Study of Astrophysical s-Process Neutron Capture Reactions at the High-Intensity SARAF-LiLiT Neutron Source

We report on recent experiments at the Soreq Applied Research Accelerator Facility Liquid-Lithium Target (SARAF-LiLiT) laboratory dedicated to the study of s -process neutron capture reactions. The kW-power proton beam at 1.92 MeV (1-2 mA) from SARAF Phase I yields high-intensity 30 keV quasi-Maxwellian neutrons (3-5×1010 n/s). The high neutron intensity enables Maxwellian averaged cross sections (MACS) measurements of low-abundance or radioactive targets. Neutron capture reactions on the important s -process branching points 147 Pm and 171 Tm were investigated by activation in the LiLiT neutron beam and γ -measurements of their decay products. MACS values at 30 keV extracted from the experimental spectrum-averaged cross sections are obtained and will be discussed. The Kr region, at the border between the so-called weak and strong s -process was also investigated. Atom Trap Trace Analysis (ATTA) was used for the first time for the measurement of a nuclear reaction cross section. After activation in the quasi-Maxwellian neutron flux at SARAF-LiLiT, isotopic ratios were determined for 81 Kr(230 ky)/80 Kr and 85g Kr(10.8 y)/84 Kr. The latter ratio was confirmed both by low-level β counting and γ spectrometry. The shorter-lived capture products 79,85m ,87 Kr were detected by γ -spectrometry and the corresponding neutron-capture MACS of the respective target nuclei 78,84,86 Kr were determined. The MACS of the 80 Kr(n, γ )81 Kr and 84 Kr(n, γ )85g Kr reactions are still under study. The partial MACS leading to 85m Kr(4.5 h) measured in this experiment has interesting implications since this state decays preferentially by γ decay (79%) to 85 Rb on a faster time scale than does 85g Kr and behaves thus as an s -process branching point.