Contaminant Dose from Incident Neutrons Associated with 22.5-Mev X-Rays from a Betatron

The flux of thermal and fast neutrons present at the patient treatment position has been determined by measurements of the activity induced in rhodium foils and in sulfur. A previously published energy spectrum of fast neutrons produced by photodisintegration reactions with 22 Mev Bremsstrahlung in heavy elements has been used in evaluating the activation information. There are 9 × 105 fast neutrons/cm.2 and 9 × 103 thermal neutrons per roentgen in the center of the collimated and compensated betatron x-ray beam at a treatment distance of 80 cm, from the target. The fast neutron spectrum has a peak intensity at 0.8 Mev. The dose produced by thermal neutrons is completely negligible. A calculation of the fast neutron dose induced in tissue has been carried out for two typical irradiation conditions: (1) four-port irradiation of the trunk; (2) bilateral irradiation of head.