Gamma Reaction History Backgrounds at the NIF

The Gamma Reaction History (GRH) diagnostic at NIF detects gamma-rays, emitted directly from DT fusion reactions (DT$\gamma )$, through the use of four Gas Cherenkov detectors with adjustable gamma-ray energy thresholds. It is primarily used to determine bang time, burn width and total DT yield of the implosion. Background interference to the signal is insignificant when capsules are driven directly by the lasers, but can be significant during indirect-drive using a hohlraum, forming an approximately 20 ns plateau under the narrow $\sim $200 ps FWHM fusion signal. This background is independent of fusion yield and most likely the result of laser-plasma interaction (LPI) induced hot electron bremsstrahlung radiation. These hard x-rays stream out target chamber ports and take multiple scatter paths to reach the GRH photomultiplier tubes (PMT), where they then bypass the Cherenkov conversion process and generate signal by direct interaction with the PMT microchannel plates. An examination of this background contribution to the GRH signal and possible mitigation strategies will be presented.