High resolution measurement of the16O(γ,pn)14N0,1,2,…reaction

Abstract

The exclusive ${}^{16}\mathrm{O}(\ensuremath{\gamma}{,pn)}^{14}{\mathrm{N}}_{0,1,2,\dots{}}$ reaction was measured for the photon energy range ${E}_{\ensuremath{\gamma}}=98.5$ to 141.0 MeV with an excitation energy resolution of 2.8 MeV. Protons were detected at $76\ifmmode^\circ\else\textdegree\fi{}$ and $82\ifmmode^\circ\else\textdegree\fi{}$ and coincident neutrons were detected at the opening angle for quasifree deuteron photodisintegration. Only the $T=0,$ ${1}^{+},$ 3.95 MeV state in the residual ${}^{14}\mathrm{N}$ nucleus was significantly populated. This corresponds to a preferred $L=0$ angular momentum transfer to the recoil nucleus as is expected for quasifree kinematics. The absence of significant population to other discrete states is evidence that absorption on correlated proton-neutron pairs with an angular momentum transfer of $L>~2$ is largely suppressed for the low recoil momentum range of the current measurement. As a consequence of this fact, and the absence of unnatural parity states, only absorption on proton-neutron pairs in relative motion $l=0,2$ is expected for all possible shell model couplings. No significant population of the $T=1$ state at 2.31 MeV is consistent with previous measurements, demonstrating that absorption on proton-neutron pairs in an isospin triplet is suppressed. The results of the measurement to discrete states in the residual ${}^{14}\mathrm{N}$ nucleus are compared to microscopic calculations which include contributions above that of the mean field due to the short range and tensor parts of the nuclear potential. Clear separation in the contribution of ${(1p)}^{\ensuremath{-}2}$ and ${(1p)}^{\ensuremath{-}1}{(1s)}^{\ensuremath{-}1}$ proton-neutron pairs is seen to occur at an excitation energy of $20\ifmmode\pm\else\textpm\fi{}2 \mathrm{MeV}.$ Significant population of the ${(1p)}^{\ensuremath{-}1}{(1s)}^{\ensuremath{-}1}$ continuum above 20 MeV indicates that absorption on $L=1$ nucleon pairs is also important near quasifree kinematics.