Comparative assessment of spherical and cylindrical ionization chambers (PTW 32002 and PTW 23361) in 60Co and 137Cs reference beams: experimental and Monte Carlo evaluation for installation compliance in gamma irradiator

Accredited calibration laboratories operating under ISO/IEC 17025 must demonstrate compliance of their facilities with technical standards such as ISO 4037 when performing calibrations in gamma and X-ray fields, typically using ionization chambers as reference instruments. In this framework, the accurate determination of the air-kerma rate is a critical step to ensure both calibration reliability and international traceability in radiation protection dosimetry. This study reports a comparative evaluation of two ionization chambers with different geometries: the PTW 32002 (spherical, 1 L) and the PTW 23361 (cylindrical, 30 cm3). Both were employed to characterize 137Cs and 60Co reference radiation fields at the CNESTEN calibration laboratory. The experimental results were compared against a theoretical benchmark provided by Monte Carlo simulations using the MCNP6.1 code to perform a comparative study with the measured values. The aim was to verify the installation's compliance with the inverse square law and to assess the metrological suitability of the two detectors. The results showed overall agreement between experiment and simulation, with deviations ranging from ~0.2% to 10.5%, depending on photon energy and source-to-detector distance. The spherical PTW 32002 chamber demonstrated greater stability in 60Co beams, while the cylindrical PTW 23361 exhibited better accuracy in 137Cs fields. These findings highlight the importance of chamber selection with respect to photon energy and dose rate, and confirm the compliance of the CNESTEN facility with international standards, thereby supporting reduced uncertainties and improved metrological traceability in calibration services.