Developing a Bq/L level ^131\text I concentration in solution measurement system using NaI(Tl) crystal and SiPM

Background ^131 I is a critical radionuclide in nuclear medicine, particularly for thyroid disorder therapies. Chinese national standards mandate reducing ^131 I concentrations in radioactive waste liquid to below 10 Bq/L. To address this requirement, rapid and precise quantification of ^131 I activity is essential for compliance monitoring and clinical safety.
Methods A detection system integrating a NaI(Tl) crystal and a silicon photomultiplier array was developed to measure the ^131 I concentrations via its 364.5 keV characteristic \gamma -ray emissions. The setup incorporates 5 cm oxygen-free copper and 5 cm lead shielding to mitigate external \gamma -ray interference. Sample analysis utilizes a 50 mL aliquot, with system sensitivity evaluated under varying measurement durations.
Results Under optimized shielding conditions, the system achieved a minimum detectable activity of 8.0 Bq/L for ^131 I within a 3-hour measurement period. Extending the acquisition time to 24 hours enhanced sensitivity to 2.8 Bq/L, surpassing the regulatory threshold requirement of 10 Bq/L.
Conclusions The developed system demonstrates sufficient sensitivity and accuracy for monitoring ^131 I in radioactive waste liquids, aligning with national emission standards. The time-dependent sensitivity improvement highlights its adaptability for both rapid screening and low-concentration quantification, thus validating its applicability in nuclear medicine waste management.