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The bremsstrahlung spectra from a 25‐MeV linear accelerator and a 19‐MeV betatron have been measured using a NaI(Tl) spectrometer system. The spectra show a low energy cut‐off at 0.6 and 0.4 MeV, respectively, and the maximum photon energies were 26.8 and 19 MeV, respectively. Cross sections, thin‐ and thick‐target photon spectra and total electron energy losses (collision and radiative) were computed using a digital computer for tungsten (Z = 74) and platinum (Z = 78) targets. The measured spectra were compared to the calculated spectra for thin and thick targets using electron kinetic energies of 26.75 and 19 MeV. The photon spectra from the two therapy units are different; however, depth dose data (10 × 10 cm, 100 cm TSD) are approximately the same. In addition, narrow‐beam attenuation coefficients in lead for the two machines were measured. The effective energy derived from the first HVL was 8.1 MeV for the linear accelerator and 7.8 MeV for the betatron.

The bremsstrahlung spectrum from an 8-MeV linear accelerator has been measured using a NaI(T1) spectrometer system. The spectrum shows a low-energy cutoff at 0.4 MeV and the maximum photon energy to be approximately 6% greater than the nominal energy. The maximum emission of energy fluence was 1.6 and 1.8 MeV for measured and calculated values, respectively. The fast neutron dose in the photon beam was approximately 0.09% of the x-ray dose. The weighted mean energy was 2.3 MeV, measured value, and 2.4 MeV, calculated value.