Purpose: To establish a standard high-energy x-ray beam to act as a surrogate for Cs-137.
Methods: To achieve an x-ray beam with a high average energy, a combination filter of tin, copper and aluminum was chosen. A Varex NDI-451Be x-ray irradiator, with a maximum energy of 500 kV, was used for developing this beam. A beam model of the irradiator was created with the EGSnrc user code BEAMnrc, which was benchmarked with percent depth dose (PDD) and half-value layer (HVL) measurements. The beam model allowed the spectrum of the standard x-ray beam to be simulated to iterate the filtration. The filter thicknesses were optimized to increase the average energy of the spectrum while allowing the beam intensity to stay above an acceptable level. An air kerma rate that is higher than our highest energy, highest filtered NIST-matched x-ray beam was chosen as the beam intensity metric.
Results: The standard x-ray beam that was created has a thickness of 5.36 mm of tin, 1.28 mm of copper, and 2.01 mm of aluminum. It creates a spectrum with an average energy of 265 keV and a HVL of 6.64 mm of copper. The air kerma rate at 1 m is 2.735 mGy/s which is above the air kerma rate of our highest filtered NIST-matched beam indicating an acceptable beam intensity.
Conclusion: This work establishes a standard high-energy, more highly filtered x-ray beam than what is currently available. As clinics and research institutions are phasing out Cs-137 irradiators in favor of x-ray irradiators, a standard high-energy x-ray beam for calibration use becomes increasingly more critical.
TH- External Beam- Photons: Calibration protocol and primary standards