M.R. Anderson

Cross-section measurements and thermonuclear reaction rates for 52Cr(p, γ)53Mn and 54Fe(p, γ)55Co☆

Abstract The yields of γ-rays from the reactions 52 Cr(p, γ) 53 Mn and 52 Cr(p, p′ γ) 52 Cr have been measured as a function of bombarding energy over the range 0.84–4.00 MeV, and from the reactions 54 Fe(p, γ) 55 Co and 54 Fe(p, p′γ) 54 Fe over the range 1.05–3.69 MeV. All γ-ray yields were lower by factors of order of 2 than those predicted by statistical model calculations based on global parameter sets. Data are taken from the literature for proton induced reactions on 48 Ca, 50 Ti, and 51 V, which like 52 Cr and 54 Fe contain a closed shell of 28 neutrons, and for which statistical model calculations were also high by factors of order of 2 or more. A new prescription for obtaining the imaginary well depth for the proton channel is proposed for N = 28 nuclei and it is shown to result in a striking improvement in the agreement between experimental and calculated cross sections. Thermonuclear reaction rates for the (p, γ) reactions on 52 Cr and 54 Fe are calculated for the temperature range (0.6−10) × 10 9 K.

Resonance strength measurements and thermonuclear reaction rates for 25Mg(p, γ)26Al☆

Abstract The strengths of resonances in 25 Mg(p, γ) 26 Al in the energy range E p = 317–591 keV have been obtained. Resonance strengths at E p = 317, 387, 433 and 591 keV have been measured absolutely. The strengths of resonances at E p = 591 and 433 keV were also deduced by comparison with the strength of the resonance at E p = 620 keV in 30 Si(p, γ) 31 P. Relative measurements of 10 resonances in the range E p = 317–591 keV were made. A search for (p, γ) strengths below E p = 300 keV was undertaken and an upper limit for the sum of all possible strengths is given. Thermonuclear reaction rates are calculated for the reaction 25 Mg(p, γ) 26 Al leading to both the ground state and isomeric first excited state of 26 Al in the temperature range T = 5 × 10 7 −5 × 10 8 K and are compared with rates determined from cross sections obtained from statistical-model calculations. The superelastic and inelastic scattering of protons is investigated as a mechanism by which the 26 Al nucleus may attain a thermal distribution of excited states and is shown to be ineffective.

25Mg (α, n)28Si and 26Mg(α, n)29Si as neutron sources in explosive neon burning

Abstract The yields of neutrons from the reaction 25 Mg(α, n) 28 Si and of γ-rays from the reaction 25 Mg(α, nγ) 28 Si have been measured as a function of bombarding energy over the range 1.8–6.3 MeV, and the yield of neutrons from 26 Mg(α, n) 29 Si has been measured over the range 1.8–6.0 MeV. Cross sections for 25, 26 Mg(α, n) 28, 29 Si were extracted from the data and compared with global statistical-model calculations. The agreement is very good. Thermonuclear reaction rates under stellar conditions appropriate for explosive neon burning are calculated and their significance for the nucleosynthesis of rare neutron-rich nuclei is discussed.

Cross-section measurements and thermonuclear reaction rates for 49Ti(p, γ)50V and 49Ti(p, n)49V

Abstract The yield of γ-rays from the reaction 49 Ti(p, γ) 50 V has been measured as a function of bombarding energy over the range 0.74–2.27 MeV and the yield of neutrons from the reaction 49 Ti(p, n) 49 V has been measured from threshold to a bombarding energy of 3.25 MeV. Strong competition effects were observed in the γ-ray yield at the opening of the neutron channel. The energy dependence of these cross sections is compared with statistical-model calculations, and good agreement is achieved. Thermonuclear reaction rates are calculated for the temperature range 5 × 10 8 –5 × 10 9 K which includes the range of temperatures of interest in nucleosysnthesis calculations. The effect of a strong isobaric analogue resonance on the (p, γ) thermonuclear reaction rate is identified and its importance is discussed.

The 56Fe(α, γ)60Ni reaction cross section☆

Abstract The yield of 1.332 MeV γ-rays from the reaction 56Fe(α, γ)60Ni has been measured as a function of α-particle bombarding energy in the range 4.2–7.1 MeV. This energy region includes the (α, n) threshold at Eα = 5.46 MeV. The energy dependence of this γ-ray cross section, as well as the (α, nγ) cross section, is compared with statistical model calculations. Good agreement is achieved between these data and calculations using global optical model parameters to determine the transmission functions. In particular, the pronounced threshold effect in the (α, γ) yield, at the opening of the (α, n) channel, is well represented.

Thermonuclear reaction rates for proton induced reactions on 41K and neutron induced reactions on 41Ca

Abstract The yield of γ-rays from the reaction 41 K(p, γ ) 42 Ca has been measured as a function of bombarding energy over the range 0.68–2.48 MeV and from the reaction 41 K(p, αγ ) 38 Ar over the range 1.20–2.48 MeV, and the yield of neutrons from the reaction 41 K(p, n) 41 Ca has been measured from threshold to a bombarding energy of 2.48 MeV. The energy dependence of the cross sections is compared with statistical-model calculations with global optical-model parameters in all particle channels. The calculations seriously overestimate the cross section for the neutron channel and underestimate those for the other channels. A reduction in the imaginary well depth in the neutron channel leads to good agreement with all the data. Statistical-model calculations with this modified set of parameters are then carried out to provide cross sections for the astrophysically interesting reactions 41 Ca(n, p) 41 K, 41 Ca(n, α ) 38 Ar, and 41 Ca(n, γ ) 42 Ca. Thermonuclear reaction rates are calculated for all six reactions over the temperature range 5 × 10 8 –10 10 K which includes the range of temperatures of interest in nucleosynthesis calculations.