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

Area of Science:

• Nuclear Astrophysics
• Stellar Nucleosynthesis
• Accelerator Mass Spectrometry

Background:

• Understanding neutron-capture cross sections is crucial for modeling stellar nucleosynthesis.
• Previous data for ^{36}Ar and ^{38}Ar neutron-capture reactions at stellar energies were limited.
• Argon isotopes like ^{37}Ar and ^{39}Ar have applications as environmental tracers.

Purpose of the Study:

• To measure the Maxwellian average cross sections (MACS) for ^{36}Ar(n,γ)^{37}Ar and ^{38}Ar(n,γ)^{39}Ar reactions at stellar energies for the first time.
• To provide accurate experimental cross-section data for improving stellar models.
• To assess the impact of these cross sections on the weak s-process and the use of Argon isotopes as tracers.

Main Methods:

• Irradiation of gas samples with a quasi-Maxwellian neutron flux (kT∼47 keV) at the Soreq applied research accelerator facility.
• Determination of ^{37}Ar/^{36}Ar and ^{39}Ar/^{38}Ar ratios using accelerator mass spectrometry at the ATLAS facility.
• Measurement of ^{37}Ar activity via low-level counting at the University of Bern.

Main Results:

• Experimental MACS for ^{36}Ar and ^{38}Ar at 30 keV were determined to be 1.9(3) mb and 1.3(2) mb, respectively.
• These experimental values differ by up to an order of magnitude from previously published theoretical and evaluated data.
• The new cross-section data significantly affect calculated mass fractions of nuclides in the A=36-48 region during the weak s-process.

Conclusions:

• The study provides the first experimental MACS for ^{36,38}Ar at stellar energies, revealing discrepancies with theoretical predictions.
• These findings have important implications for understanding the production of light neutron-rich nuclides in stars.
• The revised cross-section data are also relevant for the application of ^{37}Ar and ^{39}Ar as atmospheric and hydrospheric tracers.