Following an incident involving release of radioactive iodine, it may well be necessary to monitor large numbers of people for internal contamination. In the case of iodine-125 (125I), iodine-131 (131I) and radioiodine releases from nuclear reactors (mixtures of 131I and shorter lived iodine radionuclides) screening measurements can be done with simple scintillation probe based instruments held over the thyroid.
This guidance gives a procedure for monitoring of the thyroid using Mini Instruments type 44 and type 42 probes. Factors are given to convert measured count rates to activity in thyroid, dose to the thyroid and committed effective dose for intakes by inhalation. Measurements with these instruments are capable of detecting about 2 kBq of 131I and 1 kBq of 125I in the thyroid of an adult. For measurements made 24 hours after intake, these activities correspond to values of committed effective dose of 0.2 and 0.05 mSv for 131I and 125I respectively, assuming stable iodine has not been administered.