Photodisintegration of ${}^{80}\mathrm{Se}$: Implications for the $s$-process branching at ${}^{79}\mathrm{Se}$

Photoneutron cross sections were measured for ${}^{80}\mathrm{Se}$ immediately above the neutron separation energy with quasimonochromatic $\gamma$-ray beams to experimentally constrain the $E1$ $\gamma$ strength function for ${}^{80}\mathrm{Se}$. Two sets of the $\gamma$ strength function and the level density that equally meet the experimental constraint predict largely different neutron capture cross sections for ${}^{79}\mathrm{Se}$. Based on the Maxwellian-averaged cross sections of the latest compilation complemented with the predicted cross sections for ${}^{79}\mathrm{Se}$, we calculated $\sigma N$ values of the main $s$-process component using a phenomenological model and deduced empirical abundances of the weak $s$-process component. An attempt is made of considering the implications of the abundance ratios of ${}^{80}\mathrm{Kr}$ and ${}^{82}\mathrm{Kr}$ within the framework of phenomenological models for the weak $s$ process.

Figure 1

A response function of a 120% high-purity Ge detector to the LCS $\gamma$-ray beam and the energy distribution of the LCS beam determined in a least-square analysis of the response function with the Monte Carlo code EGS4.

Figure 2

Pileup and single-photon (inset) spectra of the LCS beam measured with a large volume NaI(Tl) detector measuring 8 inches in diameter and 12 inches long.

Figure 3

Energy spectra of neutrons detected with Ring 1 of four ${}^3\mathrm{He}$ counters during the 80 ms beam-on (a) and the 20 ms beam-off (b).

Figure 4

Detection efficiencies of the three rings of 4 (Ring 1), 8 (Ring 2), and 8 (Ring 3) ${}^3\mathrm{He}$ proportional counters coaxially embedded in the polyethylene moderator at the distances of 3.8, 7.0, and 10.0 cm from the beam axis, respectively. The efficiencies calibrated with a ${}^{252}\mathrm{Cf}$ neutron source (solid circles) are well reproduced by MCNP Monte Carlo simulations (open circles). The neutron energy dependence of the efficiencies were obtained by the Monte Carlo simulation.

Figure 5

Result of the present photoneutron cross section measurement for ${}^{80}\mathrm{Se}$ in comparison with the results of the previous measurements [37, 38]. The model predictions with the microscopic and macroscopic nuclear parameters of the $\gamma$ strength function and the nuclear level density are also shown by the solid line and the dotted line, respectively (see text for details).

Figure 6

Comparison of the $E1$ transmission coefficients (left axis) and the total level density (right axis) of ${}^{80}\mathrm{Se}$ predicted by the models adopted in the present work.

Figure 7

Neutron capture cross sections for ${}^{79}\mathrm{Se}$ (a) and neutron capture rates for ${}^{79}\mathrm{Se}$ (b) predicted with the microscopic (Comb-QRPA) and macroscopic (BSFG-Lorentz) parameter sets of the $\gamma$ strength function and NLD.

Figure 8

$s$-process branching at ${}^{79}\mathrm{Se}$.

Figure 9

$\sigma N$ values for the main component of the $s$-only nuclei (open circles) calculated with the classical model with an exponential distribution of neutron exposures ${\tau }_o=0.3\hspace{0.5em}{\mathrm{mb}}^{-1}$ using the Maxwellian-averaged cross sections of Ref. [47] complemented with the macroscopic cross section for ${}^{79}\mathrm{Se}$ (a) and with the microscopic cross section (b) at $\mathit{kT}=30$ keV and the neutron density $n_n=2.7\times {10}^8\hspace{0.5em}{\mathrm{cm}}^{-3}$. For the weak component, results of the model calculations with an exponential distribution of neutron exposures ${\tau }_o=0.04\hspace{0.5em}{\mathrm{mb}}^{-1}$ at $n_n=1.6\times {10}^8\hspace{0.5em}{\mathrm{cm}}^{-3}$ for MACRO (a) and at $n_n=4.5\times {10}^9\hspace{0.5em}{\mathrm{cm}}^{-3}$ for MICRO, respectively, are shown by the filled circles on top of the main component. The empirical $\sigma N$ values obtained from the solar abundances of Ref. [47] and the cross sections of Ref. [44] are also shown by the open squares. The model calculation overproduces the weak abundance of ${}^{58}\mathrm{Fe}$.

Figure 10

$\sigma N$ values for the weak component (filled circles) calculated with the classical model with a single neutron exposure $\tau =0.23\hspace{0.5em}{\mathrm{mb}}^{-1}$ at $\mathit{kT}=26$ keV, $n_n={10}^7\hspace{0.5em}{\mathrm{cm}}^{-3}$, and $\rho ={10}^3\hspace{0.5em}\mathrm{g}/{\mathrm{cm}}^3$ are shown on top of the main component for the Maxwellian-averaged cross section MACRO (a) and MICRO (b), respectively. The model calculation rather properly produces the weak abundance of ${}^{58}\mathrm{Fe}$. For the main component (open circles) and the empirical values (open squares); see caption of Fig. 9.