Enhancing Accuracy and Reproducibility in Self-Shielded Box Calibrator

The McMaster University Health Physics Department provides radiation safety & health physics support to a wide range of users, both internal and external to the University. Recently, as part of an effort to enhance our services, we obtained a JL Shepherd & Associates Model 89 self-shielded Cs-137 box calibrator. This is an efficient and conventional approach to radiation instrument calibration, allowing for the calibration of all types of portable radiation detection instruments and probes. However, precise and repeatable positioning of instruments relative to the Cs-137source inside the calibrator itself remains a challenge, as even small variations in position can introduce variations in dose rate measurements. Upon receipt of the box calibrator, several projects were initiated to help ensure calibration accuracy and reproducibility. First, the primary initiative was the development of custom 3D-printed jigs tailored to each radiation instrument, alongside a 3D-printed grid and peg system that enable precise and repeatable spatial positioning. Second, an electronic, automated table positioning system was installed to control the movement of the instrument table inside of the calibrator, replacing the external hand crank and analog position indicator. Finally, an ion chamber appropriate for use as a calibration standard was purchased and is calibrated annually at the Ionizing Radiation Standards Group at the Canadian National Research Lab. This allows verification of the true dose rates within the calibrator, as opposed to exclusively relying on decay curves. Each of these actions have resulted in scalable and cost-effective enhancements to the existing calibrator, while modernizing and improving the device, ensuring McMaster is well equipped to perform quality calibration for our internal users and associated stakeholders.