Experimental Neutron Capture Rate Constraint Far from Stability

Nuclear reactions where an exotic nucleus captures a neutron are critical for a wide variety of applications, from energy production and national security, to astrophysical processes, and nucleosynthesis. Neutron capture rates are well constrained near stable isotopes where experimental data are available; however, moving far from the valley of stability, uncertainties grow by orders of magnitude. This is due to the complete lack of experimental constraints, as the direct measurement of a neutron-capture reaction on a short-lived nucleus is extremely challenging. Here, we report on the first experimental extraction of a neutron capture reaction rate on ${}^{69}\mathrm{Ni}$, a nucleus that is five neutrons away from the last stable isotope of Ni. The implications of this measurement on nucleosynthesis around mass 70 are discussed, and the impact of similar future measurements on the understanding of the origin of the heavy elements in the cosmos is presented.

Figure 1

Ratio of highest to lowest neutron capture rates at 1.5 GK for the isotopes from Mn ($Z=25$) to Ga ($Z=31$). The neutron capture rates were calculated with talys [9] using the Koning and Delaroche optical model potential [11] and varying the NLD and $\gamma \mathrm{SF}$. The stable isotopes are shown as dark gray squares and all have neutron capture ratios below five.

Figure 2

(a) Nuclear level density as a function of excitation energy compared to known levels given by the thin black line. High and low experimental bounds using the HFB microscopic normalizations are shown by the blue band. An upper limit from the phenomenological model [45] is provided by the open squares. (b) $\gamma \mathrm{SF}$ as a function of $\gamma$-ray energy. Experimental data are shown by the black squares with the upper and lower bounds from the microscopic normalization given by the blue band. The experimental data from Rossi et al. [46] at higher energies from Coulomb excitation are also shown. The upper bound obtained from the phenomenological model is given by the open squares. Calculations for the $E1$ photon strength of ${}^{68}\mathrm{Ni}$ and ${}^{72}\mathrm{Ni}$ [47] are also provided. (c) ${}^{69}\mathrm{Ni}(n,\gamma ){}^{70}\mathrm{Ni}$ reaction rate as a function of temperature. JINA reaclib [48] and bruslib [49] reaction rate recommendations are shown by solid and dashed lines, respectively. The light-blue shaded band corresponds to a factor of 10 uncertainty in the ${}^{69}\mathrm{Ni}(n,\gamma ){}^{70}\mathrm{Ni}$ reaction rate. The experimental result is provided by the dark blue shaded band. The upper limit using the alternative normalization from Ref. [45] is shown by the blue dot-dashed line.

Figure 3

(Left) Predicted abundance versus mass number for the weak $r$ process compared to solar $r$-process residuals (black dots) [2]. The abundance patterns are compared using JINA reaclib rates with a variation in the ${}^{69}\mathrm{Ni}(n,\gamma ){}^{70}\mathrm{Ni}$ rate of an order of magnitude (blue) and the experimentally constrained reaction rate from the present experiment (red shaded region). (Right) Comparison of the percent abundance change as a function of mass number using old reaction rate uncertainties (blue line) versus the present uncertainties (red). The present data reduce the uncertainties in the abundance predictions to less than 20% across the mass range.

Figure 4

Predicted abundances versus mass number compared to solar $r$-process residuals (black dots) [2]. The shaded bands show the variances in the ensemble of abundance patterns from 20 000 network calculation iterations. In each iteration, a Monte Carlo variation of all neutron capture rates is applied [3]. The shaded bands correspond to neutron capture rate uncertainties of a factor of 100 (light), 10 (middle), and 2 (dark). All but the largest abundance pattern features are obscured by the rate uncertainties at a factor of 100. Only with uncertainties smaller than a factor of 10 can fine features be observed.