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2. Recent HRIBF Research - Direct Measurement of 17F(p,γ)18Ne at HRIBF
(M. S. Smith, Spokesperson)

The rate of the 17F(p,γ)18Ne reaction is of significant astrophysical importance in such explosive astrophysical events as novae and X-ray bursts. The decay of radioactive 17F is thought to help drive the expansion of the envelope ejected in the nova outburst, and the 17F(p, γ)18Ne reaction affects the production of 18F, a target of γ-ray astronomy, and influences the 17O/18O ratio produced in these explosions. It is also an important link in the alpha-p reaction chain during the ignition phase of X- ray bursts. The 17F(p,γ)18Ne reaction rate was thought to be dominated by the contribution from an unnatural parity 3+ state that was predicted to exist from analog nuclei but was missing in decades of spectroscopic studies of 18Ne. In 1999, this "missing" 3+ state was found at 599.8 keV in the center of mass via a measurement of the 17F(p,p)17F reaction at HRIBF, which was the first measurement with a reaccelerated beam in North America [1]. However, the strength of this resonance for (p,γ) was still unknown, resulting in more than an order of magnitude uncertainty in the capture reaction rate.

To address this, the 17F(p,γ)18Ne reaction has now been measured directly, to roughly 40% statistics. The HRIBF measurement used a mixed beam of 35 - 70% radioactive 17F and the remainder stable 17O at beam intensities of typically more than 5X106 pps of 17F ions. This was a factor of ~50 increase in low-energy 17F beam from previous measurements. The radioactive beam bombarded a differentially-pumped windowless hydrogen-gas target, and the Daresbury Recoil Separator was used to separate 18Ne recoils from unreacted primary beam. The spectra below show the excellent separation of recoils in the gas-filled ionization chamber at the focal plane. Resonant proton capture cross sections, gamma widths, and strengths for two resonances at 1178 keV and 599 keV in the center of mass have been determined, as well as an upper limit on the direct capture cross section at 800 keV in the center of mass. The strength of the 599-keV level was found to be more than a factor of two larger than a recent estimate based on shell model calculations of the gamma width of the analog level in 18O [2], resulting in an increase in the total 17F(p,γ)18Ne reaction rate. A set of nova simulations have been run with this new rate, and preliminary results indicate that over 400 times more 18F and 17O can be made in the hot, inner zones of one nova model when compared to simulations run with the most widely used 17F(p,γ)18Ne reaction rate. This work was the Ph.D. thesis for Kelly Chipps from the Colorado School of Mines. Future work may include a measurement of the direct capture cross section to three states just below the proton threshold in 18Ne.

[1] D.W. Bardayan et al., Phys. Rev. Lett. 83 (1999) 45.
[2] A. Garcia et al., Phys. Rev. C. 43 (1991) 2012.

Figure 2-1: Ionization-counter spectrum from direct measurement of 17F(p, γ)18Ne at HRIBF showing identification of 18Ne recoils and scattered 17F and 17O beam particles at the focal plane of the Daresbury Recoil Separator (DRS). This yield is at the dominant resonance at an energy of 599 keV in the center of mass.

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