Abstract
The medical electron linear accelerator has been in general use for radiotherapy since the early nineteen-sixties, but in recent years a number of manufacturers have increased the maximum electron energy above the 8 MeV threshold for neutron production. The International Electrotechnical Commission (IEC) formed Sub Committee 62 C to institute safety recommendations for high energy machines, and in September 1975, the principle of a maximum neutron contamination level, was established. The level chosen was an arbitrary, interim value and it was recognised by the Committee that this value would change when more information became available. The lack of immediate information was due to the extreme difficulty in measuring pulsed neutrons of a wide range of energies in a very high gamma ray and RF environment. This thesis is a record of all aspects of neutron production of one particular make of machine, the MEL SL75-20, in order to satisfy the IEC requirement for information. The results show that the maximum interim value of neutron contamination was optimistically low, and unless measures were taken to reduce the neutron production, it was unlikely that any machine, irrespective of manufacturer, using standard target, primary collimator and flattening filter materials, could comply with the recommendations. The results also show that if a treatment room labyrinth was designed on traditional principles considering only high energy treatment beam gamma scattering, the facility would fail to meet the Sealed Sources regulations for an adequately shielded compound at typical treatment beam intensities, in view of the additional radiation from neutron streaming and subsequent neutron absorption effects. It is shown that both the patient dose and the hazard to staff, arising from neutron production, can be substantially reduced at very little cost. The ability to produce neutrons in a hospital environment could be most useful, and it is shown that using the experimental beam facilty as a neutron source, the linear accelerator could be used for diagnostic investigations on body tissues and fluids.