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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 [1], direct [2,3], and time-reversed [4,5] studies, significant uncertainties remain. In particular, a recent direct measurement [2] 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 [6] 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. [2] 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. [2] was correct.

References:

[1] K. I. Hahn, et al., Phys. Rev. C54, 1999 (1996).
[2] M. Notani, et al., Nucl. Phys. A746, 113c (2004).
[3] S. Kubono, et al., Eur. Phys. J. A27, 327 (2006).
[4] J. C. Blackmon, et al., Nucl. Phys. A688, 142c (2001).
[5] B. Harss, et al., Phys. Rev. Lett. 82, 3964 (1999).
[6] D. W. Bardayan, et al., Phys. Rev. C81, 065802 (2010).




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