A Monte Carlo Simulation of the in vivo measurement of lung activity in the Lawrence Livermore National Laboratory torso phantom.

A torso phantom was developed by the Lawrence Livermore National Laboratory (LLNL) that serves as a standard for intercomparison and intercalibration of detector systems used to measure low-energy photons from radionuclides, such as americium deposited in the lungs. DICOM images of the second-generation Human Monitoring Laboratory-Lawrence Livermore National Laboratory (HML-LLNL) torso phantom were segmented and converted into three-dimensional (3D) voxel phantoms to simulate the response of high purity germanium (HPGe) detector systems, as found in the HML new lung counter using a Monte Carlo technique. The photon energies of interest in this study were 17.5, 26.4, 45.4, 59.5, 122, 244, and 344 keV. The detection efficiencies at these photon energies were predicted for different chest wall thicknesses (1.49 to 6.35 cm) and compared to measured values obtained with lungs containing (241)Am (34.8 kBq) and (152)Eu (10.4 kBq). It was observed that no statistically significant differences exist at the 95% confidence level between the mean values of simulated and measured detection efficiencies. Comparisons between the simulated and measured detection efficiencies reveal a variation of 20% at 17.5 keV and 1% at 59.5 keV. It was found that small changes in the formulation of the tissue substitute material caused no significant change in the outcome of Monte Carlo simulations.

Effect of a Simulation of 241Am Deposition Pattern in the Leg Bones on the Detection Efficiency of a High Purity Germanium Detector.

A computational model using an MCNPX version 2.6.0 code and a leg voxel phantom was previously constructed and validated against the in vivo measurements of the United States Transuranium and Uranium Registries (USTUR) case 0846 leg. Using the MCNPX model, different simulation scenarios of Am distribution in the bones and tissue material of a leg were performed, and their effects on the detection efficiency and activity calculation were examined. The purpose of this work is to ensure and increase the simulation sensitivity of real contaminated human bones and reduce the simulated efficiency error associated with the distribution of Am activity within the leg bones when using a high purity germanium [HP(Ge)] detector. The results showed that the simulated detection efficiency obtained from the uniform distribution of Am in the leg bones was underestimated by a factor of up to 0.3 compared with the measured and simulated detection efficiency obtained from the non-uniform distribution of Am in different sections of the leg bones. The p-value of a one-way analysis of variance (ANOVA) F-test among the mean values of the simulated detection efficiencies was calculated and provided evidence of a significant difference. The uncertainty in the bone activity estimate could be quite large (25% to 30%) if calibration of detection efficiency is based on assuming a uniform distribution of Am in the phantom to estimate the USTUR case 0846 leg activity. It is therefore recommended that during calibration of detectors, a non-uniform distribution of Am in different sections of the bones should be used rather than a uniform distribution. Additionally, an assumption of a uniform distribution of Am will simulate Am activity deposited in the leg bones of a real contamination case inadequately.