Attenuation measurements of a fast neutron radiotherapy beam.

Attenuation measurements have been made for pressed wood, steel and lead in the neutron beam generated by 35 MeV deuterons on a thick Be target, using narrow-beam, semibroad-beam and broad-beam geometries. In addition, the measurements were extended to the neutron beams from 25 and 15 MeV D+ on Be, for semibroad-beam geometry only. A tissue-equivalent plastic ionization chamber was used as the detector; hence the measured parameter approximated the total (n + gamma) dose in tissue. After correcting for radiation scattering around the absorbers, the remaining curves were nearly exponential in most cases, and a build-up factor could be employed to describe the varying degree of scattered radiation dose as a function of absorber thickness and geometry. The results given should be applicable in designing primary-beam shields and collimators, as well as providing a useful basis for extending calculations to other geometries through Monte Carlo techniques.