Experimentally determined tissue air ratios and scatter air ratios for collimated beams of 14 mev neutrons.

Tissue air ratios (TAR) and scatter air ratios (SAR) for collimated beams of 14 MeV neutrons were determined from depth dose curves measured in air and water using a tissue equivalent proportional counter. Central axis depth dose data were obtained as a function of field size and depth in a water phantom for an SSD or 125 cm. The collimator was composed of steel and polyethylene and was continuously variable from a 3 times 3 cm to a 20 times 20 cm field size at 125 cm SSD. The field scans for this collimator were flat within plus or minus 2 per cent over 80 per cent of the field dimension in air and were reduced to 20 per cent of the central axis dose at 1-2 field radii in air. Relative depth dose data in air and phantom were measured with an accuracy of plus or minus 2 per cent and were used for the TAR and SAR calculations. The resulting scatter air ratios for 14 MeV neutrons have roughly the same characteristics as 60-Co SAR, but are 60 per cent larger at maximum build-up and reach that maximum at about 60 per cent of the peak depth for 60-Co for all field sizes. The effects of collimator throat scatter on the central axis depth dose characteristics of this particular collimator design are discussed in detail using the measured tissue and scatter air ratios.