Degree

Doctor of Philosophy (PhD)

Department

Physics and Astronomy

Document Type

Dissertation

Abstract

Certain (α,p) reactions on proton-rich nuclei are among the most important nuclear reactions occurring during Type I X-ray bursts, as well as being impactful in stellar explosions during the Asymptotic Giant Branch (AGB) phase and on accreting white dwarfs (WD). However, large uncertainties remain for reaction rates on some radioactive nuclei due to the lack of direct measurements. The Array for Nuclear Astrophysics and Structure with Exotic Nuclei (ANASEN) is a gas target and charged-particle detector designed for direct measurements of (α,p) reactions. ANASEN was redesigned with two hexagonal barrels of 100-µm-thick silicon strip surrounding the beam axis over a length of 14 cm, and two arrays of 1000-µm-thick silicon detectors far from the beam axis to track outgoing protons. The new design, which improves the center-of-mass energy resolution from 1.5 MeV to 0.18 MeV, was used to measure the 18F(α,p)21Ne excitation function over Ecm = 1.9–4 MeV at TRIUMF Laboratory in inverse kinematics. The combined ground and 1st-excited final states were separated from the 2nd-excited final state, and the cross sections were fit with the R-matrix formalism. The total measured cross section agrees with Hauser–Feshbach statistical model predictions within 30%. The new reaction rate, calculated over T = 0.1–3 GK, is up to a factor of 2 larger than the REACLIB rate at T ≳ 1 GK. Large uncertainties remain in the rate at low energies (Ecm < 1.5 MeV) that could not be probed in this direct measurement. The impact on AGB stars and WDs is discussed.

Date

1-9-2026

Committee Chair

Deibel, Catherine

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