The Determination of Mega-Volt Electron Spectra from Dose Deposition Profiles

First, the equations are developed which are needed to compute a dose deposition profile from a given electron spectrum. The dissipation distribution in aluminum as a function of electron energy is considered as the fundamental parameter used in the calculation. Modified Nakai values of the dissipation distributions are used. Next, a brief account of the actual calculations used in the computer program is presented with a sample case for illustration. The uniqueness of the spectral determination is discussed by considering the results obtained by three different initial spectra. Although the final spectra are different and contain many of the properties of their respective initial spectra, the calculated doses correspond to the measured doses extremely well and the same mean electron energy is obtained in all three cases. The use of the resulting three final spectra for the calculation of dose deposition in high atomic number materials is illustrated by showing comparison deposition calculations in aluminum and tantalum for the three spectra. The agreement of the calculated dose deposition for the three spectra is very good. Next, the validity of the monoenergetic electron dissipation distributions which are used in the calculation is strengthened by comparing the experimentally obtained dose-depth measurements for aluminum, copper, and tantalum with the values calculated from modified Nakai, Nakai, and Spencer dissipation distributions. Finally, an appendix is included on the experimental measurement of dose from a pulsed electron source using thin calorimeters and blue cellophane.