Simulation studies of 4-MV x-ray spectral reconstruction by numerical analysis of transmission data

The use of transmission measurements with combinations of attenuating materials has been investigated as a method of determining the x-ray energy spectrum from a 4-MV linear accelerator. Simulation studies have been made with three 4-MV x-ray spectra of slightly different shapes. From each of these spectra, the narrow beam transmission curve was calculated, using lead to attenuate the beam for the first several points. Aluminum was then added to attenuate the beam to 0.001 of its initial intensity. A computer program using an iterative least-squares technique has been developed to analyze the transmission data and determine the spectrum by applying some physically reasonable sensible constraints. The calculated transmission values agreed with the simulated data within the estimated experimental uncertainties, and it is concluded that the technique is reliable for the spectral reconstruction and is sensitive to small spectral changes.