Monte Carlo simulation of a cobalt-60 beam.

We have used the Stanford Electron Gamma Shower (EGS) Monte Carlo code to compute photon spectra from an AECL Theratron 780 cobalt-60 unit. Particular attention has been paid to the careful modeling of the geometry and material construction of the cobalt-60 source capsule, source housing, and collimator assembly. From our simulation, we conclude that the observed increase in output of the machine with increasing field size is caused by scattered photons from the primary definer and the adjustable collimator. We have also used the generated photon spectra as input to a pencil beam model to calculate the tissue-air ratios in water and compared it to a model which uses a monochromatic photon energy of 1.25 MeV.