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A modified method for the evaluation of bremsstrahlung spectra on the basis of transmission measurements is presented. The method is tested for two 4 MeV therapy accelerators. The calculated spectra are compared with other experimental results and Monte Carlo simulation.

The X-ray energy spectrum from a 15 MeV linear accelerator (Siemens Mevatron 77/20) was determined combining attenuation and photoactivation analysis. The X-ray flux of the therapy linac is produced in a thick target and filtered through several materials. The theoretical expression for a thin-target spectrum was corrected for absorption in the target and materials in the accelerator head. The low-energy part of the spectrum was calculated from attenuation measurements and the high-energy part of the spectrum was determined by photoactivation.