Ground state photoneutron reactions in 17O.

Abstract

Photoneutron angular distributions and differential cross sections have been measured for the reaction $^{17}\mathrm{O}$(\ensuremath{\gamma},${\mathrm{n}}_{0}$${)}^{16}$O over the excitation energy region from 10 to 24 MeV. The angle-integrated ground state cross section was obtained and compared to the total photoneutron cross section. The comparison indicates that the ground state channel dominates the pygmy resonance region below ${E}_{\ensuremath{\gamma}}$=16 MeV but contributes very little (<10%) to the giant resonance region (near 22 MeV). Legendre coefficients extracted from the angular distribution data suggest that electric dipole transitions make up nearly all of the absorption strength in the region studied except in narrow regions near 10.8, 15.1, 17.3, and 22.3 MeV, where small but significantly nonzero values of the ${a}_{1}$ coefficient are observed. Values of the ${a}_{2}$ coefficient, interpreted in an E1 approximation, suggest a relatively pure single-particle composition of many of the states observed in the pygmy resonance region from 10 to 16 MeV.