Stellar $(n,\gamma )$ cross sections of neutron-rich nuclei: Completing the isotope chains of Yb, Os, Pt, and Hg

The $(n,\gamma )$ cross sections of the most neutron-rich stable isotopes of Yb, Os, Pt, and Hg have been determined in a series of activation measurements at the Karlsruhe $3.7$ MV Van de Graaff accelerator, using the quasistellar neutron spectrum for $kT=25$ keV that can be produced with the ${}^7\mathrm{Li}(p,n){}^7\mathrm{Be}$ reaction. In this way, Maxwellian averaged cross sections could be directly obtained with only minor corrections. After irradiation the induced activities were counted with a HPGe detector via the strongest $\gamma$-ray lines. The stellar neutron capture cross sections of ${}^{174,176}\mathrm{Yb}$, ${}^{190,192}\mathrm{Os}$, ${}^{196,198}\mathrm{Pt}$, and ${}^{202,204}\mathrm{Hg}$, extrapolated to $kT=30$ keV, were found to be $157\pm 6$ mb, $114\pm 8$ mb, $278\pm 11$ mb, $160\pm 7$ mb, $171\pm 19$ mb, $94\pm 4$ mb, $62\pm 2$ mb, and $32\pm 15$ mb, respectively. In the case of ${}^{196}\mathrm{Pt}$ the partial cross section to the isomeric state at 399.5 keV could be determined as well. With these results the cross section data for long isotopic chains could be completed for a discussion of the predictive power of statistical model calculations towards the neutron-rich and proton-rich sides of the stability valley.

Figure 1

Comparison of the experimental neutron energy distribution with Maxwellian distribution of $kT=25$ keV [11].

Figure 2

Schematic sketch of the experimental setup.

Figure 3

Efficiency curves for the three HPGe detector setups used in the measurements. The dashed line for the Clover detector represents a geant4 simulation, which was normalized to the measured data points (black symbols and solid line).

Figure 4

Typical examples of $\gamma$-ray spectra taken with the HPGe Clover array after activation of sample Pt-1 (top) and with the smaller single HPGe detector after activation of sample Os-3 (bottom).

Figure 5

Present experimental MACS values at $kT=30$ keV for the even-even nuclei compared to the KADoNiS compilation (KADoNiS 2010: [29], shaded areas corresponding to upper and lower bounds of the recommended values) and to statistical model calculations with the non-smoker code (non-smoker 2000: [38], dashed lines).