Long-Term Stability and Performance Assessment of 137Cs Gamma Beams for Calibration of Radiation Protection Instruments According to ISO 4037 and IAEA Standards

Abstract

This study presents the experimental characterization of gamma radiation beams generated by a collimated irradiator equipped with two ¹³⁷Cs sources at the Gamma and X-ray Calibration Laboratory of CNESTEN. The facility is mainly used for calibrating radiation protection instruments and supporting dosimetric assessments. The irradiator provides air kerma rates (K_air) from 60 µGy/h to 60 mGy/h at distances up to three meters. Calibrations were performed in terms of ambient dose equivalent using K_air measurements and appropriate conversion coefficients, with corrections applied for source decay to maintain accuracy over time. A long-term evaluation of the irradiator’s stability over 16 years showed consistent performance, with Kair rate deviations within 5% after decay correction. This confirms the reliability of the reference beams and supports extending calibration intervals. To ensure calibration validity, a beam homogeneity study was also conducted by measuring the transversal profile at various distances from the source. The results showed a uniform beam width of approximately ±8 cm at 1 meter and up to ±21 cm at 3 meters from the central axis, confirming the homogeneity of the irradiator for precise and reliable measurements across different distances. Finally, the system’s performance meets the requirements of ISO 4037:2019 and IAEA SRS-16 standards. The expanded uncertainty of ±3.7% (k = 2) reflects the high reliability of dosimetric evaluations at the laboratory.