Proton-unbound States in ^24 Al Relevant for the ^23 Mg(p,γ) Reaction in Novae

Background: The nucleosynthesis of several proton-rich nuclei is determined by radiative proton-capture reactions on unstable nuclei in nova explosions. One such reaction is ^23 Mg(p,γ)^24 Al, which links the NeNa and MgAl cycles in oxygen-neon (ONe) novae. Purpose: To extract ^23 Mg(p,γ) resonance strengths from a study of proton-unbound states in ^24 Al, produced via the ^24Mg(^3He,t) reaction. Methods: A beam of ^3 He^2+ ions at 50.7 MeV was used to produce the states of interest in ^24Al. Proton-triton angular correlations were measured with a K=600 QDD magnetic spectrometer and a silicon detector array, located at iThemba LABS, South Africa. Results: We measured the excitation energies of the four lowest proton-unbound states in ^24Al and place lower-limits on Γ_p/Γ values for these four states. Together with shell-model calculations of partial gamma widths, the experimental data are also used to determine resonance strengths for the three lowest ^23 Mg(p,γ)^24 Al resonances. Conclusions: The energy of the dominant first ^23 Mg(p,γ) resonance is determined to be E_r = 478 ± 4 keV, with a resonance strength ωγ = 19 ± 9 meV.