J. S. Thomas

New constraints on the 18F(p,α) 15O rate in novae from the (d, p) reaction

The degree to which the (p,gamma) and (p,alpha) reactions destroy 18F at temperatures 1-4x10^8 K is important for understanding the synthesis of nuclei in nova explosions and for using the long-lived radionuclide 18F, a target of gamma-ray astronomy, as a diagnostic of nova mechanisms. The reactions are dominated by low-lying proton resonances near the 18F+p threshold (E_x=6.411 MeV in 19Ne). To gain further information about these resonances, we have used a radioactive 18F beam from the Holifield Radioactive Ion Beam Facility to selectively populate corresponding mirror states in 19F via the inverse d(18F,p)19F neutron transfer reaction. Neutron spectroscopic factors were measured for states in 19F in the excitation energy range 0-9 MeV. Widths for corresponding proton resonances in 19Ne were calculated using a Woods-Saxon potential. The results imply significantly lower 18F(p,gamma)19Ne and 18F(p,alpha)15O reaction rates than reported previously, thereby increasing the prospect of observing the 511-keV annihilation radiation associated with the decay of 18F in the ashes ejected from novae.

19Ne Levels Studied with the 18F(d,n)19Ne*(18F+p) Reaction

A good understanding of the level structure of 19Ne around the proton threshold is critical to estimating the destruction of long-lived 18F in novae. Here we report the properties of levels in 19Ne in the excitation energy range of 6.9 Ex 8.4 MeV studied via the proton-transfer 18F(d, n)Ne reaction at the Holifield Radioactive Ion Beam Facility. The populated 19Ne levels decay by breakup into p + 18F and + 15O particles. The results presented in this manuscript are those of levels that are simultaneously observed from the breakup into both channels. An s-wave state is observed at 1468 keV above the proton threshold, which is a potential candidate for a predicted broad J = 1/2+ state. The proton and partial widths are deduced to be p = 228 50 keV and = 130 30 keV for this state.

Search for the alpha + (6)He decay of (10)Be via the (16)O((18)O, (10)Be*)(24)Mg reaction

A search for the a + He decay of Be has been performed using the Li 2 O( 18 O , α 6 He) reaction at 80 and 100 MeV. An array of two Si-Si-CsI telescopes was used for the coincident detection of the breakup fragments. No evidence for the 16 O( 18 O,α 6 He) 24 Mg reaction was obtained, the cross-section being determined as a < 1.9 μb and <3.9 μb at 80 and 100 MeV, respectively. The α + 15 N decay of 19 F was observed via the 7 Li( 18 O,α 15 N) 6 He reaction. For the 19 F excitation energy and centre-of-mass scattering angle ranges covered, the cross-sections are a = (31.7 ± 6.6) μb at 80 MeV and (31.8 ± 6.6) μb at 100 MeV.

Studies of Nuclei Close to 132Sn Using Single-Neutron Transfer Reactions

Neutron transfer reactions were performed in inverse kinematics using radioactive ion beams of 132Sn, 130Sn, and 134Te and deuterated polyethylene targets. Preliminary results are presented. The Q‐value spectra for 133Sn, 131Sn and 135Te reveal a number of previously unobserved peaks. The angular distributions are compatible with the expected lf7/2 nature of the ground state of 133Sn, and 2p3/2 for the 3.4 MeV state in 131Sn.

Development of ORRUBA: A Silicon Array for the Measurement of Transfer Reactions in Inverse Kinematics

The development of high quality radioactive beams has made possible the measurement of transfer reactions in inverse kinematics on unstable nuclei. Measurement of (d,p) reactions on neutron-rich nuclei yield data on the evolution of nuclear structure away from stability, and are of astrophysical interest. Experimentally, (d,p) reactions on heavy (Z=50) fission fragments are complicated by the strongly inverse kinematics, and relatively low beam intensities. Consequently, ejectile detection with high resolution in position and energy, a high dynamic range and a high solid angular coverage is required. The Oak Ridge Rutgers University Barrel Array (ORRUBA) is a new silicon detector array optimized for the measurement of (d,p) reactions in inverse kinematics.

Single‐Particle Structure of Neutron‐Rich Nuclei

The d(82Ge,p) reaction has been measured in inverse kinematics at the Holifield Radioactive Ion Beam facility, enabling a study of the evolution of single‐particle structure above the N=50 shell gap for neutron‐rich nuclei.

Neutron Transfer Reactions: Surrogates for Neutron Capture for Basic and Applied Nuclear Science

Neutron capture reactions on unstable nuclei are important for both basic and applied nuclear science. A program has been developed at the Holifield Radioactive Ion Beam Facility at Oak Ridge National Laboratory to study single-neutron transfer (d,p) reactions with rare isotope beams to provide information on neutron-induced reactions on unstable nuclei. Results from (d,p) studies on {sup 130,132}Sn, {sup 134}Te and {sup 75}As are discussed.

Spectroscopy of 16O Using α+12C Resonant Scattering in Inverse Kinematics

A measurement of the {alpha}({sup 12}C,{alpha}){sup 12}C reaction has been performed using resonant scattering with a gas target. Beam energies of 46, 51, 56 and 63 MeV were used to populate resonances in the excitation energy range of 11.6 to 22.9 MeV in {sup 16}O. The angular distributions of the elastic scattering were measured at zero degrees using an array of segmented silicon strip detectors with a minimum range of 0 deg. to 30 deg. in the centre of mass. The spins of 8 resonances between 14.1 and 18.5 MeV were obtained, confirming spin assignments made using elastic scattering in normal kinematics. An R-matrix analysis of the data was performed which indicates that the present understanding of {sup 16}O in this region is good, but not complete.

Neutron Transfer Reactions on Neutron‐Rich N = 50 and N = 82 Nuclei Near the r‐Process Path

Neutron transfer (d,p) reaction studies on the N = 50 isotones, 82Ge and 84Se, and A≈130 nuclei, 130,132Sn and 134Te, have been measured. Direct neutron capture cross sections for 82Ge and 84Se (n,γ) have been calculated and are combined with Hauser‐Feshbach expectations to estimate total (n,γ) cross sections. The A≈130 studies used an early implementation of the ORRUBA array of position‐sensitive silicon strip detectors for reaction proton measurements. Preliminary excitation energy and angular distribution results from the A≈130 measurements are reported.

Development of the ORRUBA Silicon Detector Array

High quality radioactive beams have recently made possible the measurement of (d,p) reactions on unstable nuclei in inverse kinematics, which can yield information on the development of single-neutron structure away from stability, and are of astrophysical interest due to the proximity to suggested r-process paths. The Oak Ridge Rutgers University Barrel Array (ORRUBA) is a new high solid-angular coverage array, composed of two rings of silicon detectors, optimized for measuring (d,p) reactions. A partial implementation has been used to measure (d,p) reactions on nuclei around the N = 82 shell closure.