Direct and Indirect Studies of Astrophysical Reactions

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The vast majority of the elements making up the world around us were formed through sequences of nuclear reactions and decays occurring in stars. These nuclear processes are responsible for energy generation in quiescent stellar burning, as well as stellar explosions such as novae, supernovae, and X-ray bursts. At the same time, these reaction/decay chains transmutate the initial "seed" elements (hydrogen and helium, formed in the Big Bang) into heavier species - eventually filling nearly the entire periodic table. The overarching goal of this research project is to better understand the rates of key reactions which have a significant impact on nucleosynthesis and/or energy generation in stellar burning and stellar explosions. The rates of key reactions - which have been identified in sensitivity studies involving computer simulations of stellar processes - will be studied through direct and indirect measurements using stable and rare-isotope beams from the TRIUMF/ISAC accelerator facility. Direct measurements will employ the world-leading DRAGON recoil separator, including an upgraded LaBr ?-ray detector array, to measure the strengths and energies of key resonances that dominate stellar proton and alpha capture rates. In these direct-measurement experiments, resonance strengths will be determined by impinging a heavy-ion beam onto an extended, windowless hydrogen or helium gas target, with the beam energy tuned to be on resonance. The resulting reaction products (heavy recoils and ? rays) will be counted and used to determine the reaction yield, which is proportional to the resonance strength. At the same time, the position of the reaction within the extended target will be used to precisely determine the resonance energy. In many cases, the resonance energy measurement will make use of a novel "resonance timing" technique, which capitalizes on the superior timing resolution of LaBr scintillators. For reactions that are too weak to measure in the laboratory, or for which sufficient rare-isotope beam intensities are not available, indirect techniques will be used to determine the resonance strengths of interest. In particular, transfer reactions will be used to measure spectroscopic factors for important resonances - quantities which are directly proportional to the resonance strength. The indirect-measurement component of the proposal will include development of a new program of inverse-kinematics (d,n) experiments at TRIUMF, using the IRIS solid H2 target and the TexNEUT neutron detector array currently under construction. This promising technique offers an opportunity to indirectly determine key resonance strengths for a wide array of reactions for which direct measurements are not presently feasible.