Experimental and calculated bremsstrahlung spectra from a 25 MeV linear accelerator and a 19 MeV betatron

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.