Measurement of the angular correlations for the16O(e,e′n)15Oreaction in the giant resonance region

Abstract

Angular correlations have been measured for the ${}^{16}\mathrm{O}{(e,e}^{\ensuremath{'}}{n}_{0})$ and ${}^{16}\mathrm{O}{(e,e}^{\ensuremath{'}}{n}_{3})$ reactions in the energy region $\ensuremath{\omega}=20--31 \mathrm{MeV}$ at a momentum transfer of 0.33 ${\mathrm{fm}}^{\ensuremath{-}1}.$ They indicate dominance of the giant dipole resonance. The data are compared with a random-phase approximation calculation. A predicted backward-peaked angular correlation in the neutron decay, due to interference between the $T=0$ and $T=1$ resonances, was not observed. Legendre coefficients extracted from the angular correlation data suggest the existence of a broad $E2$ resonance over the giant dipole resonance region. The opposite signs of the correlation parameters ${b}_{1}$ and ${b}_{3}$ for the ${}^{16}\mathrm{O}{(e,e}^{\ensuremath{'}}{n}_{0})$ and ${}^{16}\mathrm{O}{(e,e}^{\ensuremath{'}}{p}_{0})$ reactions suggest that the $E2$ resonance below about 22 MeV is isoscalar. Above about 22 MeV, large positive values for ${b}_{1}$ and ${b}_{2}$ in ${}^{16}\mathrm{O}{(e,e}^{\ensuremath{'}}{p}_{0})$ compared to small ones in ${(e,e}^{\ensuremath{'}}{n}_{0})$ reflect the contribution resulting from the direct-knockout process in ${(e,e}^{\ensuremath{'}}{p}_{0}).$ The angular correlations for ${}^{16}\mathrm{O}{(e,e}^{\ensuremath{'}}{n}_{3})$ and ${}^{12}\mathrm{C}{(e,e}^{\ensuremath{'}}{n}_{0})$ were confirmed to be similar as expected from the similarity of their particle-hole configurations. The good agreement including the fine structure between the form factors for ${}^{16}\mathrm{O}{(e,e}^{\ensuremath{'}}{n}_{0})$ and ${}^{16}\mathrm{O}(\ensuremath{\gamma}{,n}_{0})$ confirmed the dominance of the giant dipole resonance in this energy region.