Electroexcitation of the giant resonance of 17O

Abstract

The 11 to 30 MeV excitation region of 17O has been investigated by means of inelastic electron scattering, with special emphasis placed on the giant resonance. The scattering angle employed was 75.1°, and the incident energies were 64.9, 83.3, 101.3, 113.6, and 124.0 MeV. These conditions correspond to momentum transfers in the range 0.33 to 0.77 fm-1. The data reveal a broad resonance centred at 22 - 23 MeV excitation, with strength extending down to 10 - 12 MeV excitation. Some fine structure is observed which correlates well with the positions of previously observed levels. A particle-hole model with harmonic oscillator wavefunctions and Kuo-Brown residual interaction is found to predict the strength distribution and q-dependence of the giant resonance form factor very well. However, a scale factor of 1.85 is required to achieve agreement in magnitude. A fit to the Helm model prediction for a C1 transition yields reasonable values for the skin thickness and transition radius parameters. No clear evidence is found for isospin splitting of the giant resonance. The particle-hole model predicts only a slight degree of splitting in the transverse (E1) component of the form factor, and none at all in the longitudinal (C1) component. A smaller structure between 17.5 and 19.6 MeV excitation is reported for the first time. The Helm model for a C2 transition using parameters determined by the fit to the giant resonance form factor is found to predict the form factor for this structure extremely well. Levels at 16.5 and 19.6 MeV excitation are also reported for the first time. The former is found to have a natural width of about 300 keV, while the latter appears to be considerably narrower.