Stellar 36,38^{36,38}Ar(n,γ)37,39(n,\gamma)^{37,39}Ar reactions and their effect on light neutron-rich nuclide synthesis

The $^{36}$Ar$(n,\gamma)^{37}$Ar ($t_{1/2}$ = 35 d) and $^{38}$Ar$(n,\gamma)^{39}$Ar (269 y) reactions were studied for the first time with a quasi-Maxwellian ($kT \sim 47$ keV) neutron flux for Maxwellian Average Cross Section (MACS) measurements at stellar energies. Gas samples were irradiated at the high-intensity Soreq applied research accelerator facility-liquid-lithium target neutron source and the $^{37}$Ar/$^{36}$Ar and $^{39}$Ar/$^{38}$Ar ratios in the activated samples were determined by accelerator mass spectrometry at the ATLAS facility (Argonne National Laboratory). The $^{37}$Ar activity was also measured by low-level counting at the University of Bern. Experimental MACS of $^{36}$Ar and $^{38}$Ar, corrected to the standard 30 keV thermal energy, are 1.9(3) mb and 1.3(2) mb, respectively, differing from the theoretical and evaluated values published to date by up to an order of magnitude. The neutron capture cross sections of $^{36,38}$Ar are relevant to the stellar nucleosynthesis of light neutron-rich nuclides; the two experimental values are shown to affect the calculated mass fraction of nuclides in the region A=36-48 during the weak $s$-process. The new production cross sections have implications also for the use of $^{37}$Ar and $^{39}$Ar as environmental tracers in the atmosphere and hydrosphere.Comment: 18 pages + Supp. Mat. (13 pages) Accepted for publication in Phys. Rev. Let

Stellar s -process neutron capture cross sections on 78,80,84,86Kr determined via activation, atom trap trace analysis, and decay counting

We present a detailed account of neutron capture experiments of astrophysical relevance on 78,80,84,86Kr (n ,γ ) reactions at the border between weak and main s process. The experiments were performed with quasi-Maxwellian neutrons from the Liquid-Lithium Target (LiLiT) and the mA-proton beam at 1.93 MeV (2-3 kW) of the Soreq Applied Research Accelerator Facility (SARAF). The setup yields high-intensity ≈40 keV quasi-Maxwellian neutrons (3-5 ×1010 n /s) closely reproducing the conditions of s -process stellar nucleosynthesis. A sample of 100 mg of atmospheric, pre-nuclear-age Kr gas contained in a Ti spherical shell was activated in the LiLiT neutron field. The abundances of long-lived Kr isotopes (81,85 gKr) were measured by atom counting via atom trap trace analysis (ATTA) at Argonne National Laboratory and low-level counting (LLC) at University of Bern. This work is the first measurement of a nuclear cross section using atom counting via ATTA. The activities of short-lived Kr isotopes (79,85 m,87Kr) were measured by γ -decay counting with a high-purity germanium detector. Maxwellian-averaged cross sections for s -process thermal energies are extracted. By comparison to reference values, our nucleosynthesis network calculations show that the experimental cross sections have a strong impact on calculated abundances of krypton and neighboring nuclides, in some cases improving agreement between theory and observations. </SUP

Neutron Capture on the s-Process Branching Point 171^{171}Tm via Time-of-Flight and Activation

The neutron capture cross sections of several unstable nuclides acting as branching points in the s process are crucial for stellar nucleosynthesis studies. The unstable $^{171}$Tm(t$_{1/2}$=1.92 yr) is part of the branching around mass A∼170 but its neutron capture cross section as a function of the neutron energy is not known to date. In this work, following the production for the first time of more than 5 mg of $^{171}$Tm at the high-flux reactor Institut Laue-Langevin in France, a sample was produced at the Paul Scherrer Institute in Switzerland. Two complementary experiments were carried out at the neutron time-of-flight facility (n_TOF) at CERN in Switzerland and at the SARAF liquid lithium target facility at Soreq Nuclear Research Center in Israel by time of flight and activation, respectively. The result of the time-of-flight experiment consists of the first ever set of resonance parameters and the corresponding average resonance parameters, allowing us to make an estimation of the Maxwellian-averaged cross sections (MACS) by extrapolation. The activation measurement provides a direct and more precise measurement of the MACS at 30 keV: 384(40) mb, with which the estimation from the n_TOF data agree at the limit of 1 standard deviation. This value is 2.6 times lower than the JEFF-3.3 and ENDF/B-VIII evaluations, 25% lower than that of the Bao et al. compilation, and 1.6 times larger than the value recommended in the KADoNiS (v1) database, based on the only previous experiment. Our result affects the nucleosynthesis at the A∼170 branching, namely, the $^{171}$Yb abundance increases in the material lost by asymptotic giant branch stars, providing a better match to the available pre-solar SiC grain measurements compared to the calculations based on the current JEFF-3.3 model-based evaluation

Stellar s-process neutron capture cross sections on 78,80,84,86Kr determined via activation, atom trap trace analysis, and decay counting

We present a detailed account of neutron capture experiments of astrophysical relevance on reactions at the border between weak and main process. The experiments were performed with quasi-Maxwellian neutrons from the Liquid-Lithium Target (LiLiT) and the mA-proton beam at 1.93 MeV (2-3 kW) of the Soreq Applied Research Accelerator Facility (SARAF). The setup yields high-intensity keV quasi-Maxwellian neutrons (3-5 /s) closely reproducing the conditions of -process stellar nucleosynthesis. A sample of 100 mg of atmospheric, pre-nuclear-age Kr gas contained in a Ti spherical shell was activated in the LiLiT neutron field. The abundances of long-lived Kr isotopes () were measured by atom counting via atom trap trace analysis (ATTA) at Argonne National Laboratory and low-level counting (LLC) at University of Bern. This work is the first measurement of a nuclear cross section using atom counting via ATTA. The activities of short-lived Kr isotopes () were measured by -decay counting with a high-purity germanium detector. Maxwellian-averaged cross sections for -process thermal energies are extracted. By comparison to reference values, our nucleosynthesis network calculations show that the experimental cross sections have a strong impact on calculated abundances of krypton and neighboring nuclides, in some cases improving agreement between theory and observations

Stellar 30-keV neutron capture in 94, 96Zr and the Zr90(γ,n)Zr89 photonuclear reaction with a high-power liquid-lithium target

A high-power Liquid-Lithium Target (LiLiT) was used for the first time for neutron production via the thick-target Li7(p,n)Be7 reaction and quantitative determination of neutron capture cross sections. Bombarded with a 1–2 mA proton beam at 1.92 MeV from the Soreq Applied Research Accelerator Facility (SARAF), the setup yields a 30-keV quasi-Maxwellian neutron spectrum with an intensity of 3–5×1010 n/s, more than one order of magnitude larger than present near-threshold Li7(p,n) neutron sources. The setup was used here to determine the 30-keV Maxwellian averaged cross section (MACS) of 94Zr and 96Zr as 28.0±0.6 mb and 12.4±0.5 mb respectively, based on activation measurements. The precision of the cross section determinations results both from the high neutron yield and from detailed simulations of the entire experimental setup. We plan to extend our experimental studies to low-abundance and radioactive targets. In addition, we show here that the setup yields intense high-energy (17.6 and 14.6 MeV) prompt capture γ rays from the Li7(p,γ)Be8 reaction with yields of ∼3×108γs−1mA−1 and ∼4×108γs−1mA−1, respectively, evidenced by the Zr90(γ,n)Zr89 photonuclear reaction

Bismuth activation with quasi-Maxwellian neutrons at kT ∼ 30 keV

Bismuth capture of neutrons is the termination point of the s-process cycle of nucleosynthesis in stellar environments. A new measurement is reported here for neutron activation of bismuth with an intense quasi-Maxwellian neutron source at kT ∼ 30 keV. The measurement was performed at the SARAF phase I accelerator facility by bombarding a 1.5-mA proton beam on the liquid-lithium larget. The cross section of the 209Bi(n,γ ) capture reaction leading to the 210Bi ground state was determined by combining β measurements from the 210gBi decay and α and γ from the subsequent 210Po decay, along with detailed Monte Carlo simulations of the 7Li(p,n) reaction kinematics and the activation experimental setup. Deduced Maxwellian averaged cross sections (MACS) for 209Bi(n,γ )210gBi at kT = 30 keV using the ENDF, JEFF, and JENDL databases for the corrections and extrapolations yielded a value of 1.84 ± 0.09 mb. A comparison is made with previous measurements, including time-of-flight (TOF) measurements of the total bismuth capture cross section. Plans for obtaining the MACS for capture to the bismuth-210 metastable state in the reaction 209Bi(n,γ )210mBi are discussed, along with estimates based on our results in comparison with TOF measurements. The bismuth neutron activation cross section is also of importance for design of GenIV reactor coolant and subcritical accelerator driven systems, especially in light of the 3 million year half-life of the 210mBi isomer.JRC.G.2-Standards for Nuclear Safety, Security and Safeguard