Neutron Radiation from Medical Electron Accelerators*

Electron accelerators operating above about 10 MeV produce significant quantities of photoneutrons. This is not a new problem and was first reported by Laughlin1 in 1951 for a 23 MeV betatron. In recent years it has drawn considerable attention because of the increased use of higher-energy electron linear accelerators for radiation therapy and also because of the increased awareness of radiation risks. There?s an unavoidable exposure of the patient to these neutrons and in some cases there can be significant radiation fields outside the therapy room door. The regulations concerning the use of medical accelerators are issued by the various state governments in the United States. Several of the states have regulations concerning accelerator treatment head leakage that were extended to include the dose equivalent from neutrons but wit-hout an increase in the magnitude of the permitted leakage. That is, the total leakage radiation one meter from the target still has to be less than 0.1% of the useful beam at one meter from the source. As a result, most of the electron accelerators above about 14 MeV cannot be sold in those states. I began a study of neutron head leakage because I felt that it was poorly understood, that many reported measurements were wrong, and that it was highly desirable that patients get the benefit of the higher-energy accelerators. I also wished to develop a method of measuring these neutrons which was simple enough to be used in