Photon beam spectrum characterization using scatter radiation analysis

This work presents a method to empirically determine the photon spectrum of a megavoltage bremsstrahlung beam. The method makes use of the fact that scatter cross sections vary in a known fashion with incident photon energy. The distribution of scatter produced by a scattering object placed in a good geometry photon beam was measured. The scatter distribution was simulated for a series of monoenergetic good geometry photon beams. A system of linear equations was generated to combine the polyenergetic measurements with the monoenergetic simulations. Regularization techniques were applied to solve the system for the incident photon spectrum.;Monte Carlo simulations were used to predict the ratio of primary to scattered photons for narrow mono-energetic photon beams at 9 different locations, with 10 degree increments and 15 cm from a scattering material. Measurements were performed in the same geometry using the photon beams produced by linear accelerators. A linear matrix system, AxF=T, was developed to describe the scattering interactions and their relationship to the primary spectrum. A is the monoenergetic scatter kernel determined from the Monte Carlo simulations, F is the incident photon spectrum, and T represents the scatter distribution characterized by empirical measurement. Direct matrix inversion methods produce results that are not physically consistent due to errors inherent in the system. Tikhonov regularization methods were applied to address the effects of these errors and solve the system for physically consistent bremsstrahlung spectra.;The results of this research provide a method to empirically characterize the primary radiation energy spectrum produced by a linear accelerator.;Keywords. Photon energy, spectrum, scatter analysis, regularization, Tikhonov.