Development and performance evaluation of portable thyroid monitor for <i>in vivo</i> monitoring of 131I

A compact, portable, and tabletop thyroid monitor has been developed for in vivo measurement of 131I activity in the thyroid. This system is designed for use in laboratory settings for monitoring of occupational workers and is also suitable for onsite deployment by radiological emergency response teams during radiation emergency situations for thyroidal measurement of the general public. The monitor incorporates a 51 mm diameter × 51 mm-thick NaI(Tl) scintillation detector coupled with a standalone 1 K multi-channel analyzer, housed within a shielded collimator. The system allows for variable counting geometries, accommodating diverse measurement requirements. Key design elements include a vertical movement system, a chin rest, a calibrated neck-to-collimator distance scale, and an Light Emitting Diode-based alignment indicator to ensure precise positioning of the thyroid relative to the detector. A specialized software has been developed for the quantification of thyroidal 131I activity, intake estimation, and committed effective dose calculation. Under normal radiation background conditions (~70 nSv/h), the system achieves a minimum detectable activity of 60 Bq with a 5-min counting time. The monitor is capable of accurately assessing thyroid equivalent doses below 50 mSv, even under conditions of elevated environmental radiation following radiation emergency situations. This system enhances rapid and in situ thyroidal 131I monitoring, supporting radiation protection efforts in both routine monitoring and emergency response scenarios.