Stellar ($n,\gamma$) cross sections of ${}^{23}\mathbf{Na}$

The cross section of the ${}^{23}\mathrm{Na}(n,\gamma ){}^{24}\mathrm{Na}$ reaction was measured via the activation method at the Karlsruhe 3.7 MV Van de Graaff accelerator. NaCl samples were exposed to quasistellar neutron spectra at $kT=5.1$ and 25 keV produced via the ${}^{18}\mathrm{O}(p,n){}^{18}\mathrm{F}$ and ${}^7\mathrm{Li}(p,n){}^7\mathrm{Be}$ reactions, respectively. The derived capture cross sections ${\langle \sigma \rangle }_{\mathrm{kT}=5\mathrm{keV}}=9.1\pm 0.3$ mb and ${\langle \sigma \rangle }_{\mathrm{kT}=25\mathrm{keV}}=2.03\pm 0.05$ mb are significantly lower than reported in literature. These results were used to substantially revise the radiative width of the first ${}^{23}\mathrm{Na}$ resonance and to establish an improved set of Maxwellian average cross sections. The implications of the lower capture cross section for current models of $s$-process nucleosynthesis are discussed.

Figure 1

Experimental neutron distributions at $kT=5.1$ keV (left) and 25 keV (right) compared to the respective Maxwell-Botzmann spectra.

Figure 2

$\gamma$ Spectrum for sample Na-1 after irradiation at $kT=25$ keV.

Figure 3

The measured spectrum averaged cross sections (full squares) and the best-fit solution obtained with the R-matrix code sammy [32], which is fully consistent with the data points (solid line). The FWHM of the neutron spectra are indicated by open boxes. The comparison with the ENDF/B-VII.1 evaluation [33] (dashed line) shows that it was sufficient to correct only the first resonance at 2.8 keV.

Figure 4

Calculated relative $s$-abundance distribution at the end of C-shell burning for a 25 ${\mathrm{M}}_{\odot }$ star compared to the distribution obtained with the MACS of ${}^{23}\mathrm{Na}$ from the kadonis compilation [39]. (Bottom) Isotopic ratios emphasizing the reduced neutron poison effect from the smaller MACS of ${}^{23}\mathrm{Na}$ from this work. (Isotopes of the same element are connected by solid lines.)

Figure 5

The ratio of $s$-process yields of a 1.5 ${\mathrm{M}}_{\odot }$ star with Z = 0.0001 obtained with the present and old MACS for ${}^{23}\mathrm{Na}$. Pure $s$ nuclei are highlighted by full circles; crosses outside the overall distribution are from branchings in the reaction path. (Top) For neutron captures in the ${}^{13}\mathrm{C}$ pocket ${}^{23}\mathrm{Na}$ acts as an additional seed. This contribution is reduced by the smaller MACS of this work. (Bottom) In less efficient ${}^{13}\mathrm{C}$ pockets the poisoning effect of ${}^{23}\mathrm{Na}$ dominates. Consequently, more free neutrons are available from the smaller MACS, thus relaxing the poisoning effect.