P.V. Magnus

Measurement of 15O(α,γ)19Ne resonance strengths

Abstract States in 19 Ne above the 15 O + α threshold were populated by means of the 19 F( 3 He,t) 19 Ne ∗ reaction, and their alpha-particle decays to the 15 O ground state were measured. The branching ratios Γ α /Γ total for the E c . m . = 850-, 1020-, 1971-, 1183- and 1563- keV resonances in 19 O + α were determined. This information together with alpha-particle and/or gamma-ray partial widths (determined from knowledge of these quantities for the mirror states in 19 F) determines the strengths of these 15 O ( α , γ ) 19 Ne resonances and the 15 O ( α , γ ) 19 Ne reaction rate for temperatures between 7 × 10 8 and3 × 10 9 K.

Particle decays in 28Si: The destruction of 27Al in red giants and novae

Abstract The 27 Al( 3 He, d) 28 Si reaction has been used to populate states near the 27 Al+p threshold and the ensuing proton and alpha decay has been measured. No evidence for new 27 Al(p , α) 24 Mg resonance strength was observed and consequently revised limits have been placed on the thermonuclear reaction rate. As a result, the 27 Al(p , α) 24 Mg reaction is found to be astrophysically unimportant at red-giant and nova temperatures.

A high-resolution study of the 20Ne(3He, t)20Na reaction and the 19Ne(p, γ)20Na reaction rate

A high-precision measurement of the 20Ne(3He,t)20Na reaction has been made using implanted 20Ne transmission targets to obtain pertinent information on the low-energy resonances in the 19Ne(p, γ)20Na reaction. Resonance energies (447±5, 658±5, 787±5, and 857±5 keV) and upper limits on total intrinsic widths (<10, <6, <10, and <16keV) have been measured for four excited states above the 2.199 MeV proton threshold in 20Na. The stellar 19Ne(p, γ)20Na reaction rate is calculated for temperatures between 1×108 and 1×109 K. When combined with a recent study of the 15O(α, γ)19Ne reaction, a new estimate is made of the conditions required for breakout from the Hot CNO cycle to the rapid proton capture process.

Measurement of the 15N(α, γ)19F resonances at Ec.m. = 536 keV and 542 keV☆

Abstract The E c.m. = 536 keV and E c.m. = 542 keV resonances in the 15 N(α, γ) 19 F reaction have been studied using a thick Ti 15 N target, determining ωγ = (9.7 ± 2.0) × 10 −5 eV and ωγ −5 eV for these two resonances, respectively. For the 542 keV resonance this value is substantially smaller than had been assumed in earlier model calculations, and this reduces the rate of the analog 15 O(α, γ) 19 Ne reaction (connecting the Hot CNO cycle and the rp-process) by a factor of ≈ 3 for temperatures in the region of T 9 = 1.

Threshold states in 26Al revisited

Abstract Threshold states in 26 Al have been re-examined using the 27 Al( 3 He, α) 26 Al and 25 Mg( 3 He, dγ) 26 Al reactions in order to resolve apparent ambiguities in some of the previously reported properties of these states. In particular, the s-wave resonance strength reported at E c.m. = 37 keV is now found to be located at E c.m. = 57.54 keV, and the proton width for the 374 keV resonance has been revised to Γ p = 0.82 eV. These results have been used to calculate a new resonance strength of ωγ = 1.6 × 10 −13 eV for the 57.54 keV resonance. As a result, the stellar production rate for 26 Al is increased by a factor ∼- 3–38 for temperatures T 9 = 0.05–0.1.

The effect of weak resonances on the 25Mg(p, γ)26Al reaction rate☆

Abstract The 25 Mg( 3 He, d) 26 Al reaction has been used to estimate proton spectroscopic factors for states which could be weak 25 Mg+p resonances located near the proton-capture threshold. One of these states (corresponding to a resonance energy E c.m =92.2 keV) is found to have a significant effect on the 25 Mg(p, γ) 26 Al reaction rate for temperatures characteristic of Wolf-Rayet stars or late-stage red giants.

Measurements of differential cross-section ratios for single-nucleon transfer reaction pairs near A = 25

Abstract Differential cross sections for the (d, p), (3He, d), (α, t), and (α,3He) reactions involving seventy-one residual states in 23Na, 25Mg, 25Al, and 27Al have been measured at a forward angle with incident energies of 17.5, 20.2, 34.8, and 34.8 MeV, respectively. The ratio of cross-section pairs involving formation of the same residual state is determined for forty-five cases where both the angular momentum transfer and single-particle spectroscopic strength have been previously established. These are compared to values calculated with conventional distorted-wave Born approximation analyses, and the utility of this technique for identifying some levels which are possible s- or p-wave resonances is demonstrated and discussed for states in the vicinity of proton thresholds. An application is made involving proton threshold states in 27Al.

Proton threshold states in 27Si and the destruction of 26Al at low stellar temperatures

Abstract Using the 27Al(3He, t)27Si reaction, we have located and measured accurate excitation energies for seven states in 27Si between the 26Al + p threshold and the lowest observed 26 Al ( p , γ) 27 Si resonance. By examining the mirror states in 27Al via the 27Mg(3He, d)27Al and 26 Mg (α, t ) 27 Al reactions, we find that in the 26AI + p channel there can be at most only one s-wave resonance that lies below the lowest directly observed resonance; this possible l = 0 resonance most probably corresponds to either the 7589, 7557 or 7533 keV state in 27Si. The remaining six states are most probably formed via l ⩾ 2 proton capture. Upper limits for the reduced proton widths of these states have been used to estimate an upper limit for the 26 Al ( p , γ) 27 Si thermonuclear reaction rate.

Proton-threshold states in27Al and the production of 27Al at low stellar temperatures

Abstract The 26 Mg( 3 He, d) 27 Al reaction has been employed to measure excitation energies and proton spectroscopic factors for states corresponding to 26 Mg + p resonances in the vicinity of the proton-capture threshold. The width ratio Γ γ /Γ was measured for three previously established resonances via a study of the 26 Mg( 3 He, d γ ) 27 Al reaction, and corresponding values of the proton widths were obtained. Combining this information establishes strengths for four of the states lying within 150 keV of the proton threshold. A 26 Mg + p reaction rate is deduced, and its astrophysical implications are discussed.