Investigation of the impact of the $^{39}$Ar(n , $\alpha)^{36}$S reaction on the nucleosynthesis of the rare isotope $^{36}$S

The origin of the rare, neutron rich isotope $^{36}$S remains a debated question. One of the key reactions in the s-process nucleosynthesis network leading to $^{36}$S is $^{39}$Ar(n , $\alpha) ^{36}\!$S. This reaction has never been studied so far, which is due to the fact that $^{39}$Ar is a radioactive (T$_{1/2}$ = 269 y) gas, which is not commercially available. During a three days experimental campaign, an optimized $^{39}$Ar sample was prepared at ISOLDE. A dedicated titaniumoxide target (8 g/cm$^{2}$) was bombarded with 1 GeV protons from the PS Booster. In order to obtain a pure argon beam, a water-cooled transfer line was used to freeze-out less volatile isobars before they can reach the ion source. Adding stable argon with a calibrated leak to the ion source enabled to determine the ionization efficiency (3.5%). For the isotope separation, the low-mass side (GLM) of the General Purpose Separator was used. After magnetic separation, $^{39}$Ar ions (1+) were implanted at 60 keV in a 12 mm thick aluminium foil. The yield was about 5 $\times$10$^{8}$ $^{39}$Ar ions per $\mu$C. The number of atoms was derived from a measurement of the 0.6 MeV $\beta$'s emitted during the decay of $^{39}$Ar using a 4$\pi$ pressurized proportional counter. This yielded a number of 3$\times$10$^{14}\,^{39}\!$Ar atoms on the sample. With this sample, a first experiment has been performed at a thermal neutron beam of the high flux reactor of the Institut Laue-Langevin in Grenoble (F). From this measurement an upper-limit of 0.26$\beta$ could already be determined for the $^{39}$Ar(n$_{th} , \alpha)^{36}\!$S reaction cross section, which is one order of magnitude lower than the theoretical estimation.