HOW ACCURATELY CAN EGS4 PRESTA CALCULATE ION-CHAMBER RESPONSE/

Ion-chamber responses are calculated for graphite, PMMA, and aluminum- walled ion chambers free-in-air in Co-60 beams and 200-keV beams for a graphite chamber. The EGS4 Monte Carlo system is used with various el ectron step-size algorithms, in particular the PRESTA algorithm, the m uch simpler ESTEPE constraint on the energy loss per step, and in comb ination. Contrary to previous reports, it is found that there are vari ations in the calculated ion chamber response of up to 3% in Co-60 bea ms and up to 8% in the 200-keV beam. It is recommended that all ion ch amber calculations be done with the PRESTA algorithm plus an additiona l constraint on the energy losses per step of 1% (or less for higher-Z materials). The systematic uncertainty in the calculations for Co-60 beams is found to be 1% (1sigma) and somewhat higher in 200-keV beams because of sensitivity to transport parameters such as AE and ECUT.