The National Ignition Facility at Lawrence Livermore National Laboratory is the world's leading
facility to study the physics of igniting plasmas. Plasmas of hot deuterium and tritium, undergo
d(t,n)α reactions that produce a 14.1 MeV neutron and 3.5 MeV a particle, in the center of mass.
As these neutrons pass through the materials surrounding the hot core, they may undergo
subsequent (n,x) reactions. For example, 12C(n,n'γ)12C reactions occur in remnant debris from
the polymer ablator resulting in a significant fluence of 4.44 MeV gamma-rays. Imaging of these
gammas will enable the determination of the volumetric size and symmetry of the ablation; large
size and high asymmetry is expected to correlate with poor compression and lower fusion yield.
Results from a gamma-ray imaging system are expected to be complimentary to a neutron
imaging diagnostic system already in place at the NIF. This paper describes initial efforts to
design a gamma-ray imaging system for the NIF using the existing neutron imaging system as a
baseline for study. Due to the cross-section and expected range of ablator areal densities, the
gamma flux should be approximately 10-3 of the neutron flux. For this reason, care must be taken
to maximize the efficiency of the gamma-ray imaging system because it will be gamma starved.
As with the neutron imager, use of pinholes and/or coded apertures are anticipated. Along with
aperture and detector design, the selection of an appropriate scintillator is discussed. The volume
of energy deposition of the interacting 4.44 MeV gamma-rays is a critical parameter limiting the
imaging system spatial resolution. The volume of energy deposition is simulated with GEANT4,
and plans to measure the volume of energy deposition experimentally are described. Results of
tests on a pixellated LYSO scintillator are also presented.