DEPENDENCE OF THE PHOTON BACKSCATTER FACTOR FOR WATER ON SOURCE-TO-PHANTOM DISTANCE AND IRRADIATION FIELD SIZE

The Monte Carlo method was used to calculate backscatter factors for a cylindrical water phantom (diameter, 33 cm; thickness along the beam direction, 20 cm) irradiated by X-rays of various radiation qualities generated at voltages between 10 and 280 kV and three monoenergetic radiations with energies up to 661 keV. To study the influence of the irradiation geometry on the backscatter factor, the calculations were performed for conical photon beams of field diameters between 1 and 20 cm at the phantom front face and at source-to-phantom distances between 10 and 100 cm. The numerical results-the statistical uncertainties of which are less than 0.5%-are compared with other known data.