Backscatter factors for x-rays generated at voltages between 10 and 100 kV.

Using the Monte Carlo method, backscatter factors were calculated for cylindrical phantoms (diameter, 33 cm; thickness along the beam direction, 20 cm) of various composition (water, graphite, acrylic glass, and M3) for x-ray beams of a mean energy between 7.5 and 52.0 keV, corresponding to first half-value layers in aluminium between 0.036 and 4.3 mm, generated by voltages between 10 and 100 kV. In contrast with the current literature, the backscatter factor in this work is defined as the ratio of the kerma rate with respect to a specified reference material at the intersection of the beam axis with the phantom surface to the kerma rate at the same point in space in the absence of the phantom. The calculations were performed for a source-surface distance of 100 cm and three field diameters at the phantom surface, namely 2 cm, 4 cm and 11.2 cm, where the latter corresponds to a square field of 10 cm X 10 cm.