High-resolution measurement of the ¹⁶O(γ,pn) reaction

Sammanfattning: The 16O(gamma,pn) reaction has been measured with a resolution high enough to resolve individual low-lying states in the residual 14N nucleus. The relative population of the states provides detailed information about the quantum numbers of the proton-neutron pairs on which photons are absorbed. Partial cross-sections, available to the acceptance of the detector system, have been extracted for the individual states, and compared to a recent calculation based on absorption on one-pion exchange currents and the Delta-resonance current. The experiment was performed at the Maxlab accelerator laboratory in Lund, Sweden, using tagged photons at an energy of 67-76 MeV. The proton detector angular range was 60-100 degrees and the corresponding number for the neutron detector was 81-103 degrees. A missing-energy resolution of 1.5 MeV was obtained. The relative population of the states in the residual 14N nucleus indicates that the (gamma,pn) reaction takes place predominantly on proton-neutron pairs coupled to (J^pi,T) = (1^+,0), where J is the total angular momentum of the pair, pi is the parity and T is the isospin. The cross-section for absorption on (0^+,1) pairs is strongly suppressed. Furthermore, the relative population of the states indicates that both L=0 and L=2 pairs participate in the reaction, where L is the angular momentum of the center-of-mass of the pair.