Modeling the wall effects of a tissue equivalent proportional counter using MCNP

Tissue equivalent proportional counters (TEPCs) utilize tissue equivalent materials to depict homogeneous microscopic volumes of human tissue. Although both the walls and gas simulate the same medium, they respond to radiation differently. Differences between the densities of the two materials cause distortions in the measured spectra, called wall effects. The most common type of wall effect is caused by delta rays given off when charged particles strike the TEPC. The number of delta rays that enter the gas cavity of the TEPC is often greater than the number that appear in the microscopic volume. A Monte Carlo transport code, MCNP, is chosen to simulate the transport of secondary electrons, or delta rays, within a TEPC The TEPC modeled in this study is composed of A-150 tissue equivalent plastic and propane-based tissue equivalent gas. The Rudd model is used to describe the secondary electron energy spectrum obtained by the impact of two iron beams, 250 and 1050 MeV nucleon^-1, on water. The wall effects in a TEPC are characterized by changing the source location, on either side of the TEPC wall, and by excluding the solid wall.

A comparison of flux responses with and without delta rays or re-entrant particles shows that the delta rays or re-entrant particles in either the gas/wall consistently directed the flux response for the gas/wall much higher than would be normally seen in a microscopic volume of tissue. Another comparison between a solid wall TEPC and a wall-less one illustrates that the wall attenuates the lower energy electrons at a much higher rate than the higher energy electrons. This causes a lower initial flux of particles that enter the gaseous volume for the lower end of the secondary electron energy spectrum

In addition to characterizing the flux response, the energy deposition as a function of impact parameter within the TEPC is evaluated. The wall-less TEPC has a higher lineal energy as a function of impact parameter than the solid-wall TEPC The wall-less TEPC gives a much better approximation of a microscopic volume. The most important conclusion about the lineal energy within a solid-wall TEPC is that MCNP has the ability to adequately model wall effects in a TEPC.