Investigation of the 24Mg(α, γ)28Si reaction and some 27Al(p, γ)28Si resonances

Abstract

The yield of 8–13 MeV γ radiation from the 24Mg(α, γ)28Si reaction has been measured in the Eα = 1.2–3.2 MeV range, making use of natural Mg and enriched 24Mg targets. Twenty resonances were observed, corresponding to 28Si levels in the Eχ = 11.3–12.8 MeV region. Six resonances show a relatively strong ground-state transition (γ0); the spin and parity (either 1-, or 2+) of these resonances was uniquely determined through angular distribution measurements. At nine other resonances the angular distribution was measured of γ1, the transition to the 28Si first excited state at 1.77 MeV; supplemented with angular correlation measurements in several different geometries, this uniquely determined the Jπ of these resonances. The remaining five resonances are weak.

The strongest resonances have Jπ=2+ or 4+. These are deexcited through E2 or strongly mixed M1-E2 transitions.

In both the 24Mg(α, γ)28Si and 27Al(p, γ)28Si reactions a level in 28Si at Eχ=12.24 MeV is excited. The (unique) spin determinations from these reactions, however, do not agree, yielding Jπ = 4+ and Jπ = 3+, respectively. Although the γ-ray single spectra are closely analogous, pronounced differences exist in the γ-γ coincidence spectra measured from the two reactions, showing that actually two closely spaced (ΔEχ ≈ 0.6 keV) levels in 28Si are excited. A new, extremely weak, resonance in the 27Al(p, γ)28Si reaction has been observed at Ep = 202.3 ± 0.9 keV.

The data obtained from the 24Mg(α, γ)28Si reaction are compared with the information from the 27Al(p, γ)28Si and 27Al(p, α)24Mg reactions. From the measured yields of these three reactions, the partial widths Γα, Γp and Γγ are computed for most resonance levels.