Nuclear Regulatory Authority low energy germanium detection system: performance for the uranium individual monitoring

Abstract

The lung counter facility of the Nuclear Regulatory Authority (ARN) is presented. A calibration was carried out using the Lawrence Livermore National Laboratory (LLNL) phantom. This phantom is provided with a pair of lungs and lymph nodes containing uranium homogeneously distributed and a set of four overlay plates covering a chest wall thickness (CWT) ranging from 1.638 to 3.871 cm. Individual organ calibration factors were acquired for 235U photopeaks energies and for each effective chest thickness. Using these factors, a collection of theoretical fitting curves were found. A counting efficiency formulae and a curve for simultaneously active lymph nodes and lung was obtained and checked through measures. Background measurements of the chamber with and without volunteer persons were performed in order to obtain the detection limits (DL) of the system. As this task involves the knowledge of the volunteers CWTs, these magnitudes were determined through formulae selected from the literature taking into account the detection system characteristics. The deviation in the CWT assigned to an individual, generated by applying different equations, produces variations up to 33% in the estimations of the incorporated activity and DL. An analysis of the changes in efficiencies as consequences of the detectors locations and CWT was also performed. This reveals that the DL of the camera (detectors, shield and blank phantom) is between 2.7 and 6.4 Bq of 235U, which implies 4.9 and 11.5 mg lung burden of natural uranium. An estimation of the minimum detectable intake performed with the DL considering blank persons shows that a system with the characteristics described is only adequate for non-routine individual monitoring.

Study of the Influence of Radionuclide Biokinetics on the Efficiency of In Vivo Counting Using Monte Carlo Simulation

“…Consequently, an internal source of activity is seldom limited to one organ or to one body region but rather follows a time-dependent distribution among different organs. Several studies have already demonstrated the influence of the biokinetics on in vivo counting by experimental means (Spinella et al 2007) and by Monte Carlo simulation in voxel phantoms (Hunt et al 2003;de Carlan et al 2007). However, no general method has been available to practically take this information into account for calibration of in vivo counting systems currently in use by health physics laboratories.…”