Cross-Section Measurements of the ${}^{86}\mathrm{Kr}(\gamma ,n)$ Reaction to Probe the $s$-Process Branching at ${}^{85}\mathrm{Kr}$

We have carried out photodisintegration cross-section measurements on ${}^{86}\mathrm{Kr}$ using monoenergetic photon beams ranging from the neutron separation energy, $S_n=9.86\mathrm{MeV}$, to 13 MeV. We combine our experimental ${}^{86}\mathrm{Kr}(\gamma ,n){}^{85}\mathrm{Kr}$ cross section with results from our recent ${}^{86}\mathrm{Kr}(\gamma ,{\gamma }^{\prime })$ measurement below the neutron separation energy to obtain the complete nuclear dipole response of ${}^{86}\mathrm{Kr}$. The new experimental information is used to predict the neutron capture cross section of ${}^{85}\mathrm{Kr}$, an important branching point nucleus on the abundance flow path during $s$-process nucleosynthesis. Our new and more precise ${}^{85}\mathrm{Kr}(n,\gamma ){}^{86}\mathrm{Kr}$ cross section allows us to produce more precise predictions of the ${}^{86}\mathrm{Kr}$ abundance from $s$-process models. In particular, we find that the models of the $s$ process in asymptotic giant branch stars of mass $<1.5{\mathrm{M}}_{\odot }$, where the ${}^{13}\mathrm{C}$ neutron source burns convectively rather than radiatively, represent a possible solution for the highest ${}^{86}\mathrm{Kr}:{}^{82}\mathrm{Kr}$ ratios observed in meteoritic stardust SiC grains.

Figure 1

Nuclidic chart near the $s$-process branching at ${}^{85}\mathrm{Kr}$. Nuclides shown as shaded (open) squares are stable (radioactive). The blue (red) arrows depict the $s$-process path at low (high) neutron densities. The ground state of ${}^{85}\mathrm{Kr}$ can either $\beta$ decay ($T_{1/2}=10.75\mathrm{y}$) to ${}^{85}\mathrm{Rb}$ or capture a neutron to form ${}^{86}\mathrm{Kr}$. The isomeric state of ${}^{85}\mathrm{Kr}$ can either $\beta$ decay ($T_{1/2}=4.48\mathrm{h}$) or decay to the ground state via $\gamma$-ray emission; its neutron capture rate is negligible in $s$-process models. Another $s$-process branching occurs at ${}^{86}\mathrm{Rb}$. The dashed vertical line connects nuclides with a magic number of neutrons ($N=50$).

Figure 2

Level schemes of ${}^{85}\mathrm{Kr}$ and ${}^{86}\mathrm{Kr}$. The blue arrows depict the ($n$, $\gamma$) reaction, while the red arrows correspond to the ($\gamma$, $n$) and ($\gamma$, ${\gamma }^{\prime }$) reactions above and below the neutron threshold (thick dashed line), respectively. The blue-shaded region depicts the increasing level density at higher excitation energies. See text.

Figure 3

The blue point-up triangles represent the total cross section of the ${}^{86}\mathrm{Kr}(\gamma ,n){}^{85}\mathrm{Kr}^{g+m}$ reaction while the green point-down triangles denote the ${}^{86}\mathrm{Kr}(\gamma ,n){}^{85}\mathrm{Kr}^m$ cross section, both from the present measurements. The calculated cross section for the ${}^{86}\mathrm{Kr}(\gamma ,n){}^{85}\mathrm{Kr}^{g+m}$ reaction, using the TALYS code, is represented by the red line. The magenta points denote the total photoabsorption cross section of ${}^{86}\mathrm{Kr}(\gamma ,{\gamma }^{\prime })$ from Ref. 26.

Figure 4

Cross section of ${}^{85}\mathrm{Kr}^g(n,\gamma ){}^{86}\mathrm{Kr}$ calculated with TALYS using experimental dipole (in black) and three-phonon quasiparticle-phonon model (QPM) strength functions (in red) from Ref. [26]. The predicted uncertainties (shaded area) are derived from the experimental errors of the dipole strength function and from variations in the nuclear level density parameters.

Figure 5

Number abundance ratio ${}^{86}\mathrm{Kr}:{}^{82}\mathrm{Kr}$ from different stellar models of AGB stars. Results shown as red circles and blue squares are obtained using the present and previous ${}^{85}\mathrm{Kr}^g(n,\gamma ){}^{86}\mathrm{Kr}$ reaction rate, respectively. Values observed in stardust SiC grains are shown on the right-hand side; the numbers next to the symbols denote the size of the grains in units of $\mu \mathrm{m}$. The solar ratio is shown as a horizontal dashed line.