Dead layer thickness characterization of an HPGe detector by measurements and Monte Carlo simulations.

To fully characterize the front dead layer (DL) of an HPGe detector at low photon energy range, its intrinsic efficiency curve was measured using a (241)Am radioactive source in 10-60 keV energy range. A comparison between experimental efficiency and MCNPX results showed that the DL value of 0.4 µm initially quoted by the manufacturer has to be changed to 7.5 µm to reproduce measurements.

Radiation protection in inhomogeneous beta-gamma fields and modelling of hand phantoms with MCNPX.

The usage of beta-radiation sources in various nuclear medicine therapies is increasing. Consequently, enhanced radiation protection measures are required, as medical staff more frequently handle high-activity sources required for therapy. Inhomogeneous radiation fields make it difficult to determine absorbed dose reliably. Routine monitoring with dosimeters does not guarantee any accurate determination of the local skin dose (LSD). In general, correction factors are used to correct for the measured dose and the maximum absorbed dose received. However, strong underestimations of the maximum exposure are possible depending on the individual handling the process and the reliability of dose measurements. Simulations can be used as a tool for a better understanding of the maximum possible exposure depending on the individual-related handling. While measurements reveal the overall dose during the entire irradiation time of the dosimeter, simulations help to analyse sequences of action. Hence, simulations allow for tracking the points of highest absorbed dose received during the handling process. In this respect, simulations were performed using the MCNPX software. In order to investigate the LSD, two hand phantoms were used, a model based on geometrical elements and a voxel hand. A typical situation of radiosynoviorthesis, i.e. handling a syringe filled with (90)Y, was simulated. The results of the simulations show that the annual dose limit may be exceeded within minutes at the position of maximum absorbed dose received and that finger-ring dosimeters measure significantly different doses depending on their wearing position. It is of essential importance to wear the dosimeter properly and to use suitable correction factors with respect to the individual. Simulations are a suitable tool for ensuring reliable dose determination and may help to derive recommendations regarding radiation protection measures.

Validation of a Monte Carlo efficiency calibration procedure for a partial body counter system with a voxel model of the LLNL torso phantom.

Virtual models of real phantoms used with Monte Carlo methods facilitate the calibration and other studies associated with whole-body and partial-body counting systems. In this investigation, a voxel model of an LLNL torso phantom, available physically in the in vivo laboratory at KIT, was created from computed tomography scans. Series of measurements with a high-purity germanium detector and the real torso phantom, loaded with different radioactive organs, have been carried out. Computer simulations of these measurement setups were performed with the aid of MCNPX, using a coarsened voxel phantom and a validated model of the germanium detector. The results of simulations were compared with data from the measurements and an agreement within the uncertainties was found. The voxel model could therefore be validated. The results of the simulations were then used to quantify the activity of (241)Am impurities detected in the liver loaded with (239)Pu.

A Monte Carlo study of lung counting efficiency for female workers of different breast sizes using deformable phantoms.

There are currently no physical phantoms available for calibrating in vivo counting devices that represent women with different breast sizes because such phantoms are difficult, time consuming and expensive to fabricate. In this work, a feasible alternative involving computational phantoms was explored. A series of new female voxel phantoms with different breast sizes were developed and ported into a Monte Carlo radiation transport code for performing virtual lung counting efficiency calibrations. The phantoms are based on the RPI adult female phantom, a boundary representation (BREP) model. They were created with novel deformation techniques and then voxelized for the Monte Carlo simulations. Eight models have been selected with cup sizes ranging from AA to G according to brassiere industry standards. Monte Carlo simulations of a lung counting system were performed with these phantoms to study the effect of breast size on lung counting efficiencies, which are needed to determine the activity of a radionuclide deposited in the lung and hence to estimate the resulting dose to the worker. Contamination scenarios involving three different radionuclides, namely Am-241, Cs-137 and Co-60, were considered. The results show that detector efficiencies considerably decrease with increasing breast size, especially for low energy photon emitting radionuclides. When the counting efficiencies of models with cup size AA were compared to those with cup size G, a difference of up to 50% was observed. The detector efficiencies for each radionuclide can be approximated by curve fitting in the total breast mass (polynomial of second order) or the cup size (power).