Collection efficiencies of cylindrical and plane parallel ionization chambers: analytical and numerical results and implications for experimentally determined correction factors.

&#xD;&#xD;To derive a collection efficiency formula, f_Gauss, for cylindrical ionization chambers in pulsed radiation beams from a volume recombination model of Boag et al (1996) including free electrons. To validate f_Gauss and a parallel plate chamber formula f_exp using an ion transport code and calculate changes in collection efficiencies caused by electric field charge screening at 0.1-100 mGy doses-per-pulse. And to determine collection efficiencies CE∞ predicted at infinite voltage in the absence of avalanche effects by fitting scaled formulae to efficiencies computed for 100-400 V chamber voltages and 10 and 100 mGy doses-per-pulse. &#xD;&#xD;Methods.&#xD;&#xD;Calculations were performed for an idealized parallel plate chamber with 2 mm electrode separation d, and an idealized cylindrical chamber with 0.5 and 2.333 mm inner and electrode radii r_in and r_out. &#xD;&#xD;Results.&#xD;&#xD;f_Gauss and f_exp predict the same collection efficiencies for cylindrical and parallel plate chambers satisfying d^2=((r_out^2-r_in^2) ln(r_out⁄r_in))⁄2, an equivalence condition met by the chambers studied. Without charge screening, efficiencies computed using the code equalled f_Gauss and f_exp. With screening, efficiencies changed by <=0.03%, <=1.1% and <=21.3% at 1, 10 and 100 mGy doses-per-pulse, and differed between the chambers by <=0.9% and <=19.6% at <=10 and 100 mGy dose-per-pulse. For fits of f_exp and f_Gauss, CE∞ values were <=1.2% and <=17.6% from unity at 10 and 100 mGy per pulse respectively, closer than for other formulae tested. &#xD;&#xD;Conclusions.&#xD;&#xD;Allowing for screening, f_Gauss and f_exp described computed collection efficiencies to within 0.03%, 1.1% and 21.3% at doses-per-pulse <=1, 10 and 100 mGy. Equivalence of the two chambers broke down at 100 mGy per pulse. Departures of CE∞ values from unity suggest that collection efficiencies determined experimentally by fitting f_Gauss or f_exp to readings made at multiple voltages will be accurate to <=1.2% and <=17.6% at 10 and 100 mGy per pulse respectively.&#xD.