The effect of lead underlying water on the backscatter of X-rays of beam qualities 0.5 mm to 8 mm Al HVT

Direct measurement is made for X-ray beams with first-half-value thicknesses in the range 0.5 mm to 8 mm Al and for field sizes ranging from 15 mm diameter to 70 mm square at 100 mm SSD. The measurements were made in a water phantom using a 0.2 cm 3 thimble ionization chamber. The surface dose relative to that for full backscatter conditions decreases as the depth of water with lead beneath decreases. Further, for a given thickness of water with lead beneath, the surface dose relative to that for full backscatter conditions decreases with increasing beam energy and field size. The measured data is well described by a simple exponential function, the parameter values of which for the range of beam qualities and field sizes considered in this study are given. The measurements are compared with Klevenhagen's measurements of the build-up of backscatter with thickness of polystyrene.