First on-line results for As and F beams from HRIBF target/ion sources

The first on-line tests of the ion sources to provide radioactive ion beams of 69/7OAs and 17>18F for the Holifield Radioactive Ion Beam Facility have been performed using the UNISOR facility at HRIBF. For 70As the measured efficiency is 0.8+ 0.3% with a hold-up time of 3.6 ± 0.3 hours as measured with ^As at a target temperature of 1270°C. For 17F the efficiency for A117F is 0.0024 + 0.0008% with a hold-up time of 16.4 + 0.8 m as measured with A118F at a target temperature of 1470°C

Identification of new states in 26Si using the29Si(3He,6He)26Si reaction and consequences for the 25Al(p,y)26Si reaction rate in explosive hydrogen burning environments

We have studied the [Formula Presented] reaction and have identified new states in [Formula Presented] at [Formula Presented] and [Formula Presented] Based on these measurements and other recent evidence, we suggest spin-parity assignments of [Formula Presented] for the 5.678 MeV state and [Formula Presented] for the 5.945 MeV state, which would account for all the “missing” unnatural parity states in [Formula Presented] in the excitation energy region important to hydrogen burning in novae. New reaction rates are presented for the [Formula Presented] reaction based on this possible assignment of states

Destruction of 18F via 18F(p,α) 15O burning through the Ec.m.=665 keV resonance

Knowledge of the astrophysical rate of the 18F(p,α)15O reaction is important for understanding the γ-ray emission expected from novae and heavy-element production in x-ray bursts. The rate of this reaction is dominated at temperatures above ∼0.4 GK by a resonance near 7.08 MeV excitation energy in 19Ne. The 18F(p,α)15O rate has been uncertain in part because of disagreements among previous measurements concerning the resonance strength and excitation energy of this state. To resolve these uncertainties, we have made simultaneous measurements of the 1H(18F,p)18F and 1H(18F,α)15O excitation functions using a radioactive 18F beam at the ORNL Holifield Radioactive Ion Beam Facility. A simultaneous fit of the data sets has been performed, and the best fit was obtained with a center-of-mass resonance energy of 664.7±1.6 keV (Ex = 7076±2 keV), a total width of 39.0±1.6 keV, a proton branching ratio of Γp/Γ = 0.39±0.02, and a resonance strength of ωγ= 6.2±0.3 keV

Kinematically complete measurement of the 1H(18F,p)18F excitation function for the astrophysically important 7.08-MeV state in 19Ne

Knowledge of the astrophysical [Formula Presented] rate is important for understanding gamma-ray emission from novae and heavy-element production in x-ray bursts. A state with [Formula Presented] in [Formula Presented] provides an s-wave resonance and, depending on its properties, could dominate the [Formula Presented] rate. By measuring a kinematically complete [Formula Presented] excitation function with a radioactive [Formula Presented] beam at the ORNL Holifield Radioactive Ion Beam Facility, we find that the [Formula Presented] state lies at a center-of-mass energy of [Formula Presented] has a total width of [Formula Presented] and a proton partial-width of [Formula Presented]

Breakout from the hot CNO cycle: The 18F(p,γ) vs 18F(p,α) branching ratio

We have studied the properties of low-lying 18Fp resonances as excited states in 19Ne. Three new levels have been found in the range 0Ec.m.1 MeV just above the 18Fp threshold, and partial decay widths and isospin-mirror connections are suggested to known states in 19F for each of the nine states in this energy range. The properties of these resonances have been used to calculate the reaction rate NAvfor the 18F(p,)19Ne and 18F(p,)15O reactions in the temperature range 108T109. A comparison of these rates indicates that in this temperature range, the 14O(,p)17F(p,)18Ne(e)18F(p,)19Ne reaction sequence is not as fast as the 15O(,)19Ne reaction

Low energy scattering cross section ratios of N 14 (p,p) N 14

Background: The slowest reaction in the first CNO cycle is N14(p,γ)O15, therefore its rate determines the overall energy production efficiency of the entire cycle. The cross section presents several strong resonance contributions, especially for the ground-state transition. Some of the properties of the corresponding levels in the O15 compound nucleus remain uncertain, which affects the uncertainty in extrapolating the capture cross section to the low energy range of astrophysical interest. Purpose: The N14(p,γ)O15 cross section can be described by using the phenomenological R matrix. Over the energy range of interest, only the proton and γ-ray channels are open. Since resonance capture makes significant contributions to the N14(p,γ)O15 cross section, resonant proton scattering data can be used to provide additional constraints on the R-matrix fit of the capture data. Methods: A 4 MV KN Van de Graaff accelerator was used to bombard protons onto a windowless gas target containing enriched N14 gas over the proton energy range from Ep=1.0 to 3.0 MeV. Scattered protons were detected at θlab=90, 120°, 135°, 150°, and 160° using ruggedized silicon detectors. In addition, a 10 MV FN Tandem Van de Graaff accelerator was used to accelerate protons onto a solid Adenine (C5H5N5) target, of natural isotopic abundance, evaporated onto a thin self-supporting carbon backing, over the energy range from Ep=1.8 to 4.0 MeV. Scattered protons were detected at 28 angles between θlab=30.4° and 167.7° by using silicon photodiode detectors. Results: Relative cross sections were extracted from both measurements. While the relative cross sections do not provide as much constraint as absolute measurements, they greatly reduce the dependence of the data on otherwise significant systematic uncertainties, which are more difficult to quantify. The data are fit simultaneously using an R-matrix analysis and level energies and proton widths are extracted. Even with relative measurements, the statistics and large angular coverage of the measurements result in more confident values for the energies and proton widths of several levels; in particular, the broad resonance at Ec.m.=2.21 MeV, which corresponds to the 3/2+ level at Ex=9.51 MeV in O15. In particular, the s- and d-wave angular-momentum channels are separated. Conclusion: The relative cross sections provide a consistent set of data that can be used to better constrain a full multichannel R-matrix extrapolation of the capture data. It has been demonstrated how the scattering data reduce the uncertainty through a preliminary Monte Carlo uncertainty analysis, but several other issues remain that make large contributions to the uncertainty, which must be addressed by further capture and lifetime measurements

Observation of the astrophysically important 3+ state in 18Ne via elastic scattering of a radioactive 17F beam from 1H

The 17F(p, γ)18 reaction is important in stellar explosions, but its rate has been uncertain because of an expected 3+ state in 18Ne that has never been conclusively observed. This state would provide a strong l = 0 resonance and, depending on its excitation energy, could dominate the stellar reaction rate. We have observed this missing 3+ state by measuring the 1H(17F, p)17F excitation function with a radioactive 17F beam at the ORNL Holifield Radioactive Ion Beam Facility. We find that the state lies at a center-of-mass energy of Er = 599.8 ± 1.5stat ± 2.0sys keV (Ex = 4523.7 ± 2.9keV) and has a width of Γ = 18 ± 2stat ± 1sys keV

The astrophysically important 3+ state in 18Ne and the 17F(py)18Ne stellar rate

Knowledge of the [Formula Presented] reaction rate is important for understanding stellar explosions, but it was uncertain because the properties of an expected but previously unobserved [Formula Presented] state in [Formula Presented] were not known. This state would provide a strong s-wave resonance for the [Formula Presented] system and, depending on its excitation energy, could dominate the stellar reaction rate at temperatures above 0.2 GK. We have observed this missing [Formula Presented] state by measuring the [Formula Presented] excitation function with a radioactive [Formula Presented] beam at the ORNL Holifield Radioactive Ion Beam Facility (HRIBF). We find that the state lies at a center-of-mass energy of [Formula Presented] keV [Formula Presented] and has a width of [Formula Presented] The measured properties of the resonance are only consistent with a [Formula Presented] assignment

Three-body structure of low-lying 18Ne states

We investigate to what extent 18Ne can be descibed as a three-body system made of an inert 16O-core and two protons. We compare to experimental data and occasionally to shell model results. We obtain three-body wave functions with the hyperspherical adiabatic expansion method. We study the spectrum of 18Ne, the structure of the different states and the predominant transition strengths. Two 0+, two 2+, and one 4+ bound states are found where they are all known experimentally. Also one 3+ close to threshold is found and several negative parity states, 1-, 3-, 0-, 2-, most of them bound with respect to the 16O excited 3- state. The structures are extracted as partial wave components, as spatial sizes of matter and charge, and as probability distributions. Electromagnetic decay rates are calculated for these states. The dominating decay mode for the bound states is E2 and occasionally also M1.Comment: 17 pages, 5 figures (version to appear in EPJA

S-wave scattering lengths for the Be 7 +p system from an R-matrix analysis

The astrophysical S factor for the radiative proton capture reaction on Be7 (S17) at low energies is affected by the s-wave scattering lengths. We report the measurement of elastic and inelastic scattering cross sections for the Be7+p system in the center-of-mass energy range 0.474-2.740 MeV and center-of-mass angular range 70-150. A radioactive Be7 beam produced at Oak Ridge National Laboratory's (ORNL) Holifield Radioactive Ion Beam Facility was accelerated and bombarded a thin polypropylene (CH2)n target. Scattered ions were detected in the segmented Silicon Detector Array. Using an R-matrix analysis of ORNL and Louvain-la-Neuve cross-section data, the s-wave scattering lengths for channel spins 1 and 2 were determined to be 17.34-1.33+1.11 and -3.18-0.50+0.55 fm, respectively. The uncertainty in the s-wave scattering lengths reported in this work is smaller by a factor of 5-8 compared to the previous measurement, which may reduce the overall uncertainty in S17 at zero energy. The level structure of B8 is discussed based upon the results from this work. Evidence for the existence of 0+ and 2+ levels in B8 at 1.9 and 2.21 MeV, respectively, is observed