Differential cross-section measurements of the ¹⁸O(γ,n) + ¹⁸O(γ,2n) reactions at 23.7 and 32.0 MeV

Photoneutron reactions on Oxygen-18 offer an opportunity to examine the neutron-neutron interaction and may be used as a surrogate reaction for planning a tritium photodisintegration experiment at the Triangle Universities Nuclear Laboratory (TUNL).  The relatively large ¹⁸O(γ,2n) reaction cross section allows measurements of differential cross-section data for neutron-neutron coincidences at various opening angles between the neutrons similar to the ³H(γ,nn) reaction without the risks associated with handling a radioactive gas target.  Cross sections for the sequential and simultaneous emission of the two neutrons in the ¹⁸O(γ, nn) reaction are measured.  The simultaneous neutron emission provides information about neutron-neutron correlations in the ¹⁸O nucleus.

The measurements were performed at the High Intensity γ-ray Source (HIγS) using a 23.7 and 32.0 MeV circularly polarized photon beam.  Neutrons were detected using a neutron detector array comprised of 30 liquid scintillator detectors at reaction angles of 65°, 90°, and 135°.  Neutron energies were determined using time-of-flight techniques. These are the first differential cross-section data for ¹⁸O(γ,n) and ¹⁸O(γ,2n).  A description of experimental techniques and preliminary results will be presented.