Scattered radiation from a neutron collimator.

Fast-neutron beams are being employed in radiotherapy trials and associated radiobiology studies at numerous centers in the U.S., Europe, and Japan. Since collimated beams of various sizes and shapes are employed, it is desirable to know the composition of the scattered radiation component contributed by the collimator. A simple method is shown for deducing the field composition in terms of a three-component model, from measurements made with three ionization chambers (tissue-equivalent, graphite, and magnesium). The dose contributed by the scattered radiation in the present example was found to be predominantly due to fast neutrons indistinguishable from those in the primary spectrum (from 35-MeV D+ on Be). This method may prove useful for measurements in phantoms as well.