Internal dosimetry

The estimation of the internal contamination is done by direct measurement of the gamma rays emitted from inside the body and by indirect methods (e.g. activity in urine samples). In the anthropogammametry laboratory of SCK•CEN we focus on the direct measurements; people working in nuclear facilities are screened for internal contaminations. The individual is placed in a shielded room to reduce the background radiation and gamma rays emitted from the body are detected with large NaI(Tl) scintillators (whole body counting) or with germanium detectors (lung or thyroid measurements).

The main challenges of these measurements are:

• Reducing the ambient background by using shielded rooms
• Increasing the photo-peak detection efficiency
• Reducing the uncertainty on the reported activity by reducing the uncertainties on the efficiency calibration of the full detection system

The efficiency of the detection system is significantly influenced by the size of the person which is monitored. This is valid both for the whole body counting and for the lung measurements. As a first approach we developed a new modular phantom for whole body counting so that efficiency curves are obtained for different body geometries (length, weight).

Nowadays, with the Monte Carlo methods it is possible to precisely simulate complex radiation interactions and energy depositions in human body. Also different body geometries can be simulated to study the influence of this on the measurement efficiency. Therefore we try to use Monte Carlo simulations to improve the reliability of the calibration of a counting facility, particularly for non-standard measurements (obese people, children, etc). Anthropomorphic voxel phantoms are used in the Monte Carlo simulations for a better representation of the body geometry.

Figure 1: Example of whole body counting set-up and its Monte Carlo simulation.

Figure 2: Example of a physical phantom, its voxel representation of it and the simulation of a set-up consisting of the phantom and 4 germanium detectors as seen from two different angles.

Contact: Lebacq Anne Laure, Fonseca Telma, Vanhavere Filip