are also included.

Figure 4

[(a)–(e)] $\gamma \mathrm{SF}$ for the ${}^{59,60,61,64,65}\mathrm{Ni}$ isotopes. The red triangles correspond to the upper and lower limits of the $\gamma \mathrm{SF}$ extracted from the present Oslo data and the red open squares to the present NewSUBARU photoneutron data. The dashed blue curve represents the D1M + QRPA $E1$ strength and the black dotted (blue full) line the D1M + QRPA + 0lim $E1+M1$ dipole strength obtained with $C=3\times {10}^{-8}{\mathrm{MeV}}^{-3}\left(C={10}^{-7}{\mathrm{MeV}}^{-3}\right)$. The $\gamma \mathrm{SF}$ of ${}^{64,65}\mathrm{Ni}$ are taken from Refs. [32, 33] (red triangles). The $\gamma \mathrm{SF}$ extracted from the ${}^{60}\mathrm{Ni}(\gamma ,n)$ data of Fultz et al. [23] (black diamonds) is also shown in panel (b).

Figure 5

Radiative neutron capture cross section for the (a) ${}^{58}\mathrm{Ni}$, (b) ${}^{60}\mathrm{Ni}$, (c) ${}^{63}\mathrm{Ni}$, and (d) ${}^{64}\mathrm{Ni}$. The full (dotted) line corresponds to the talys calculation obtained with the D1M + QRPA + 0lim dipole strength obtained with $C=3\times {10}^{-8}{\mathrm{MeV}}^{-3}\left(C={10}^{-7}{\mathrm{MeV}}^{-3}\right)$. Experimental data are taken from Refs. [49, 50, 51, 52, 53, 54]. The hashed areas correspond to the prediction uncertainties associated with different nuclear level density models.

Figure 6

Same as Fig. 5 for the radiative neutron MACS. Experimental data are taken from [49, 55, 56, 57, 58, 59].

Figure 7

(a) Calculated ${}^{59}\mathrm{Ni}(n,\gamma ){}^{60}\mathrm{Ni}$ cross section. (b) Same for the MACS (red solid lines). The open black squares correspond to the theoretical calculation recommended in Ref. [2] and the blue dashed line corresponds to the Brussels library (BRUSLIB) prediction [60].