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2. Recent HRIBF Research - Proton-Transfer Study of Unbound 19Ne States via 2H(18F,alpha + 15O)n
(C. R. Brune, Spokesperson)

The 18F(p,alpha)15O reaction in the temperature range (1-4)x108 K plays an important role in classical novae. The rate of this reaction affects the production of heavier elements and is also critical for calculating the flux of 511-keV gamma rays which result from 18F decay (a major target of gamma-ray observatories). In order to better determine this reaction rate, several measurements of the 18F(p,alpha)15O reaction have been undertaken or are planned for Ec.m. > 330 keV at ORNL and elsewhere in recent years (e.g., RIB-099). The neutron transfer reaction 18F(d,p)19F has also been studied at ORNL to determine properties of states in the mirror nucleus (RIB-044). While these studies have contributed significantly to our understanding of the situation, there remains considerable uncertainty in the extrapolation of the cross section to lower energies and the assignment of analog states between 19F and 19Ne. Of particular importance are the two 3/2+ states just above the proton threshold in 19Ne (potential 8- and 38-keV resonances): it is still not clear which state, if either, has a significant proton spectroscopic factor. The answer to this question can change the 18F(p,alpha)15O reaction rate by up to an order of magnitude, with an even larger potential change in the amount of 18F synthesized in novae.

In order to address these questions, an experiment was carried out to measure the proton transfer reaction 18F(d,n)19Ne at ORNL/HRIBF in summer 2005 (RIB-145). With this approach, it is possible to determine resonance energies and the distribution of proton strength in 19Ne without resorting to mirror symmetry. A 150-MeV 18F9+ beam was used to bombard a 720-μg/cm2 CD2 target. A total of 14 shifts of 18F beam were used for the measurements, with an average intensity of about 2.5x106/s on target. The 19Ne excited states of interest decay by breaking up into alpha + 15O. The alpha and 15O were detected in coincidence with six position-sensitive Si detector telescopes. The energy and position determination allowed the momenta of both reaction products to be reconstructed. The excitation energy of the decaying state relative to the alpha + 15O threshold (relative energy) and the momentum of the undetected neutron can then also be calculated. The resolution in the relative energy is only minimally broadened by the effects of energy loss in the target and finite beamspot size. A schematic diagram of the target and detector layout is shown in Fig. 2-1. Backgrounds from other reactions are strongly suppressed by utilizing the coincidence requirement and particle identification on the alpha and 15O.

Fig. 2-1. Schematic diagram of the experimental apparatus. Each position-sensitive Si detector telescope is 5 cm x 5 cm; they were located approximately 45 cm downstream from the target and covered lab angles between 2.5 and 17 degrees. The 15O recoils were detected in the inner telescopes while alphas were primarily detected in the outer telescopes.

A particle identification spectrum obtained from an inner telescope is shown in Fig. 2-2. In order to optimize the resolution and calibration of the relative energy, the position and energy calibrations of the detectors must be well known. We thus also carried out stable beam calibrations using the elastic scattering of alpha and 16O beams. The resolution for the relative energy achieved is approximately 70 keV. Backgrounds were measured from a carbon target (i.e., without deuterium) and found to be negligible. We have also simultaneously measured the mirror reaction 2H(18F,alpha + 15N)p. The comparison of these mirror reactions will be helpful for determining isospin mirror assignments. The analysis of these data is ongoing. This project is a part of the Ph.D. dissertation of Aderemi Adekola from Ohio University.

Fig. 2-2. Particle identification spectrum obtained from an inner telescope.

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