Assessment of thyroid counter accuracy using Caesium −137 and Europium −152 and personnel effective dose

Occupational exposure to unsealed radionuclides in nuclear medicine and industrial environments underscores the critical need for precise internal dose assessments to ensure radiation protection and regulatory compliance. The thyroid is particularly susceptible during radioiodine (I-131) treatment, underscoring the need for reliable monitoring systems. This study evaluates the reliability of the WBC-T ORTEC thyroid counter, a crucial system for occupational dosimetry. This system comprises of 2" × 2″ NaI(Tl) detector with lead-lined copper shielding, which makes it stable and sensitive. The study's originality is its robust and systematic method for validating results. This method combines background analysis, quality control testing, chi-square reliability checks, and an in-depth investigation at the Minimum Detectable Activity (MDA). This study examined a variety of geometries, acquisition times, and energies. It used Cs-137 (662 keV) and Eu-152 (121–1408 keV) as reference sources for energy calibration. The source is mounted inside a neck phantom. Background measurements, conducted in both open and closed laboratory settings, consistently produced coefficients of variation below 5 %, thereby validating the system's reproducibility. Quality control trials utilising Cs-137 over several sessions demonstrated percentage deviations within the ±5 % tolerance stipulated by international standards. Using longer acquisition times, up to 1800 s, helped reduce background variability and improve measurement accuracy. The results of the chi-square test showed stable values (4.47–4.58), which fell within the manufacturer's approved range (4.1–14.7). This confirmed statistical reliability and compliance with Poisson expectations. MDA findings demonstrated a strong relationship with counting duration and the distance between the detector and phantom, with optimal sensitivity recorded at closer geometries and extended acquisition periods. At the energy peaks of this study, the observed MDAs are attributed to improved intrinsic detection efficiency, while increased photon energy and distance diminish sensitivity. Assessments of linearity and consistency further confirmed robust long-term stability. The assessments confirmed that the WBC-T ORTEC thyroid counter demonstrates notable reliability, stability and sensitivity under optimised measurement conditions. Notably, the validated performance directly supports its application in occupational settings for detecting thyroidal I-131 uptake in nuclear medicine personnel. By enabling accurate activity quantification and subsequent committed practical dose estimation, the system ensures compliance with regulatory dose limits, thereby reinforcing its suitability for routine monitoring of personnel.