D. C. Powell

Reaction rate of 24Mg(p,γ)25Al

Abstract The proton-capture reaction on 24 Mg has been investigated in the bombarding energy range of E p =0.2–1.7 MeV. Resonance properties (strengths, branching ratios and lifetimes) of low-energy resonances have been measured. From the experimental results, accurate proton partial widths, γ -ray partial widths and total widths ( Γ p , Γ γ , and Γ ) have been deduced. The present experimental information establishes the 24 Mg+p reaction rates over the temperature range T =0.02–2.0 GK with statistical uncertainties of 5% to 21%. Our recommended reaction rates deviate from previous estimates by 18% to 45%. Based on our results, we can rule out the recent suggestion that the total width of the E R =223 keV resonance has a significant influence on the reaction rates. We also discuss several effects that might give rise to systematic uncertainties in the reaction rates. The astrophysical implications for hydrogen burning of 24 Mg at low stellar temperatures are presented.

Low-energy resonance strengths for proton capture on Mg and Al nuclei

Abstract Absolute strengths of selected low-energy resonances for proton captures on the nuclei 24 Mg, 25 Mg, 26 Mg and 27 Al have been determined. The experiments were carried out by measuring the number of resonant γ-rays, integrated over the yield curve, simultaneously with the number of Rutherford scattered protons. The method applied in the present work is independent of target stoichiometries and uniformities, stopping powers, beam straggling and current integration. The ratio of charged-particle and γ-ray detection efficiencies was measured by using the 19 F(p,α 2 γ) 16 O reaction at E R =340 keV. A new set of reliable resonance strength standards at low bombarding energies is presented and our results are compared with previous work.

Investigation of the 23Na(p, γ)24Mg and 23Na(p, α)20ne reactions via (3He, d) spectroscopy

States near the {sup 23}Na+p threshold in {sup 24}Mg were investigated using the {sup 23}Na({sup 3}He,d){sup 24}Mg reaction over the angular range of 5 deg. {<=}{theta}{sub lab}{<=}35 deg. at E({sup 3}He)=20 MeV. Spectroscopic factors were extracted for states corresponding to resonances in the {sup 23}Na(p,{gamma}){sup 24}Mg and {sup 23}Na(p,{alpha}){sup 20}Ne reactions. We find that one state, corresponding to a previously unobserved resonance at E{sub c.m.}=138 keV, may make a significant contribution to the rates of both reactions at low temperatures. Another state, corresponding to a possible resonance at E{sub c.m.}=37 keV may make a small contribution to the {sup 23}Na(p,{alpha}){sup 20}Ne reaction. New rates for the {sup 23}Na(p,{gamma}){sup 24}Mg and {sup 23}Na(p,{alpha}){sup 20}Ne reactions are presented and the astrophysical implications are discussed.

Measurement of 7Li(n,γ0)8Li cross sections at En=1.5-1340 eV

The {sup 7}Li({ital n},{gamma}){sup 8}Li cross section is important in inhomogeneous big bang models, and as a constraint on model parameters used to determine the solar {sup 7}Be({ital p},{gamma}){sup 8}B reaction rate. Values of the {sup 7}Li({ital n},{gamma}{sub 0}){sup 8}Li reaction cross section were measured for neutron energies between 1.5 and 1340 eV at the Oak Ridge Electron Linear Accelerator. The normalization of the cross section was determined by measuring the gamma-ray yield from the {sup 7}Li({ital n},{gamma}{sub 0}){sup 8}Li reaction relative to that from the {sup 10}B({ital n},{alpha}{gamma}){sup 7}Li reaction. The cross section was found to have the inverse neutron-velocity relationship (1/{ital v}) indicative of {ital s}-wave capture. These results help resolve ambiguities in previous measurements. {copyright} {ital 1996 The American Physical Society.}

Measurement of Low-lying states in 40Sc

In explosive hydrogen burning nucleosynthesis material is processed via the proton capture sequence 39Ca(p,γ)40Sc(p,γ)41Ti. It has been predicted that the isotope 39Ca represents a waiting point for a continuous reaction flow. Therefore, its reaction rate is of interest. The 39Ca(p,γ)40Sc reaction rate is determined by three resonances corresponding to the 2nd, 3rd and 4th excited states of 40Sc. Improved nuclear structure information of the low-lying levels of 40Sc is necessary to reduce uncertainties in the reaction rate of 39Ca(p,γ)40Sc. Results from a current measurement of 40Ca(3He,t)40Sc at TUNL indicate that the 4th excited state is a doublet and further investigation is warranted.

Investigation of the Na-23(p, gamma) Mg-24 and Na-23(p, alpha) Ne-20 reactions via (He-3, d) spectroscopy

States near the {sup 23}Na+p threshold in {sup 24}Mg were investigated using the {sup 23}Na({sup 3}He,d){sup 24}Mg reaction over the angular range of 5 deg. {<=}{theta}{sub lab}{<=}35 deg. at E({sup 3}He)=20 MeV. Spectroscopic factors were extracted for states corresponding to resonances in the {sup 23}Na(p,{gamma}){sup 24}Mg and {sup 23}Na(p,{alpha}){sup 20}Ne reactions. We find that one state, corresponding to a previously unobserved resonance at E{sub c.m.}=138 keV, may make a significant contribution to the rates of both reactions at low temperatures. Another state, corresponding to a possible resonance at E{sub c.m.}=37 keV may make a small contribution to the {sup 23}Na(p,{alpha}){sup 20}Ne reaction. New rates for the {sup 23}Na(p,{gamma}){sup 24}Mg and {sup 23}Na(p,{alpha}){sup 20}Ne reactions are presented and the astrophysical implications are discussed.

Measurement of {sup 7}Li({ital n},{gamma}{sub 0}){sup 8}Li cross sections at {ital E}{sub {ital n}}=1.5{endash}1340 eV

The {sup 7}Li({ital n},{gamma}){sup 8}Li cross section is important in inhomogeneous big bang models, and as a constraint on model parameters used to determine the solar {sup 7}Be({ital p},{gamma}){sup 8}B reaction rate. Values of the {sup 7}Li({ital n},{gamma}{sub 0}){sup 8}Li reaction cross section were measured for neutron energies between 1.5 and 1340 eV at the Oak Ridge Electron Linear Accelerator. The normalization of the cross section was determined by measuring the gamma-ray yield from the {sup 7}Li({ital n},{gamma}{sub 0}){sup 8}Li reaction relative to that from the {sup 10}B({ital n},{alpha}{gamma}){sup 7}Li reaction. The cross section was found to have the inverse neutron-velocity relationship (1/{ital v}) indicative of {ital s}-wave capture. These results help resolve ambiguities in previous measurements. {copyright} {ital 1996 The American Physical Society.}

Measurement ofLi7(n,γ0)8Li cross sections atEn=1.5–1340 eV

The {sup 7}Li({ital n},{gamma}){sup 8}Li cross section is important in inhomogeneous big bang models, and as a constraint on model parameters used to determine the solar {sup 7}Be({ital p},{gamma}){sup 8}B reaction rate. Values of the {sup 7}Li({ital n},{gamma}{sub 0}){sup 8}Li reaction cross section were measured for neutron energies between 1.5 and 1340 eV at the Oak Ridge Electron Linear Accelerator. The normalization of the cross section was determined by measuring the gamma-ray yield from the {sup 7}Li({ital n},{gamma}{sub 0}){sup 8}Li reaction relative to that from the {sup 10}B({ital n},{alpha}{gamma}){sup 7}Li reaction. The cross section was found to have the inverse neutron-velocity relationship (1/{ital v}) indicative of {ital s}-wave capture. These results help resolve ambiguities in previous measurements. {copyright} {ital 1996 The American Physical Society.}

Measurement of Li-7 (n, gamma0) Li-8 cross sections at En=1.5-1340 eV

The {sup 7}Li({ital n},{gamma}){sup 8}Li cross section is important in inhomogeneous big bang models, and as a constraint on model parameters used to determine the solar {sup 7}Be({ital p},{gamma}){sup 8}B reaction rate. Values of the {sup 7}Li({ital n},{gamma}{sub 0}){sup 8}Li reaction cross section were measured for neutron energies between 1.5 and 1340 eV at the Oak Ridge Electron Linear Accelerator. The normalization of the cross section was determined by measuring the gamma-ray yield from the {sup 7}Li({ital n},{gamma}{sub 0}){sup 8}Li reaction relative to that from the {sup 10}B({ital n},{alpha}{gamma}){sup 7}Li reaction. The cross section was found to have the inverse neutron-velocity relationship (1/{ital v}) indicative of {ital s}-wave capture. These results help resolve ambiguities in previous measurements. {copyright} {ital 1996 The American Physical Society.}