Neutron Capture Cross Section Measurement of ${}^{151}\mathrm{S}\mathrm{m}$ at the CERN Neutron Time of Flight Facility (n_TOF)

The${}^{151}\mathrm{S}\mathrm{m}(n,\gamma ){}^{152}\mathrm{S}\mathrm{m}$ cross section has been measured at the spallation neutron facility n_TOF at CERN in the energy range from 1 eV to 1 MeV. The new facility combines excellent resolution in neutron time-of-flight, low repetition rates, and an unsurpassed instantaneous luminosity, resulting in rather favorable $\mathrm{\text{signal}}/\mathrm{\text{background}}$ ratios. The ${}^{151}\mathrm{S}\mathrm{m}$ cross section is of importance for characterizing neutron capture nucleosynthesis in asymptotic giant branch stars. At a thermal energy of $\mathrm{k}\mathrm{T}=30\mathrm{k}\mathrm{e}\mathrm{V}$ the Maxwellian averaged cross section of this unstable isotope ($t_{1/2}=93\mathrm{y}\mathrm{r}$) was determined to be $3100\pm 160\mathrm{m}\mathrm{b}$, significantly larger than theoretical predictions.

Figure 1

The $s$-process network in the mass region of ${}^{151}\mathrm{S}\mathrm{m}$. Branchings of the reaction path occur at ${}^{151}\mathrm{S}\mathrm{m}$, ${}^{152,154,155}\mathrm{E}\mathrm{u}$, and ${}^{153}\mathrm{G}\mathrm{d}$. The branching effect at ${}^{152}\mathrm{E}\mathrm{u}$ is completely dominated by the $\beta$-decay rate, which is strongly enhanced at stellar temperatures. Branching point nuclei and $s$-only isotopes are indicated by dashed and double boxes, respectively.

Figure 2

Capture yields for ${}^{151}\mathrm{S}\mathrm{m}$ plotted with the overall background. The inset shows a SAMMY fit [9] of the resonances between 490 and 550 eV.

Figure 3

Cumulative number of levels at the neutron separation energy of ${}^{152}\mathrm{S}\mathrm{m}$ fitted at energies below 400 eV.

Figure 4

Maxwellian averaged $(n,\gamma )$ cross section of ${}^{151}\mathrm{S}\mathrm{m}$ (shaded band) and previous calculations (symbols) [2].