Absorbed Dose and Cavity Ionization for High-Energy Electron Beams

Physical Measurements of Medical Electron Beams–A Review

“…The whole subject of cavity chamber theory has been reviewed by Burlin (1968), including the application of cavity theory to devices other than the ionization chamber. The application of the theory to the special case of high energy electron beams has been described by Kessaris (1970a), who calculated the variation of the ratio of absorbed dose to cavity ionization with the depth of penetration of energetic electron beams in water and carbon. Comparisons of computed and measured stopping power ratios have also been made by Harder (1965b), Pinkerton ( 1969), Berger and Seltzer ( 1969a), Svensson (1971) and Almond (1970).…”

“…Experimental values when compared with stopping power ratios calculated using the above equation do not deviate by more than 204. More refined calculations of the stoppinq power ratio, notably that by Kessaris (1970a) have invoked Australasian Radiology, Vol. X VIT, No.…”

“…The range of these secondary electrons is of the order of several mg/cmZ and experiments have indicated that a thickness of only 1-2mg/cm2 is sufficient to absorb the greater proportion of them. A value of 0.1 MeV has therefore been suggested for c (ICRU, 1972), although the calculations of Kessaris (1970a) assume A = 0.01 MeV (cf. Laughlin, 1969).…”

“…Laughlin, 1969). The choice of n (or cavity size) does not affect sm,q greatly and calculations by Kessaris (1970a) indicate that for an air cavity in water a change in A from 0.01 to 0.1 MeV for 45 MeV electrons results in a change in s~,~ of about 3y0, and this variation is smaller at lower energies. For a given value of n most of the variation in stopping power with energy, or for a beam of given initial energy with depth in the medium, is due to the density effect, which results in a decrease in stopping power ratio with increase in energy.…”

“…The exposure calibration factor, N, which is obtained with the normal CO-60 build-up cap includes a correction for photon attenuation in the chamber equilibrium walls, a factor commonly taken as A = 0-985 (Greene, Massey and Meredith, 1962;Svensson and Pettersson, 1967). Berger and Seltzer (1969) and Kessaris (1970a) both quantities being, in essence, constant multiples of the stopping power ratios. Values of CE recommended by the ICRU (1972) are derived from Kessaris (1970a) using A = 0.985 and -W e based upon -= 0-869, equivalent to W = 33.73 eV per ion pair.…”