Evidence for the isovector giant quadrupole resonance inO16from theO16(γ,n0)O15reaction

The cross section and angular distribution for the reaction ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$($\gamma$,$n_0$)${}_{}{}^{15}{}_{}{}^{}\mathrm{O}$ have been measured over an excitation energy range of 25-45 MeV. Neutron time-of-flight spectra were recorded at six angles (22.5°, 45°, 67.5°, 90°, 112.5°, and 135°) and at four bremsstrahlung end-point energies (30, 35, 40, and 45 MeV). Differential cross sections deduced from these spectra were fitted by a fourth-order Legendre-polynomial expansion. The cross section obtained from this expansion decreases smoothly from 3.95 mb at 25.5 MeV to 0.22 mb at 43.8 MeV. The appearance in this analysis of large first-, third-, and especially fourth-order coefficients, is strong evidence for significant $E2\mathrm{}$ cross section. An $E2\mathrm{}$ cross section extracted from the data exhausts approximately 68% of the isovector $E2\mathrm{}$ energy-weighted sum rule. The present data are compared to previous related measurements and excellent agreement is seen. In comparisons with theoretical studies on ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$, there is good general agreement but relatively poor detailed agreement.