D.W. Bardayan

{sup 19}F alpha widths and the {sup 18}F+p reaction rates

Understanding the properties of {sup 19}F levels in the range E{sub x}=6.4-7.5 MeV is important for constraining the contributions of {sup 19}Ne levels to the {sup 18}F(p,{alpha}){sup 15}O and {sup 18}F(p,{gamma}){sup 19}Ne thermonuclear reaction rates. We have reanalyzed {sup 15}N({alpha},{alpha}){sup 15}N data from H. Smotrich et al. [Phys. Rev. 122, 232 (1961)] to determine properties of {sup 19}F levels in this energy range. We find the energies and widths of broad levels to be different than previously reported and have set upper limits on the widths of postulated 3/2{sup +} resonances, analogs of which are important for the {sup 18}F+p rates.

Unbound states of 32Cl and the 31S(p,\gamma)32Cl reaction rate

The {sup 31}S(p,{gamma}){sup 32}Cl reaction is expected to provide the dominant break-out path from the SiP cycle in novae and is important for understanding enrichments of sulfur observed in some nova ejecta. We studied the {sup 32}S(3He,t){sup 32}Cl charge-exchange reaction to determine properties of proton-unbound levels in {sup 32}Cl that have previously contributed significant uncertainties to the {sup 31}S(p,{gamma}){sup 32}Cl reaction rate. Measured triton magnetic rigidities were used to determine excitation energies in {sup 32}Cl. Proton-branching ratios were obtained by detecting decay protons from unbound {sup 32}Cl states in coincidence with tritons. An improved {sup 31}S(p,{gamma}){sup 32}Cl reaction rate was calculated including robust statistical and systematic uncertainties.

Unbound states of32Cl and the31S(p,γ)32Cl reaction rate

The {sup 31}S(p,{gamma}){sup 32}Cl reaction is expected to provide the dominant break-out path from the SiP cycle in novae and is important for understanding enrichments of sulfur observed in some nova ejecta. We studied the {sup 32}S(3He,t){sup 32}Cl charge-exchange reaction to determine properties of proton-unbound levels in {sup 32}Cl that have previously contributed significant uncertainties to the {sup 31}S(p,{gamma}){sup 32}Cl reaction rate. Measured triton magnetic rigidities were used to determine excitation energies in {sup 32}Cl. Proton-branching ratios were obtained by detecting decay protons from unbound {sup 32}Cl states in coincidence with tritons. An improved {sup 31}S(p,{gamma}){sup 32}Cl reaction rate was calculated including robust statistical and systematic uncertainties.

Unbound states of {sup 32}Cl and the {sup 31}S(p,gamma){sup 32}Cl Reaction Rate

The {sup 31}S(p,{gamma}){sup 32}Cl reaction is expected to provide the dominant break-out path from the SiP cycle in novae and is important for understanding enrichments of sulfur observed in some nova ejecta. We studied the {sup 32}S(3He,t){sup 32}Cl charge-exchange reaction to determine properties of proton-unbound levels in {sup 32}Cl that have previously contributed significant uncertainties to the {sup 31}S(p,{gamma}){sup 32}Cl reaction rate. Measured triton magnetic rigidities were used to determine excitation energies in {sup 32}Cl. Proton-branching ratios were obtained by detecting decay protons from unbound {sup 32}Cl states in coincidence with tritons. An improved {sup 31}S(p,{gamma}){sup 32}Cl reaction rate was calculated including robust statistical and systematic uncertainties.

Design and Construction of a Gas Jet Target for RIB Experiements

PNNL is now part of the JENSA collaboration to produce a gas jet system for the Facility for Rare Isotope Beams (FRIB). This document is a status report for the gas jet working group to be delivered to the FRIB scientific advisory council (SAC). It briefly describes PNNL’s capability at constructing cost efficient and high detection efficiency HPGe arrays.

The

The production of 26Al in novae is uncertain, in part, because of the uncertain rate of the 25Al(p,g)26Si reaction at novae temperatures. This reaction is thought to be dominated by a longsought 3+ level in 26Si, and the calculated reaction rate varies by orders of magnitude depending on the energy of this resonance. We present evidence concerning the spin of a level at 5.914 MeV in 26Si from the 28Si(p,t)26Si reaction studied at the Holifield Radioactive Beam Facility at ORNL. We find that the angular distribution for this level implies either a 2+ or 3+ assignment, with only a 3+ being consistent with the mirror nucleus, 26Mg. Additionally, we have used the updated 25Al(p,g)26Si reaction rate in a nova nucleosynthesis calculation and have addressed the effects of the remaining uncertainties in the rate on 26Al production.

^{25}

The 14O( ,p)17F reaction is an important trigger reaction leading to the p process in x-ray bursts. The inclusion of reaction channels populating excited 17F levels may significantly increase the calculated 14O( ,p)17F reaction rate. A radioactive 17F beam was used at the Oak Ridge National Laboratory Holifield Radioactive Ion Beam Facility to search for a 18Ne resonance at Ec.m.(17F + p) 3.1MeV that had been previously suggested to decay strongly to the first excited level in 17F. No evidence, however, of inelastic 17F + p scattering was observed at this energy, and an upper limit of 10 mb has been set on the inelastic-scattering cross section.

Al

Several resonances in 25 Al(p,γ) 26 Si have been studied via the 28 Si(p,t) 26 Si reaction. Triton energies and angular distributions were measured using a segmented annular detector array. An additional silicon detector array was used to simultaneously detect the coincident protons emitted from the decay of states in 26 Si above the proton threshold in order to determine branching ratios. A resonance at 5927 ± 4 keV has been experimentally confirmed as the first l = 0 state above the proton threshold, with a proton branching ratio consistent with one.

(p,\gamma)^{26}

The {sup 18}Ne({alpha},p){sup 21}Na reaction plays a crucial role in the ({alpha},p) process, which leads to the rapid proton capture process in x-ray bursts. The reaction rate depends upon properties of {sup 22}Mg levels above the {alpha} threshold at 8.14 MeV. Despite recent studies of these levels, only the excitation energies are known for most with no constraints on the spins. We have studied the {sup 24}Mg(p,t){sup 22}Mg reaction at the Oak Ridge National Laboratory (ORNL) Holifield Radioactive Ion Beam Facility (HRIBF), and by measuring the angular distributions of outgoing tritons, we provide some of the first experimental constraints on the spins of astrophysically important {sup 18}Ne({alpha},p){sup 21}Na resonances.

Si Reaction Rate in Novae

The magnitude of the {sup 15}N(n,{gamma}){sup 16}N reaction rate in asymptotic giant branch stars depends directly on the neutron spectroscopic factors of low-lying {sup 16}N levels. A new study of the {sup 15}N(d,p){sup 16}N reaction is reported populating the ground and first three excited states in {sup 16}N. The measured spectroscopic factors are near unity as expected from shell model calculations, resolving a long-standing discrepancy with earlier measurements that had never been confirmed or understood. Updated {sup 15}N(n,{gamma}){sup 16}N reaction rates are presented.