Measurement of ${}^{34}\mathrm{S}({}^3\mathrm{He},p){}^{36}\mathrm{Cl}$ cross sections for nuclide enrichment in the early solar system

Isotopic studies of meteorites have provided ample evidence for the presence of short-lived radionuclides (SLRs) with half-lives of less than 100 Myr at the time of the formation of the solar system. The origins of all known SLRs are heavily debated and remain uncertain, but the plausible scenarios can be broadly separated into either local production or outside injection of stellar nucleosynthesis products. The SLR production models are limited in part by reliance on nuclear theory for modeling reactions that lack experimental measurements. Reducing uncertainty on critical reaction cross sections can both enable more precise predictions and provide constraints on physical processes and environments in the early solar system. This goal led to the start of a campaign for measuring production cross sections for the SLR ${}^{36}\mathrm{Cl}$, where Bowers et al. found higher cross sections for the ${}^{33}\mathrm{S}(\alpha ,p){}^{36}\mathrm{Cl}$ reaction than were predicted by Hauser-Feshbach based nuclear reaction codes talys and non-smoker. This prompted re-measurement of the reaction at five new energies within the energy range originally studied, resulting in data slightly above but in agreement with talys. Following this, efforts began to measure cross sections for the next most significant reaction for ${}^{36}\mathrm{Cl}$ production, ${}^{34}\mathrm{S}({}^3\mathrm{He},p){}^{36}\mathrm{Cl}$. Activations were performed to produce nine samples between 1.11 MeV/nucleon and 2.36 MeV/nucleon. These samples were subsequently measured with accelerator mass spectrometry at two labs. The resulting data suggest a sharper-than-expected rise in cross sections with energy, with peak cross sections up to 30% higher than predictions from talys.

Figure 1

Reproduced from [18], 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

The results from a srim simulation of ${10}^4$ ions of ${}^{34}\mathrm{S}$ passing through the ${}^3\mathrm{He}$ filled gas cell for the lowest energy sample (68 MeV). 99.9% of recoils are caught within the 9 cm diameter.

Figure 3

The FN Tandem accelerator at the NSL and the AMS beamline are shown along with key elements. The AMS beamline underwent a redesign during a scheduled accelerator shutdown to improve transmission, and a high energy offset Faraday cup was installed after the analyzing magnet to improve AMS measurements.

Figure 4

Experimental data from AMS measurements are compared to predictions from the code talys and the adopted cross sections by Gounelle et al. [23]. The red points are the calculated integrated cross sections from NSL AMS data, and the black points are from PRIME Lab AMS data. In solid black are the cross sections used by Gounelle, in solid green is the predicted cross section curve with talys's default parameters, and the rest of the curves are results using one of the six different level density (LD) models available in the code. The dash-dotted lines are results using global/phenomenological models, with LD1 and LD2 (which produce the same results across this energy range) shown in red, and LD3 in blue. The dotted lines are results using microscopic level density models with LD4 in black, LD5 in red, and LD6 in blue.