Energy and Angular Distribution of Photoneutrons from Oxygen-18

Abstract

The energy and angular distribution of photoneutrons from ${\mathrm{O}}^{18}(\ensuremath{\gamma},n){\mathrm{O}}^{17}$ are investigated. An ${\mathrm{O}}^{18}$ target enriched to 90% is irradiated with bremsstrahlung of 20-MeV maximum energy from the University of Pennsylvania betatron. The photoneutrons are detected by recoil protons in nuclear emulsions at angles between 30 and 150\ifmmode^\circ\else\textdegree\fi{} with the beam. Distinct peaks in the neutron spectrum at 1.3, 3.1, 7.4, and possibly at 1.7, 5.5, 9.2, and 10.5 MeV are observed. The peaks at 1.3 and 3.1 MeV can be accounted for in terms of single-particle excitation modes ($2{S}_{\frac{1}{2}}\ensuremath{\rightarrow}2{P}_{\frac{3}{2},\frac{1}{2}}$) of a valence neutron without disturbing the ${\mathrm{O}}^{16}$ core. These correspond to ${1}^{\ensuremath{-}}$ levels in ${\mathrm{O}}^{18}$ at 10.3 and 12.2 MeV with transitions leaving ${\mathrm{O}}^{17}$ in the ${\mathrm{\textonehalf{}}}^{+}$ excited state at 0.871 MeV. The angular distribution of the neutrons is predominantly ${sin}^{2}\ensuremath{\Theta}$ at low energies. At high energies, there is an indication of interference effects between electric dipole and quadrupole absorption.