Measurement of beam current and evaluation of scatter production in an 18-MeV accelerator.

Measurements of dose rate in a 10-MV x-ray beam may indicate considerable variation with collimator setting. This variation is observed even when the peak scatter factor is fixed by holding the field size at the phantom surface constant. The resulting output factor variation may be due to radiation backscattered from the collimators into the monitor chamber. Measurements of charge deposited in the accelerator target were compared with ionization readings for several field sizes. Evaluation of the data indicates that radiation backscattered into the monitor chamber contributes only a small amount to the output factor.