Title page for ETD etd-01192016-134111

Type of Document Dissertation
Author Afanasieva, Liudmyla
URN etd-01192016-134111
Title Nuclear Structure of 26Si and 32Cl for Astrophysics
Degree Doctor of Philosophy (Ph.D.)
Department Physics & Astrononomy
Advisory Committee
Advisor Name Title
Blackmon, Jeffery Committee Chair
Deibel, Catherine Committee Member
Lee, Hwang Committee Member
Tzanov, Martin Committee Member
Tyagi, Mayank Dean's Representative
  • astrophysics
  • nuclear
Date of Defense 2015-12-07
Availability unrestricted
We studied the nuclear structure of two isotopes, 26Si and 32Cl, important for understanding stellar explosions like novae and Type I X-ray bursts. The 31S(p,γ)32Cl reaction rate influences the enrichment of sulfur observed in some nova ejecta, but the uncertainty in the rate spans as much as an order of magnitude and arises from uncertainties in the properties of resonances corresponding to excited states in 32Cl.

We populated states in 32Cl via the 10B(24Mg,2n)32Cl reaction using the Argonne Tandem-Linac Accelerator System (ATLAS), with a 75 MeV beam of 24Mg bombarding a 200 μg/cm2 10B target. Gamma rays emitted from recoiling heavy nuclei were detected by Gammasphere, and the Argonne Fragment Mass Analyzer (FMA) was used to separate heavy ions. We built the level scheme for 32Cl from gamma-gamma coincidences, determining energies for 6 states, including 2 levels at Ex = 1738.1 (6) and 2130.5 (10) keV that correspond to the most important resonances in the 31S(p,γ)32Cl reaction at Ecm = 156.3(7) and 549.9(8) keV. With the resonance energies established, the single uncertainty dominating the 31S(p,γ)32Cl reaction rate is the strength of the 549.9 keV resonance.

The 22Mg(α,p)25Al reaction plays an important role in type I X-ray bursts. We studied the structure of states in 26Si corresponding to potential resonances in the 22Mg(α,p)25Al reaction by measuring 25Al+p elastic scattering at the John D. Fox Superconducting Accelerator Laboratory at Florida State University. A secondary 25Al radioactive ion beam at 102.5 MeV bombarded a 2.05 mg/cm2 polypropylene target. Scattered protons were detected using an array of silicon strip detectors, and the heavy ions were detected in a gas ionization chamber.

The center-of-mass energy for each event was reconstructed from the measured energy and angle of the protons, and the differential cross section for 25Al+p scattering was determined for center of mass energies of 2.7-4.0 MeV. We observe one strong s-wave resonance at a resonance energy of about 2.8 MeV, below the alpha particle threshold in 26Si. While no strong resonances are conclusively observed at higher energies, there may be indications for weaker resonances in the excitation energy range between 8.5-9.5 MeV.

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