Abstract
High energy X-ray machines used in radiation therapy have a fast neutron contamination. The authors have measured the neutron dose in several medical linear accelerators and betatrons. Passive silicon diodes having a relative gamma -ray to neutron sensitivity of 10-6, were exposed in air to about 10000 tissue rad of X-rays at the treatment distance normally utilized. In the treatment field neutron doses of 0.5% of the X-ray dose were found. Outside the treatment field, the neutron doses were an order of magnitude lower. The dependence of the contamination on X-ray energy, target and compensating filter material, and field size was discussed.