SU‐E‐T‐313: Ionization Chamber Measurements in a Small, Non‐Uniform Beam ‐ Applications for Synchrotron Beam Dosimetry

Purpose: The aim of this ongoing work is to develop a dosimetric method suitable for the biomedical beamlines at the Canadian Light Source (CLS), which offer novel imaging and therapy modalities. As a step in this process, we analyzed the response of a clinical, cylindrical ionization chamber to small, non‐uniform synchrotron beams to determine absolute dose rates in air based on clinical calibration factors obtained under broad‐beam conditions.Methods: Three detectors (Gafchromic EBT2 film, Capintec PR06C ion chamber, and PTW diamonddetector) were used to determine the vertical profile of the filtered beam produced by the CLS's bending magnet beamline. The radial response function of the ion chamber was determined using a 100 micrometer slit beam. Matlab code incorporated this information to predict the relative ion chamber output as a function of nominal vertical beam collimation (0.1 – 8.7 mm) for both stationary and scanning ion chamber geometries. The beam‐size dependent output was also measured directly with the ion chamber.Results: The beam profile was Gaussian in shape, and measurements with all three detectors were in substantive agreement. The ion chamber response function showed the walls and inner electrode of the chamber. Predicted and measured values for the ion chamber response with varying field size were in excellent agreement for both the scanning and stationary measurements, validating the MATLAB algorithm. Based on the algorithm and using an interpolated clinical calibration factor, the dose rates for the filtered bending magnet beam ranged from 1.7 – 1.9 Gy/s at the maximum CLS storage ring current Conclusions: The excellent agreement between the predicted and measured results provides confidence that the developed model of ion chamber response to the small, non‐uniform beam is valid and suggests that absolute dose rates can be measured accurately using clinical ion chambers.