Stellar ${}^{36,38}\mathrm{Ar}(n,\gamma ){}^{37,39}\mathrm{Ar}$ Reactions and Their Effect on Light Neutron-Rich Nuclide Synthesis

The ${}^{36}\mathrm{Ar}(n,\gamma ){}^{37}\mathrm{Ar}$ ($t_{1/2}=35\mathrm{d}$) and ${}^{38}\mathrm{Ar}(n,\gamma ){}^{39}\mathrm{Ar}$ (269 yr) reactions were studied for the first time with a quasi-Maxwellian ($kT\sim 47\mathrm{keV}$) 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 ${}^{37}\mathrm{Ar}/{}^{36}\mathrm{Ar}$ and ${}^{39}\mathrm{Ar}/{}^{38}\mathrm{Ar}$ ratios in the activated samples were determined by accelerator mass spectrometry at the ATLAS facility (Argonne National Laboratory). The ${}^{37}\mathrm{Ar}$ activity was also measured by low-level counting at the University of Bern. Experimental MACS of ${}^{36}\mathrm{Ar}$ and ${}^{38}\mathrm{Ar}$, 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 ${}^{36,38}\mathrm{Ar}$ 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 ${}^{37}\mathrm{Ar}$ and ${}^{39}\mathrm{Ar}$ as environmental tracers in the atmosphere and hydrosphere.

Figure 1

(Right) Diagram of the LiLiT and activation target assembly. The ($\sim 1.5\mathrm{mA}$, $\sim 9\mathrm{mm}$ full width) proton beam (open red arrow) impinges on the free-surface lithium film (cyan) (see [19] for details). The Ar-filled sphere and Au foil are positioned in the outgoing neutron cone (green dotted lines) in a vacuum chamber separated from the LiLiT chamber. (Top left) Simulated neutron spectrum incident on the ${}^{36}\mathrm{Ar}$ sample (black) and a fit in the range $E_n\sim 0–110\mathrm{keV}$ with a Maxwell-Boltzmann flux (red) at $kT\sim 47\mathrm{keV}$. (Bottom left) Count rate (left $y$ axis) of fission chamber (see text) and calibration to proton current (right $y$ axis) during the ${}^{36}\mathrm{Ar}$ run.

Figure 2

Comparison of the ${}^{37}\mathrm{Ar}/{}^{36}\mathrm{Ar}$ ratio (at the end of irradiation) measured by AMS (black) and LLC (red).

Figure 3

Identification spectrum of ${}^{39}\mathrm{Ar}$ ions in the detector measured for the LiLiT irradiated ${}^{38}\mathrm{Ar}$ gas (top) and for nonirradiated ${}^{38}\mathrm{Ar}$ gas (bottom). The horizontal axis represents dispersion along the focal plane and the vertical axis represents a differential energy loss signal measured in the fourth anode of the focal-plane ionization chamber [36].

Figure 4

Comparison of the ${}^{36}\mathrm{Ar}$ (top) and ${}^{38}\mathrm{Ar}$ (bottom) $(n,\gamma )$ reaction rates ($N_A⟨\sigma v⟩$) extracted from this Letter (red) to the Kadonis [49] recommended values (black). The dashed curves encompass the estimated $1\sigma$ uncertainty.

Figure 5

(Top) Comparison of the mass fractions calculated for stable nuclei between ${}^{34}\mathrm{S}$ and ${}^{58}\mathrm{Fe}$ changing by $>10\%$ at the end of a single-zone calculation modeling He burning in a massive star, using literature rates [49] (solid circles) or replacing the ${}^{36,38}\mathrm{Ar}$$(n,\gamma )$ rates with the experimental values from this Letter (solid squares). We observe a smoother distribution of mass fractions in the vicinity of ${}^{36}\mathrm{Ar}$ when using the experimental cross sections. (Bottom) Ratio of the mass fractions using experimental and literature reaction rates as above.