Masters Theses
Date of Award
12-2002
Degree Type
Thesis
Degree Name
Master of Science
Major
Nuclear Engineering
Major Professor
Laurence Miller
Abstract
X-ray spectral reconstruction from x-ray transmission data was investigated using graphite as an attenuating material which has a monotonic declining mass attenuation coefficient for energies up to 30 MeV, and by employing the numerical method for unfolding spectra. No experimental investigation of energies above 10 MeV using this technique has been performed prior to this research. Previous works utilizing this method have used material such as lead and aluminum which exhibit an increase in attenuation coefficients in the 8 to 10 MeV range and beyond. This increased attenuation leads the Simpson technique to produce a number of non-unique fractional fluencies representing dual energies. Photon energy spectra for 6 MV and 18 MV beams produced by Varian 6100 and Varian 2100C linear accelerators were determined to validate the consistency of the measured data and the modeling method. The resulting Simpson unfolded spectra were compared to the published 6 MV Mohan spectrum and an interpolated 18MV spectrum. Both spectra are Monte Carlo based and used in the ADAC Laboratories three dimensional Pinnacle treatment planning system (3DPRTP). Modal energies for the Simpson technique were determined and compared to the modal energies for the Mohan spectrum and the interpolated 18 MV spectrum. Our method's modal energies for the 6MV beams were similar to the Mohan 6MV spectrum. The modal energies for this work are 0.73 MeV for 2100-6 MV beam and 0.84 MeV for the 6100. For the 2100-18 MV beam, the modal energy for the determined Simpson spectrum was approximately 4.2 MeV. The Pinnacle (ADAC 3DPRTP) interpolated 18 MV spectral model yield a modal energy of 1.0 MeV. All of the above spectra were also compared to a Huang modified Jones-fit Laplace transform generated energy fluence spectra. The Laplace technique produced modal energies of 1 MeV for the 2100-6 MV beam and .7 MeV for the 6100 photon beams. The 2100-18 MV modal energy for the Laplace technique was 2.5 MeV. The relatively large differences in the modal energies for the 18 MV spectra lead the author to compare the results with a fourth spectral model generated by Francois and Catala through a technique of direct resolution of a matrix system of transmission data. The Francois - Catala technique yields a number of modal energies for 15MV to 20 MV beam spectra ranging from 2.76 to 3.58 MeV. These modal energies agree with the Simpson model as opposed to the interpolated 18 MV model. The HVL data also suggests that the modal energy of the beam is substantially larger than 1 MeV. The disagreement of all the above models leads the author to conclude that further work and unification of methodology is needed to verify true photon energy spectra.
Recommended Citation
Alhakeem, Robbie, "Determination of the bremsstrahlung spectra of a clinical linear accelerator using a Simpson iteration technique. " Master's Thesis, University of Tennessee, 2002.
https://trace.tennessee.edu/utk_gradthes/5881