Charged particle equilibrium corrections for photon sources from 400 keV to 1.4 MeV.

摘要

Lack of charged particle equilibrium (CPE) has practical importance in radiological health protection, in nuclear medicine, and radiobiology where small radioactive point sources irradiate the human body accidentally or may be introduced into the body for diagnostic, therapeutic, or analytical purposes. The absorbed dose under CPE is readily calculated from knowledge of the photon energy fluence and mass-absorption coefficient of the material. When estimating absorbed dose rates at points close to the source, the primary radiation field varies appreciably over the region within the range of secondary particles. Under such conditions, CPE does not exist and prediction of absorbed dose becomes difficult. However, if one applies correction factors for non-CPE conditions, absorbed dose rates can be calculated fairly easily. In this dissertation, a CPE model was developed for non-CPE conditions to predict the fraction of charged particle equilibrium (Γ_CPE) attained in a water medium for point sources of energies in the range from 400 keV to 1.4 MeV using EGS4-DOSRZ Monte Carlo calculation. A new methodology to calculate absorbed dose and kerma along the central axis of the cylindrical phantom was presented and the results were found to be in excellent agreement with published values. In order to corroborate with the EGS4-DOSRZ calculation, another model based on the Klein-Nishina single scattering cross section was developed to quantify the Γ_CPE attained in water for point sources. A CPE path length coefficient (μ cm−1) was found for each photon energy and compared with published values. This coefficient was used to determine dose rates averaged over 1 cm2 at depths that are of interest in skin dose exposures. Experimental measurements of CPE were carried out for a Co-60 point source using GAFCHROMIC® MD-55 film (1990) as the dosimetry media. The films were read using a document scanner. Dose rates obtained using the scanner method were compared with those obtained using the single scattering model and excellent agreement was observed. Finally, the empirical model for Γ_CPE was implemented in a point kernel dose calculation code WISE and SIMPLE. A Windows-based user interface was also created for the modified code. The code was tested for various geometries and the results compared against those obtained using a very popular commercial code, MICROSHIELD©.