We have developed an extended direct-semidirect (DSD) model for fast-nucleon capture to virtual single-particle configurations that subsequently damp into the compound nucleus or (at sufficiently high excitation energies) escape into the continuum. The inclusion of final-state fluctuation effects is an important feature of this model. To test the model we have measured the spectra of gamma rays from approximately 10 MeV to the endpoint in the ${}_{}{}^{89}{}_{}{}^{}\mathrm{Y}$(p→,γ) reaction with 19.6 MeV polarized protons from the TUNL tandem accelerator. Gamma spectra were measured with a pair of 25.4 cm×25.4 cm anticoincidence-shielded NaI detectors at angles of 30°, 55°, 90°, 125°, and 150° with respect to the incident beam. The spectra show significant analyzing powers and forward peaking of the angular distributions. These features allow for the discrimination between compounds processes and direct processes. Analyzing powers and fore-aft asymmetries were observed for gamma energies below those associated with direct-semidirect transitions to known bound final states. We have also performed Hauser-Feshbach calculations of the statistical component of the gamma emission, which dominates below approximately 15–16 MeV. The extended DSD model reproduces the spectral shapes and analyzing powers above this energy quite well. There is no evidence in the present reaction that additional mechanisms, such as multistep compound or multistep direct emission, are required.

• Received 11 January 1995

DOI:https://doi.org/10.1103/PhysRevC.52.252