Thyroidal 131I Monitoring System Using Whole-Body Counter Capabilities

The counting efficiency and detection limit of a thyroidal 131I monitoring system using a whole body counter facility is described. Calibration factors are presented for a collimated high purity Ge detector (HPGe) of 12.5% efficiency and thyroid phantoms for two groups of people of different ages: children of 10 and 15 years old and ICRP reference man and woman. The construction of a neck human phantom with a human like thyroid gland of various volumes used in calibration permits the correction of measured thyroidal 131I activity to the thyroid depth in neck tissue. Counting efficiencies of the proposed thyroid realistic phantom are compared with calibration data from the often used cylindrical thyroid phantom of two separated lobes. This comparison shows that the counting efficiency of the proposed phantom for measuring 131I content in human thyroids of various sizes is approximately 20% higher than the efficiencies of the cylindrical phantom. The relative error in activity measurement as well as the minimum detectable activity are determined for the 364 keV gamma ray energy of 131I, 1000 s measuring time and 68.27% confidence level. The sensitivity of the monitoring system for the given counting parameters is about 2 Bq.