Using GEANT4 to investigate performance of the NIF Gamma Imaging System

The National Ignition Facility at Lawrence Livermore National Laboratory is aiming to achieve the self-heated and sustained fusion of deuterium and tritium using over 2 MJ of laser energy. The required pressure and temperatures are generated at the centre of an imploded DT capsule surrounded by a thin ablator material, typically high-density carbon. During the burn history, neutrons stream out through the remnants of the ablated carbon shell and undergo 12C(n,n’γ)12C, resulting in the emission of 4.4 MeV gammas. Imaging of the carbon 4.4 gammas has been achieved through a modification to the NIF neutron imaging system, enabling determination of the spatial distribution and volumetric size of the remaining shell. Large size and significant asymmetries in the gamma images are expected to correlate with poor compression and reduced fusion yield, making the gamma imaging system a critical tool for diagnosing implosion performance. In this work we discuss how the GEANT4 Monte Carlo code has been used to investigate key parameters of the system, such as the spatially and energy-dependent point spread functions and scintillator sensitivity, and how synthetic images can be used to stress the likelihood maximization image analysis routines.