X-Ray Spectra from Accelerators in the range 2 to 6 MeV

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Abstract
The photon flux spectra from two 6 Mev Linear Accelerators and a 2 Mev Van de Graaff Generator which are in normal use for radiotherapy, have been measured using a 7.5 x 7.5 cm Nal (T1) crystal spectrometer. The X-ray beam was scattered by aluminium sheets to reduce the intensity and energy range, and the primary spectra calculated from the measured scattered spectra using the Klein-Nishina formula. The crystal spectrometer was calibrated from theoretical Monte Carlo calculations which were compared with experimentally obtained isotope spectra. The measured pulse height spectra were converted to photon flux spectra by a digital computer. Photon flux spectra were obtained for different field sizes and under different operating conditions, and the variation in the spectra over the X-ray field was investigated. The results were compared with theoretically calculated thick target spectra using published formulae for the bremsstrahlung cross-sections.