Using background subtraction for measuring the dose distribution of an electron pencil beam

The spread of a narrow electron beam with depth in a phantom has been measured in the past using either a magnetically focused beam or one that has been collimated. The former approach is usually impractical with therapy accelerators and the latter must contend with the bremsstrahlung photons and scattered electrons that are introduced into the beam. This technical report describes a simple technique in which data taken with a "closed collimator" are subtracted from those measured with the collimator "open." Such a subtraction process removes much of the background contaminants.