Neutron and gamma ray measurements carried out around a Varian Clinac 2500 linear accelerator

Abstract

Auckland Hospital, New Zealand, has recently installed a Varian Clinac2500 linear accelerator. This unit is capable of producing 24 MV peak photon energies. It is well known (Fox et al, 1977; Ing et al, 1982; Palta et al, 1984) that such photons are capable of producing neutrons via a photonuclear reaction. The sources of such neutrons from this accelerator have been described (LaRiviere, 1985). In this study a survey was carried out around this new installation to observe some of the characteristics of the neutrons produced, and to assess likely radiological hazards to radiotherapy staff. Equipment and methods Equivalent neutron dose was measured using an Alnor neutron dose rate meter (rem-meter) model 2202D. Remote pulse counting was carried out using an Elscint model INS-30 nuclear spectrometer. The manufacturer of this unit claims that the detector exhibits 200 R/h gamma ray discrimination. Initially this would appear satisfactory for carrying out a survey, but owing to the fact that linear accelerators are operatedin pulsed mode, peak leakage and scatter photon dose rates within the treatment room could well exceed 200 R/h. This was confirmed by operating the accelerator in 6 MV photon mode (i.e. producing no expected neutrons) and showing that the neutron meter did in fact respond. However, in the entrance maze to the treatment room the scattered X-ray dose rate was attenuated sufficiently so that it was not contributing to detector response.