title: Head-scatter factors and effective x-ray source positions in a 25-MV linear accelerator.
abstract: The behavior of the effective source position and the correction factor associated with the collimator opening (head-scatter factor) were investigated for the 6- and 25-MV x-ray beams of a linear accelerator. The primary photon fluence was measured in air for square field sizes from 5 x 5 cm to 40 x 40 cm at distances from the nominal source of 80 to 140 cm, for open and wedged fields (wedge angle 60 deg). An inverse-square analysis shows that, for open fields, the effective source position of the accelerator is about the same (approximately 1 cm downstream) at 6 and 25 MV, for all field sizes. For the wedged fields, the effective source position depends on field size and ranges from about 2 to 4 cm. The head-scatter correction factors for given collimator settings were found to be essentially independent of distance at both energies.
authors.affiliation: Department of Radiation Medicine, Brown University, Providence, Rhode Island 02908.
authors.forename: M
authors.initials: M
authors.lastname: Tatcher
authors.forename: B
authors.initials: B
authors.lastname: Bjärngard
mesh_terms: D008433:Mathematics; D008962:Models, Theoretical; D010315:Particle Accelerators; D012542:Scattering, Radiation; D014965:X-Rays