EXPERIENCES AT TROMBAY IN MONITORING ACTINIDE INTAKES BY OCCUPATIONALWORKERS BY DIRECT EXTERNAL COUNTING

Direct assessment of lung/liver/skeletal burdens of actinides (Pu/Am/U ) for monitoring intakes forms part of the ongoing internal dosimetry programmes for occupational workers at Trombay. The majority of the me asurements are performed with a 200 mm diameter phoswich detector oper ating with pulse-shape discrimination based on ORTEC electronics. Addi tional detectors (NaI(Tl), a miniature CdTe and a special coaxial HPGe ) are also employed whenever needed. All the detection systems are loc ated inside a graded lined 20 cm thick steel chamber. Calibration tech niques include both in vivo methods as well as the use of a realistic torso phantom. Utilising these facilities, routine, special/operationa l, confirmatory and follow-up monitoring programmes have been extensiv ely conducted. A brief description is given of the facilities, procedu res employed and methodologies for computation of committed effective dose equivalent (CEDE). Monitoring data have also been analysed to obs erve elimination patterns with respect to actinide aerosols deposited in the lung. These have been compared with the metabolic models curren tly in use. In particular, unusually long retention half-times for nat ural uranium, class Y behaviour of (AmO2)-Am-241 (embedded in Pu matri x), and super class Y behaviour of Pu aerosols in human lungs have bee n observed. Likely effects of these findings on the estimates of inter nal doses ale considered.