Measurement of fast neutrons produced by high-energy X-ray beams of medical electron accelerators

The fast neutron contamination associated with the 25 MV X-ray beam of a clinical linear accelerator and with the 19 MV X-ray beam of a betatron has been measured at the patient treatment location, using both fission fragment track detectors and silicon diode dosemeters. Measurements are made of the neutron fluence and dose, both in and out of the primary photon beam, at distances up to 60 cm from the central axis. Neutron distributions are similar for the linac and the betatron, with approximately ten times greater neutron dose in the X-ray field than at 20 cm outside it.