Differential cross-section measurements of $^{18}$O(g,n)$+^{18}$O(g,2n) reactions at 23.7 and 32.0 MeV

Photoneutron reactions on $^{18}$O allow examination of the neutron-neutron (nn) interaction and may be used as a surrogate for planning a $^{3}$H photodisintegration experiment at TUNL. The relatively large $^{18}$O($\gamma $,2n) cross section allows measurements of differential cross-sections for nn coincidences at various opening angles between the neutrons without risks associated with handling a radioactive gas target. Cross sections for the sequential and simultaneous emission of the two neutrons in the $^{18}$O($\gamma $, nn) reaction are measured. The simultaneous neutron emission provides information about neutron-neutron correlations in the $^{18}$O nucleus. Ab-initio calculations are needed to disentangle the simultaneous contribution from the aggregate cross section. The measurements were performed at HI$\gamma $S using a 23.7 and 32.0 MeV circularly polarized photon beam. Neutrons were detected using 30 liquid scintillator detectors at reaction angles of 65, 90, and 180\textdegree . Neutron energies were determined using time-of-flight techniques. These are the first differential cross-section data for $^{18}$O($\gamma $,n) and $^{18}$O($\gamma $,2n). A description of experimental techniques and preliminary results will be presented.