A radiation safety survey on a Clinac-20 linear accelerator

The radiation protection problems associated with a 20 MV linear accelerator are discussed, in particular those due to the considerable neutron production accompanying the 18 MV photon beam. These include the potential neutron dose to the patient, the special shielding requirements and the neutron activation of the accelerator and its surroundings. The results of the structural protection survey are described. In particular, the methods recommended by the NCRP for predicting the neutron dose equivalent at the end of a maze are shown to be reliable. Dose rates due to activation of the treatment room and various parts of the accelerator were measured, and potential doses to staff were assessed. Neutron dose equivalents were measured in the specified two metre diameter patient plane using the standard NRPB neutron badges and also with the recently developed CR-39 dosimeters. The latter appear to give sensible results.