The Reduction of limits of detection in In-Vivo Counting of Low Energy Photon Emitters by optimizing the shape and sizes of detectors
Workshop on Internal Dosimetry of Radionuclides - Occupational, Public and Medical Exposure - 9-12 September 2002 - New College, Oxford, United Kingdom
J.L. Genicot1, D. Franck2 and L. de Carlan2
1 Whole Body Counting, Safeguard and Nuclear Physics Measurements Radiation Protection SCK·CEN Boeretang 200, B-2400 Mol Belgium
2 IRSN (Institut de Radioprotection et de Sûreté Nucléaire), Département de Protection de la santé de l'Homme et de Dosimétrie, BP17, 92262 Fontenay-aux-Roses Cedex, France
The in-vivo measurement of internally deposited radionuclides is in many circumstances governed by the capability of the acquisition facility to detect low deposited burdens. This sensitivity is discussed in terms of detection limits or minimum detectable activities. For many alpha-emitting radionuclides, this in-vivo detection limit has to be very low and the measurement techniques have to be adapted to guarantee the detection and quantification of these isotopes. The purpose of this paper is to compare three ways to reduce the detection limits in an in-vivo measurement of deposited radionuclides by using passive techniques: the use of shielding rooms, the use of underground laboratories and the intrinsic characteristics of the detectors (material, dimensions) to the examined energy. The study described here clearly indicates that the efficiency of the detector is not the primary parameter to achieve sensitive measurements in the low energy range but the optimization of background level. Furthermore, the reduction of background has a limited impact on the sensitivity of in vivo counting because of the presence of 40K in the body. A new concept is defined to describe the optimized dimensions and the design of a detector according to its uses.
This study shows that the deep underground laboratories could be replaced by detectors properly designed in shape and size and used at ground level in normally shielded rooms to reach the required limits of detection in the in vivo assessment of low energy photon emitters. First conclusions can be drawn from these results.