Monitoring Intakes of Pu/Am by External Counting: Current Status in India

INTRODUCTION The current practice of internal dosimetry for occupational workers handling actinides, in particular Pu/Am, involves monitoring of intakes. This, in turn, requires assessments of internal actinide contamination of the relevant body organs. Such assessments are needed: a) more often and almost on a routine basis for lungs for the recent as well as old exposures; b) comparatively less frequently for skeleton and liver for older exposures and c) those of cuts and wounds as and when encountered. Therefore, direct assessments of organ (lung/liver/skeleton) burdens of actinides have formed an important part of the on-going internal dosimetry programmes at BARC. The results of direct assessments when used in conjunction with dosimetric and metabolic models of ICRP lead to the evaluation of intakes and hence the internal doses. Once embedded under tissue or tissue-like media, the actinides, essentially, constitute sources of low energy photons (LEPs; energy < 200 keV). Therefore, the most reliable direct measurements of lung/organ burdens of the actinides in-vivo are only possible by the external detection of LEPs accompanying their nuclear decays: for example for Pu -ULX-rays (~ 17 keV) with 4.6% yield and Am NpLX-Rays ( ~ 18 keV) with yield 37.6% and 59.6 keV γ rays with yield 35.9%. Highly sensitive in-vivo monitoring facilities are required in practice in order to measure low values of lung/organ burdens through the detection of low yield, low energy X/γ rays. The external counting techniques for monitoring intakes of actinides (Pu/Am) thus, entail substantial developmental efforts and huge costs. This paper attempts to describe the current status of the direct methods of internal dosimetry of Pu/Am in India. It begins with the description of facilities established for monitoring intakes of actinides and then covers the procedures employed, monitoring philosophy, methodology for calculation of committed effective doses and lastly, gives results from the monitoring programmes as well as from follow-up studies which highlight new findings and some interpretational problems. IN VIVO MONITORING FACILITIES Shield :The shield consists of a totally shielded steel room with 20 cm thick mild steel all around and is divided into two compartments separated by a 15 cm thick steel wall. The large compartment (inner dimensions 3.8x1.8x1.9 m) is provided with 3 mm Pb lining inside and accommodates a linear scanning arrangement for the measurements of body/organ burdens of high-energy (E > 100 keV) gammaemitters. Such measurements facilitate appropriate corrections to be made in the LEP spectral region. The small compartment (inner dimensions 2.4 x 1.8 x 1.9 m ) is specially designed for in-vivo detection of LEP emitters and is provided with a graded lining inside (Pb 3 mm + Cd 2 mm + Cu 0.5mm) in order to achieve further background reduction in the low energy regions. The schematic diagram of the steel room shield is depicted in Fig. 1.