Thermonuclear Reaction Rate of 17O(p,γ)18F
University of North Carolina, Chapel Hill
Thermonuclear explosions can occur during a star′s life cycle and stars often experience cataclysmic disruptions during death throes. However, in some binary star systems, a degenerate stellar corpse — a white dwarf — can experience a non-disruptive explosive event. Prior this, a white dwarf will leach hydrogen-rich matter from its binary companion as this “host” star evolves. An accretion disk can form and layers of hydrogen build up on the surface of the compact object. Compression eventually will drive the underbelly of this hydrogen layer into degeneracy. As temperatures rise, there's no mechanism to cool the stellar plasma and a thermonuclear runaway occurs. A major element created during the explosion is fluorine-18, which decays by emitting a positron. Positron annihilation-produced radiation contributes to ejection of nuclear “ash” produced by explosive hydrogen burning during classical novae. Thus thermonuclear reactions creating and destroying fluorine-18 are key and must be studied experimentally.
One fluorine-18 production mechanism is 17O(p,γ)18F. This reaction also affects oxygen-17 abundances. Classical novae are thought to be our galaxy's dominant oxygen-17 source. The importance of the 17O(p,γ)18F reaction's non-resonant component is known and numerous studies have been performed to analyze this reaction experimentally. However, the temperature regime (Gamow window) relevant to explosive hydrogen burning during classical novae (100−400 MK) corresponds to low proton-bombarding energies. At these low energies, the Coulomb barrier suppresses the laboratory reaction yield, and many accelerator facilities can't overcome this limitation. Environmental backgrounds also dominate the detected signal, making it impossible to differentiate the direct capture γ-cascade from background. The Laboratory for Experimental Nuclear Astrophysics (LENA) has tools that can overcome these limitations. The LENA electron cyclotron resonance ion source produces intense, low-energy protons that boost the thermonuclear reaction yield. LENA also has a coincidence detector that reduces the environmental background contribution, allowing extraction of the direct capture γ fingerprint. These tools should allow me to further constrain 17O(p,γ)18F direct-capture reaction rates. My latest results will be reported.