GRH Characterization using 4.4 MeV $^{12}$C Gamma-Rays

The OMEGA Gamma Reaction History (GRH) diagnostic has been characterized using a relatively well-known source of 4.43 MeV gamma rays produced from inelastic scattering of DT-neutrons off of a graphite puck placed near an imploding capsule at the OMEGA laser facility. An independently measured neutron yield, combined with the known $^{12}$C density and $^{12}$C(n,n'$\gamma )^{12}$C cross-section, allows an in-situ calibration of the GRH detection efficiency at 4.43 MeV. GRH data were collected at two different $^{12}$C target locations to confirm the published angular distribution of gamma rays and were compared with MCNP modeling predictions. These in-situ calibrations were used to validate the GRH simulation code based on a coupled MCNP/ACCEPT Monte-Carlo method. By combining these results with other absolute calibration methods, we are able to infer a DT branching ratio for gamma to neutron production and to make an accurate plastic ablator areal density measurement.