Re-measurement of the ${}^{33}\mathrm{S}(\alpha ,p){}^{36}\mathrm{Cl}$ cross section for early solar system nuclide enrichment

Short-lived radionuclides (SLRs) with half-lives less than 100 Myr are known to have existed around the time of the formation of the solar system around 4.5 billion years ago. Understanding the production sources for SLRs is important for improving our understanding of processes taking place just after solar system formation as well as their timescales. Early solar system models rely heavily on calculations from nuclear theory due to a lack of experimental data for the nuclear reactions taking place. In 2013, Bowers et al. measured ${}^{36}\mathrm{Cl}$ production cross sections via the ${}^{33}\mathrm{S}$($\alpha ,p$) reaction and reported cross sections that were systematically higher than predicted by Hauser-Feshbach codes. Soon after, a paper by Peter Mohr highlighted the challenges the new data would pose to current nuclear theory if verified. The ${}^{33}\mathrm{S}(\alpha ,p){}^{36}\mathrm{Cl}$ reaction was re-measured at five energies between 0.78 MeV/nucleon and 1.52 MeV/nucleon, in the same range as measured by Bowers et al., and found systematically lower cross sections than originally reported, with the new results in good agreement with the Hauser-Feshbach code talys. Loss of Cl carrier in chemical extraction and errors in determination of reaction energy ranges are both possible explanations for artificially inflated cross sections measured in the previous work.

Figure 1

Reproduced from [13], a schematic of the gas cell used for the activations. A 9 cm diameter Al foil was used to catch the forward-recoiled ${}^{36}\mathrm{Cl}$ atoms.

Figure 2

Reproduced from [13], a srim simulation of ${10}^4$ ions of ${}^{33}\mathrm{S}$ passing through the ${}^4\mathrm{He}$ filled gas cell for the original highest energy sample (104.5 MeV). (a) A histogram of the lateral distribution of ions implanted in the catcher foil, with the beam incident on the gas cell at radius $=0$. (b) A 2D histogram showing the distribution of ions across the 9 cm diameter cross section of the catcher foil present to the beam.

Figure 3

A comparison of the current work's measurements (black), those of Bowers et al. (blue), and cross sections calculated by talys (red). Note the two overlapping pairs of points above 1 MeV/nucleon which show the agreement between samples 2a and 2b, and 3a and 3b.