Thyroidal 125I monitoring system using an NaI (Tl) survey meter

This paper describes the counting efficiency and detection limit of a thyroidal 125I monitoring system. Two systems were used: (1) M1 was composed simply of one survey meter for 125I (SM) having an NaI (Tl) crystal of 5.08-cm diameter by 5-mm thick, and (2) M2 was composed of the SM having an output terminal for spectroscopy and a multichannel pulse height analyzer. The counting efficiency was determined by using four simulated thyroids of 17, 20.5, 31, and 40 mL containing 125I solution. The simulated thyroids were embedded in an anthropomorphic neck phantom. The counting efficiency between 0 degrees and 45 degrees to the normal of the thyroid (front of the neck) changed by +/-2%. The efficiency of M1 ranged from 1.8 to 7.9% as the distance between the probe and the neck increased from 0 to 5 cm. Similarly, the efficiency of M2 ranged from 2.2 to 8.3%. The detection limit ranged from 7 to 34 Bq for M1 and from 1 to 5.1 Bq for M2. The M2 system was applied to monitoring a worker performing iodination with 74 MBq Na 125I. Both monitoring systems provided the necessary sensitivity to detect thyroidal 125I within the uncertainty of +/-10%.