Progress report on research in nuclear physics, August 1, 1994--June 30, 1995

The progress on Grant No. DE-FG05-87ER40314 from August 1, 1994 to June 30, 1995, is summarized in this report. The activities for the past year were focused on the rare electron capture studies, using experimental facilities at Tennessee Technological University and Montana State University. Also discussed are the PC-based multiparameter data acquisition systems and the two-dimensional position-sensitive microchannel plate detector

Electromagnetic transitions in neutron-rich Cl40

In-beam -rays from excited states of the neutron-rich (Tz=3) nucleus Cl40 have been identified in a threefold coincidence experiment in which rays and light charged particles were observed. The resulting decay scheme is presented, and implications for the structure of low-lying levels in Cl40 are discussed in light of recent data from charge-exchange and -decay work. The ordering of levels would seem to be quite different from the predictions of recent shell-model calculations

Yrast decays in K43

High-spin states in K43 were studied using the Be9(36S,pn)43K reaction. Threefold (p12) coincidence data and -ray intensity ratios were used to establish a decay scheme and identify negative- and positive-parity yrast decay chains. The 15/2- yrast state is relatively poorly aligned prior to decay. Energies of positive-parity levels predicted by Johnstone are in good agreement with experiment

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

Strength of the 18F(p, α)15O resonance at Ec.m. = 330 keV

The astrophysical rate of the 18F(p,α)15O reaction at nova temperatures is critical to understanding production of the radioisotope 18F, which may be used to constrain nova models via observations with the coming generation of satellite-based γ-ray telescopes. As such, a measurement is made of the strength of this resonance using a radioactive 18F beam at the HRIBF. As a result, it is indicated that the 18F(p,α)15O reaction rate is lower than previous estimates by a factor of ∼2

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]

Direct measurments of (p,γ) cross sections at astrophysical energies using radioactive beams and the Daresbury Recoil Separator

There are a number of astrophysical environments in which the path of nucleosynthesis proceeds through proton-rich nuclei. Radioactive nuclei have traditionally not been available as beams, and thus proton-capture reactions on these nuclei could only be studied indirectly. At the Holifield Radioactive Ion Beam Facility (HRIBF), some of the first direct measurements of (p,γ) cross sections on radioactive beams have been made. The Daresbury Recoil Separator (DRS) has been used to separate the recoils of interest from the unreacted primary beam and identify them in an isobutane-filled ionization counter. Data from 17F(p,γ)18Ne and 7Be(p,γ)8B measurements are presented

Neutron single particle strengths from the (d,p) reaction on F18

The F19 nucleus has been studied extensively. However, there have been no comprehensive experimental studies of F18+n single-particle components in F19, and no measure of neutron vacancies in the F18 ground state, as such experiments require a (radioactive) F18 target or beam. We have used the H2(F18,p)F19 reaction to selectively populate states in F19 that are of F18+n character. The 108.5-MeV radioactive F18+9 beam was provided by the Holifield Radioactive Ion Beam Facility at Oak Ridge National Laboratory. Proton-recoil coincidence data were taken for both α-decaying and particle-stable final states. Angular distributions and spectroscopic factors were measured for nine proton groups, corresponding to 13 states in F19. The results are compared to shell model calculations

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