Measurements of electron beam peak scatter factors.

We have measured the peak scatter factor (PSF) for electron beams which in analogy to photon beams is defined as the ratio of the absorbed dose to water at the depth of dose maximum in a water phantom to the absorbed dose in free air at the same location for a given incident beam. In this study we have measured the PSFs as a function of the incident electron beam energy and field size. We have used both an RK ionization chamber and a silicon diode as radiation detectors. After applying the water/air stopping power ratio and fluence correction factors to the ionization chamber readings, the values of measured PSFs by using two different detectors are in a good agreement. The results show that the value of PSF increases with the increase of the field size and decreases with the increase of the beam energy. The range of the variation with field size between 2 x 2 and 10 x 10 cm2 is 1.24-1.42, 1.15-1.30, 1.15-1.17 for 6, 11, and 20 MeV beams, respectively.