Determination of the Maxwellian-averaged cross section of 35Cl(n,gamma)

In stellar environments neutron capture reactions produce 99% of all elements heavier than iron. In the slow neutron capture process (s-process) lighter isotopes, especially those with high abundances and large neutron capture cross sections, act as “neutron poisons” by reducing the number of available neutrons and hence, decreasing the production of the heavier nuclei. 35Cl, which is produced in advanced burning phases of stars, contributes to the poisoning effect. Accordingly, its Maxwellian-averaged cross section (MACS) is of significant astrophysical interest. A combination of activation technique and accelerator mass spectrometry (AMS) was used for the determination of the MACS of 35Cl(n,gamma)36Cl. NaCl pellets were irradiated at KIT and SARAF-LiLiT (Soreq Applied Research Accelerator Facility-Liquid-Lithium Target) with a quasi-Maxwellian neutron energy distribution of 25 keV. The neutron fluence was monitored by gold foils. AMS measurements at VERA (Vienna Environmental Research Accelerator), the DREAMS (DREsden AMS) facility and the ANU Heavy Ion Accelerator Facility quantified the 36Cl/35Cl ratio of the samples. The product of the neutron fluence and the isotopic ratio gives the spectrum-averaged cross section of the reaction. By normalizing this value to a real Maxwellian spectrum a new value for the 25 keV MACS of (9.5±0.4) mb for the reaction 35Cl(n,gamma)36Cl was obtained. This is ~16% lower than the previously determined value in Guber et al [1]. [1] K. H. Guber et al., Phys. Rev. C 65, 058801 (2002).