Monte Carlo investigations of distance-dependent effects on energy deposition in K-shell x-ray fluorescence bone lead measurement

Abstract

Radiation energy deposition results are presented from a Monte Carlo code simulating the lower part of a leg during an in vivo 109Cd K-shell x-ray fluorescence (KXRF) bone lead measurement. The simulations were run for a leg phantom model representing an adult subject, assuming concentrations of 10 µg Pb per gram bone mineral and tracing 500 million photons in each simulation. Trials were performed over a range (0.5–6.0 cm) of source-to-sample (S–S) distances. Energies deposited due to Compton and photoelectric processes occurring in the bone and the soft tissue were obtained. The data show an increase in the amount of energy deposited in the bone as the sample is moved closer to the source (from 2.0 cm to 0.5 cm). However, there is a decrease in the amount of energy deposited in the soft tissue as the sample is moved closer to the source over the same distance interval. In decreasing the S–S distance from 2.0 cm to 0.5 cm, the amount of energy deposited in the sample as a whole was found to increase by 11%. By calculating the energy deposition in the bone and in the soft tissue as a fraction of the total energy deposited in the sample, the corresponding changes are quantified as a function of S–S distance. Similarly, the proportions of energy deposited via the photoelectric effect and Compton scattering are presented as a function of S–S distance.