We report a detailed calculation of the angular distribution and polarization of the photoneutrons from ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ in the giant dipole region. The electric dipole ($E1\mathrm{}$) amplitudes are obtained from a continuum shell-model calculation which reproduces the intermediate structure in the total cross section. A consistent interpretation of the angular distribution and polarization may be obtained either by (i) assuming a phenomenological giant quadrupole ($E2\mathrm{}$) resonance, or (ii) by modifying the phase difference between the $E1\mathrm{}$ amplitudes. In case (ii), we do not require any $E2\mathrm{}$ resonance to fit the data, or, alternatively, the magnitudes of the $E2\mathrm{}$ amplitudes used can be taken to be in reasonable agreement with those extracted from the polarized-proton capture experiment. As we show in case (i), there is a theoretical possibility for an $E2\mathrm{}$ resonance. On the other hand, the present experimental results indicate the absence of such a resonance in the dipole region; this possibility is studied in case (ii).

[NUCLEAR REACTIONS ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}(\gamma ,n_0)$; giant dipole resonance; angular distribution and polarization of photoneutrons. Giant $E2\mathrm{}$ resonance in ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$.]

• Received 16 November 1973

DOI:https://doi.org/10.1103/PhysRevC.9.2144