Determining Optimal Activated Graphite Thicknesses Using MCNPX

The Monte Carlo N-Particle Transport Code (MCNPX) is being used to model the detection of back-to-back 511 keV gamma rays produced by positron annihilation in a graphite disk. Once activated by fusion neutrons, the disk is placed between matching NaI detectors which count the coincident gamma rays. The objective of the simulations is to optimize the signal-to-noise ratio of the coincident count detection system by varying the thickness of the activated disk. The optimal ratio is determined by balancing the gain achieved as the disk thickness increases against the loss due to gamma ray absorption. Geometric considerations of the detection system play an important role as the detectors are separated to accommodate the thicker disk. These MCNPX results for an optimal thickness will be reported.