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

TL;DR

This study measures the neutron capture cross sections of argon isotopes at stellar energies, revealing significant differences from theoretical values and impacting models of light neutron-rich nuclide synthesis and environmental tracing.

Contribution

First experimental MACS measurements for $^{36}$Ar and $^{38}$Ar at stellar energies, showing discrepancies with prior data and influencing nucleosynthesis models.

Findings

Measured MACS values are 1.9(3) mb for $^{36}$Ar and 1.3(2) mb for $^{38}$Ar.

Experimental cross sections differ from theoretical and evaluated data by up to an order of magnitude.

Results impact understanding of light neutron-rich nuclide synthesis and environmental tracer applications.

Abstract

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.