Effect of phantom thickness variation on speed and efficiency for different MC codes

The effect of the phantom thickness on the simulation speed and efficiency of Monte Carlo (MC) codes, MCNP4c, DOSXYZnrc, DOSRZnrc and DPM, is investigated. The result shows that the in- creases of the simulation speed for different MC codes are distinct difference with dicreasing the phantom thickness, in which DOSXYZnrc is the most one, and DOSRZnrc is the least one. In the viewpoint of MC efficiency, DPM and DOSRZnrc have the remarkable advantages over others; and the efficiency for the thinner phantom shows the consistent advantage over the thicker one for these MC codes. In the viewpoint of MC efficiency increase, MCNP4c, DOSXYZnrc and DOSRZnrc show the same comparability, but DPM shows relatively low characteristics. The strange characteristics of DOSRZnrc in statistical uncertainty, speed increase are further investigated to find that it is due to the photon forcing VRT (variance reduction technique) implemented in it. This VRT reduces the statistical uncertainty, but it also weakens the speed advantage of the thinner phantom. However it is due to the remarkably under estimate of the variance on the interest region, the MC efficiency of DOSRZnrc is still increasing for the thinner phantom. (authors)