Fast and slow neutrons in an 18-MV photon beam from a Philips SL/75-20 linear accelerator

Fast- and slow-neutron contamination in an 18-MV photon beam from a Philips SL/75-20 linear accelerator has been measured. Aluminum and indium foils were activated to determine fast- and slow-neutron fluence, which were largely independent of field sizes. Measured fast-neutron fluences were typically 13.9 X 10(4) and 4.4 X 10(4) neutrons/cm2/rad of x ray inside and 5 cm outside the field, respectively. Slow-neutron fluences, 1.3 X 10(4) neutrons/cm2/rad of x ray, remained relatively constant inside and outside the field. The reported results are about three times higher than neutron fluences recently reported with a betatron operated at the same energy.