2. Recent HRIBF Research -
Inelastic 17F(p,p)17F Scattering at 3 MeV and the 14O(α,p)17F Reaction Rate
[D. Bardayan (ORNL), spokesperson]
The 14O(α,p)17F reaction is an important trigger reaction in X-ray burst nucleosynthesis, and despite several indirect , direct [2,3], and time-reversed [4,5] studies, significant uncertainties remain. In particular, a recent direct measurement  observed an unexpected peak in the thick-target excitation function at Ec.m. = 1.45 MeV. Since no known 18Ne resonances exist at that energy, their conclusion was that the peak arose from a known 4+ resonance at 1.95 MeV (~3.1 MeV in the time-reversed 17F+p frame) and populating the first excited state of 17F at 495 keV in the exit channel. This was somewhat surprising because previous studies had not found such a large branch populating the first excited state of 17F. In principle, such a resonance should also be observable in a measurement of the 17F(p,p')17F* excitation function at 3 MeV.
To confirm the existence of this resonance, the inelastic scattering reaction was studied at the HRIBF with a 17F beam bombarding a CH2 plastic foil. Several beam energies between 52-58 MeV were measured with 17F beams of intensities of ~4x105 17F/s and purities of17F/17O ~ 2/1. Protons were detected in the SIDAR Silicon Detector Array in coincidence with beam-like recoils detected at forward angles in an isobutane-filled ionization chamber. The ion chamber could distinguish 17F from 17O recoils and was thus used to unambiguously identify the reaction events of interest.
Figure 2-1: In (a)-(g), Q-value spectra are shown for 17F+p scattering events at bombarding energies of 52-58 MeV, respectively.
Data collected from SIDAR in coincidence with recoil 17F ions are shown in a Q-value plot in Fig. 2-1. The large peak at Q=0 arises from elastic scattering 17F(p,p)17F events whereas inelastic events should result in a peak at -495 keV. No evidence for inelastic scattering was observed at these energies in contradiction to data gated on 17O recoils (Fig. 2-2) which show clear evidence for inelastic scattering as a peak at Q=-871 keV. Upper limits  on the 17F(p,p')17F* cross section are shown in Fig. 2-3 in comparison with what would be expected if the Notani et al.  interpretation of their data were correct. Clearly more work is needed to understand the mysterious peak observed in their data.
Figure 2-2: The same as Fig. 2-1, but now gated on 17O recoils.
Figure 2-3: Upper limits on the 17F(p,p')17F cross section. The dashed curve shows the expected cross section if the interpretation from Ref.  was correct.
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 D. W. Bardayan, et al., Phys. Rev. C81, 065802 (2010).