K. Y. Chae

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 nuclear structure of {sup 19}Ne near the proton threshold is of interest for understanding the rates of proton-induced reactions on {sup 18}F in novae. Analogues for several states in the mirror nucleus {sup 19}F have not yet been identified in {sup 19}Ne indicating the level structure of {sup 19}Ne in this region is incomplete. The {sup 18}F(d;n){sup 19}Ne and {sup 18}F(d,p){sup 19}F reactions have been measured simultaneously at E{sub c.m.} = 14.9 MeV. The experiments were performed at the Holifield Radioactive Ion Beam Facility (HRIBF) of Oak Ridge National Laboratory (ORNL) by bombarding a 720-mg/cm{sub 2} CD{sub 2} target with a radioactive {sup 18}F beam. The {sup 19}Ne states of interest near the proton threshold decay by breakup into a and {sup 15}O particles. These decay products were detected in coincidence with position-sensitive E-{Delta}E silicon telescopes. The {alpha} and {sup 15}N particles from the break up of the mirror nucleus {sup 19}F were also measured with these detectors. Particle identification, coincidence, and Q-value requirements enable us to distinguish the reaction of interest from other reactions. The reconstruction of relative energy of the detected particles reveals the excited states of {sup 19}Ne and {sup 19}F which are populated. The neutronmorexa0» (proton) angular distributions for states in {sup 19}Ne ({sup 19}F) were extracted using momentum conservation. The observed states in {sup 19}Ne and {sup 19}F will be presented.«xa0less

Si Reaction Rate in Novae

The rate of the (17)F(p,gamma)(18)Ne reaction is important in various astrophysical events. A previous (17)F(p,p)(17)F measurement identified a 3;{+} state providing the strongest resonance contribution, but the resonance strength was unknown. We have directly measured the (17)F(p,gamma)(18)Ne reaction using a mixed beam of (17)F and (17)O at ORNL. The resonance strength for the 3;{+} resonance in (18)Ne was found to be omegagamma = 33 +/- 14(stat) +/-1 7(syst) meV, corresponding to a gamma width of Gamma_{gamma} = 56 +/- 24(stat) +/- 30(syst) meV. An upper limit on the direct capture of S(E) <or= 65 keV b was determined at an energy of 800 keV.

Inelastic {sup 17}F(p,p){sup 17}F scattering at E{sub c.m.}=3 MeV and the {sup 14}O(alpha,p){sup 17}F reaction rate

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.

The {sup 28}Si(p,t){sup 26}Si{sup *}(p) reaction and implications for the astrophysical {sup 25}Al(p,{gamma}){sup 26}Si reaction rate

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.

Astrophysically important 19Ne states studied with the 2H(18F, alpha+15O)n Reaction

We have developed a novel technique for measurements of low-energy (p,{alpha}) reactions using heavy-ion beams and a differentially pumped windowless gas target. We applied this new approach to study the 183 keV resonance in the {sup 17}O(p,{alpha}){sup 14}N reaction. We report a (center-of-mass) resonance energy of E{sub r}=183.5(+0.1/-0.4) keV and a resonance strength of {omega}{gamma}{sub p{alpha}}=(1.70{+-}0.15) meV, and we set an upper limit (95% confidence) on the total width of the state of {gamma}<0.1 keV. This resonance is important for the {sup 17}O(p,{alpha}){sup 14}N reaction rate, and we find that {sup 18}F production is significantly decreased in low-mass ONeMg novae but less affected in more energetic novae. We also report the first determination of the stopping power for oxygen ions in hydrogen gas near the peak of the Bragg curve (E=193 keV/u) to be (63{+-}1)x10{sup -15} eV cm{sup 2}.

First Direct Measurement of the {sup 17}F(p,{gamma}){sup 18}Ne Cross Section

The

Constraint on the astrophysical {sup 18}Ne({alpha},p){sup 21}Na reaction rate through a {sup 24}Mg(p,t){sup 22}Mg measurement

^{28}mathrm{Si}