Thyroidal 125I monitoring system using a survey meter for 125I

This paper describes the counting efficiency and detection limit of a thyroidal 125I monitoring system. Two systems were used: (1) M1 was composed of only a survey meter for 125I (SM) having a NaI(T1) crystal of 2 in phi X 5 mmt 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 an anthropomorphic neck phantom embedded four simulated thyroids of 17, 20.5, 31 and 40 ml including 125I solution. The counting efficiency between 0 degrees and 45 degrees to the direction from the center of the thyroid to the front of the neck coincided within -4%. The efficiency of M1 ranged from 7.9 to 1.8% as the distance between the probe and the neck increased from 0 to 5 cm. Similarly the efficiency of M2 ranged from 8.3 to 2.2%. The detection limit of M1 ranged from 7 to 34 Bq, and the limit of M2 ranged from 1 to 5.1 Bq. M2 system was applied to monitoring a worker performing iodination with Na125I of 74 MBq. Both monitoring systems proved to sensitively detect thyroidal 125I within the uncertainty +/- 10%.