Re-measurement of the S33(α,p)Cl36 cross section for early solar system nuclide enrichment

Abstract

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 Cl production cross sections via the 33 S(α,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 S(α,p) 36 Cl reaction was re-measured at 5 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.

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

“…This suggests that, while the general shape of the cross section curve is correct, the predictions peak at too low of an energy. The results observed for 33 S(α,p) [20] showed an apparent disagreement with the trend of Hauser-Feshbach codes slightly over-predicting cross sections for α-induced reactions [19], and while this reaction was 3 He-induced it may still be similarly categorized. To ensure confidence in the experimental results, all sources FIG.…”

“…Any created 36 Cl atoms were forward-recoiled and implanted into an Al catcher foil at the back of the cell. Aside from minor changes made to re-capture the target 3 He gas and minimize its losses, this method has been successfully used to produce samples from the α-induced reactions 40 Ca(α,γ) 44 Ti [21] and 33 S(α,p) 36 Cl [18,20]. The reaction energies were chosen based upon the predicted cross sections using the default parameters of TALYS, the energy loss through the gas cell as calculated by SRIM [22], and the voltage limitations of the accelerator.…”

“…The same method for sample preparation was used at both the NSL and PRIME Lab, and is identical to the method used in [20]. To prepare a sample for use in a sputtering ion source, the extracted AgCl powder was gently pressed into the face of AgBr-packed cathodes which were warmed on a hot plate to drive away moisture.…”

“…The resulting data showed a systematic under-prediction of cross sections by the Hauser-Feshbach codes TALYS and NON-SMOKER, the irregularity of which was highlighted in a following paper by Mohr [19]. In response, the reaction was re-measured at 5 points in the energy range where experimental data and theory were most discrepant between 0.78 MeV/nucleon and 1.52 MeV/nucleon, with a special focus paid to the procedures followed by Bowers et al to rule out any experimental errors [20]. The new samples were produced shortly before a scheduled maintenance shutdown, so they were measured at Purdue's Rare Isotope Measurement Laboratory (PRIME Lab).…”

“…This work represents the next step in the 36 Cl campaign with measurements of 34 S( 3 He,p) 36 Cl across as wide an energy range as was experimentally feasible given experimental equipment limitations and predicted reaction cross sections. The details of the experiment are nearly identical to those for the re-measurement of the 33 S(α,p) 36 Cl reaction discussed in [20] and are broken into three parts: activations, extraction chemistry, and sample measurement with Accelerator Mass Spectrometry (AMS).…”