UNC-CH Physics and Astronomy PhD Defense

Andrew L. Cooper

“A Low-Energy Study of the 20Ne(proton,gamma)21Na Reaction with High-Current Proton Beams at LENA”

The neon-sodium (NeNa) cycle is believed to play a pivotal role in explaining the oxygen-sodium abundance anticorrelation exhibited by members of all carefully observed globular clusters, but not seen in field stars. High-current, low-energy cross section measurements of the 20Ne(p,g)21Na reaction at the Laboratory for Experimental Nuclear Astrophysics (LENA) can clarify the rate at which the NeNa cycle occurs and enhance our understanding of stellar nucleosynthesis in the globular cluster environment. However, the primary challenge in directly measuring nuclear reaction rates at stellar energies is their extraordinarily small cross sections (often < 10^(−9) b). Combined with novel detection techniques, this hurdle can be overcome by employing high-current (mA-range), pulsed proton beams from the electron cyclotron resonance ion source (ECRIS) at LENA. Development and performance highlights of this system are described. This ion source also provided low-energy 20Ne+ beams to implant the necessary tantalum and titanium targets for this experiment. Target stoichiometries were measured via nuclear reaction and Rutherford backscattering analyses and verified with simulations in TRIM-2013. A lead-shielded, dual high-purity germanium detector arrangement in a 55-90 degree geometry was constructed for this experiment to minimize angular correlation and Doppler-shift effects within collected spectra. This detection system was characterized using Monte Carlo-based simulations in GEANT4. Experimental results and astrophysical implications from recent measurements of the strength and primary decay branching ratios of the E(lab) = 384 keV resonance in 20Ne(p,g)21Na at LENA are provided. Also, a measurement of the total cross section at E(lab) = 330 keV is presented. In closing, an experimental roadmap for future low-energy measurements of 20Ne(p,g)21Na reaction, which is based off of estimates that incorporate these new results, is described.