Modifications to MCNP to provide coincident data files for use in Compton camera geometry studies

Compton camera imaging (CCI) systems have been investigated for a number of applications. Because useful Compton events consist of coincident interactions in two or more detector element cells, a simulation code used must also produce coincidence event data information. While MCNP is an effective and well-tested Monte Carlo code, in its unmodified form it does offer provisions to produce the necessary coincidence data. This paper describes modifications to the MCNP 4A code to enable it to provide a file containing coincident event data from cells of interest for a wide variety of geometries and problems. An alternative to modification, the use of the PTRAC option was first considered and reasons for its rejection are discussed. The approach taken for extracting coincidence information was designed to avoid alteration of the program data or control variables. The modification to MCNP involves intercepting the F8 tally energy accounting data for cells of interest. A description of the modification data structures and data flow are presented. Sample input and output files are presented and limitations of this technique are discussed. The approach to validation of code integrity after modification is also discussed.