StellarAr36,38(n,γ)Ar

Abstract

The ^{36}Ar(n,γ)^{37}Ar (t_{1/2}=35 d) and ^{38}Ar(n,γ)^{39}Ar (269 yr) reactions were studied for the first time with a quasi-Maxwellian (kT∼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) 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.

Cosmogenic production of Ar39 and Ar37 in argon

“…See text for details and references. and capture cross-sections on 36 Ar (red) and 38 Ar (blue) along with experimental data points at thermal energies [70][71][72]. 40 Ar(p, 2p) 39 Cl and 40 Ar(p, pn) 39 Ar At sea-level the flux of 10-100 MeV cosmic ray protons is at least thirty times lower than that of cosmic ray neutrons due to the additional electromagnetic interactions of protons in the atmosphere.…”

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

“…where N A+1 is the number of A+1 Ar detected, is the detector efficiency, t the counting time, q the ion charge state (18 for 37 Ar and 8 for 39 Ar), e the electronic charge in C, and i A the A Ar q+ beam intensity (nA); λ = ln(2) t 1/2 is the A+1 Ar decay constant and t cool is the time between end of irradiation and counting. The A+1 Ar/ A Ar ratios measured lead to spectrum-averaged capture cross sections of σ exp = 1.4(1) and 0.95(10) mb for 36 Ar and 38 Ar (see [18]) in the conditions of our experiments. The MACS at a given thermal energy kT were extracted from these experimental cross sections with the procedure developed in [11]) as above, using Hauser-Feshbach models (e.g.…”

“…The 37 Ar activity, measured by Auger electrons, was also determined in aliquots of the same gas samples by low-level counting (LLC) in an underground laboratory [27,28] at the University of Bern. Excellent agreement was obtained between the two methods [18]. The isotopic ratios r = A+1 Ar/ A Ar at the end of irradiation are determined by…”

“…The technique is applicable for the detection of reaction products for which decay counting is not practical due either to a very long half-life or decay radiation difficult to detect. We recently used the AMS atom-counting method for the study of the 38 Ar(n, γ) 39 Ar (269 y) and 36 Ar(n, γ) 37 Ar(35.0 d) reactions after neutron irradiation at SARAF-LiLiT [18]. The 36 Ar and 38 Ar argon isotopes are among the rare stable nuclides for which the neutron-capture cross sections above thermal energy was never measured experimentally.…”

“…Calculations done using the single-zone NucNet Tools reaction network code [30] starting at the H-burning phase with solar abundances [21] and continuing into a singlezone He core burning (T= 300 MK, density of 1 kg/cm 3 ) show significant (10-50%) changes [18] in the calculated mass fractions for neutron-rich light nuclides between 34 S and 58 Fe, reminiscent of the sensitivity observed in the weak s-process region (A≈ 56-70) due to the change of a single cross section [31].…”