Investigating the O-Na Anticorrelation in Globular Clusters through Improved Low Energy Cross Section Measurements of the ²⁰Ne(p,γ)²¹Na  keV Resonance at LENA

Within every Globular Cluster (GC) there exists a star-to-star variation in the abundances of light elements. Specifically, there is an anticorrelation between oxygen (O) and sodium (Na) across all GCs. Considering the evolution of an intermediate-mass star, an overabundance of Na would be most likely to occur at the end of a star’s evolution during the Thermally Pulsing Asymptotic Giant Branch (TP-AGB) phase, where temperatures are sufficient for Na production via the neon-sodium (NeNa) cycle. However, some stars in phases as soon as the Turn-Off and early Sub-Giant Branch display this overabundance of Na. The neon-sodium (NeNa) cycle is the best candidate for creating excess Na. Although this cycle commences best in TP-AGB stars, it could contribute to the overabundance of Na and depletion of O through the simultaneous operation of the CNO cycles. Through mass loss in the AGB phase, this signature would be passed on to other, less-evolved stars within the cluster, as well as the next generation of stars. The first reaction of the NeNa cycle, ²⁰Ne(p,γ)²¹Na, determines the rate at which the cycle proceeds. Specifically, there exists a  keV subthreshold resonance that dominates the fractional contribution at relevant stellar energies. However, measuring this cross section in a laboratory presents a unique set of challenges owing to the slow reaction rate and the presence of both cosmic-ray and natural background radiation at the energies of interest. At the Laboratory for Experimental Nuclear Astrophysics (LENA), improved measurements of the  keV cross section will be taken with the aid of a coincidence spectrometer composed of a High-Purity Germanium (HPGe) detector and a thallium-doped sodium iodide (NaI(Tl)) annulus. A Cosmic Ray Veto (CRV) has also been implemented to reduce the signal-to-background ratio throughout the experiment. A state-of-the-art Singletron accelerator, capable of producing high-current, pulsed beams at low energies down to 250 keV, has also been installed at LENA to further decrease this ratio for low-energy measurements.

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Topic: Churchman Thesis Proposal Presentation

Time: Feb 12, 2024 10:00 AM Eastern Time (US and Canada)

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