Doctor of Philosophy (PhD)
Physics and Astronomy
Reactions involving radioactive nuclei play an important role in stellar explosions, but limited experimental information is available due to difficulties in producing and studying radioactive nuclei. Several facilities aim to provide greater access to these unstable isotopes at higher intensities, but more efficient and selective techniques and devices are required to be able to study many important reactions. The Array for Nuclear Astrophysics and Structure with Exotic Nuclei (ANASEN), a charged particle detector designed by Louisiana State University (LSU) and Florida State University (FSU), was created for this purpose. ANASEN is used to study the reactions important for the αp- and rp- processes with proton-rich exotic nuclei by providing essentially complete solid angle coverage through an array of 40 silicon-strip detectors backed with CsI(Tl) scintillators, covering an area of roughly 1300 cm2. ANASEN includes an active gas target/detector using a position-sensitive annular gas proportional counter that allows direct measurement of (α,p) reactions in inverse kinematics. Measurements with a partial implementation of ANASEN were first performed at the RESonator SOLenoid Upscale Transmission (RESOLUT) radioactive beam facility of FSU during the summer of 2011. This included stable beam experiments and measurements of the 17F(p,p)17F and 17F(p,α)14O reactions that are important for understanding the structure of 18Ne and the 14O(α,p)17F reaction rate. We report in detail on one experiment using a 55 MeV beam of 17F to bombard a 2 mg/cm2 thick polypropylene target. Scattered protons were detected in a silicon strip detector array in coincidence with recoiling heavy ions in a gas ionization chamber that provided clean identification of the reaction channels. The center-of-mass energy for each event was reconstructed from the proton energy and angle, allowing the 17F(p,p)17F elastic scattering cross section to be measured between center-of-mass energies of 1.95 MeV and 3.06 MeV. The measured cross sections were fit with theoretical cross sections using phenomenological R-matrix theory. We conclusively observe two resonances. One corresponds to a 1- state in 18Ne, and we determine Ex = 6142 (5)stat (8)sys keV with total width Γ = 45 (12) keV. We are also able to set a lower limit on the proton partial width to the ground state of Γp > 15 keV at the 90% confidence level. We also observe a 2- state that we determine to be at Ex = 6373 (8)stat (8)sys keV with a total width equal to the partial width to the ground state of Γ = Γp = of 87 (15) keV. While we can not resolve a previously reported 3- resonance in this same energy regime, we can set a stringent limit on its proton partial width to the 17F ground state of Γp < 4 keV at the 90% confidence level.
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Pratt, Laura, "Study of ¹⁸NE using the array for Nuclear Astrophysics and Structure with exotic nuclei" (2014). LSU Doctoral Dissertations. 97.