The photon beam characteristics of linear accelerator equipped with additional narrow beam collimator

In this paper a laboratory set-up with an electron linac installed in this Institute, and an attached narrow-beam collimator for photon beam are presented. Characteristics of the circular photon beams of diameters ranging from 10 to 30 mm at the isocentre are reported. The specific quantities measured include: relative output factors, beam profiles (off axis factors), and a central axis attenuation of beams. Measurements of these parameters were performed in a water phantom using small cylindrical ionisation chambers.