BUILDUP CAP MATERIALS FOR MEASUREMENT OF PHOTON HEAD-SCATTER FACTORS

The suitability of high-Z materials as build-up caps for head-scatter measurements has been investigated. Build-up caps are often used to en able characterization of fields too small for a mini-phantom. We have studied lead and brass build-up caps with sufficiently large wall thic knesses, as compared to the range of contaminating electrons originati ng in the accelerator head, and compared them with build-up caps made of ionization chamber equivalent materials, i.e. graphite. The results were also compared with measurements taken using square and cylindric al polystyrene mini-phantoms. Field sizes ranging from 3 cm x 3 cm up to 40 cm x 40 cm were studied for nominal photon energies of 4, 6, 10 and 18 MV. The results show that the use of lead and brass build-up ca ps produces normalized head-scatter data slightly different from graph ite build-up caps for large fields at high photon energies. At lower e nergies, however, no significant differences were found. The intercomp arison between the two different plastic mini-phantoms and graphite ca ps showed no differences.