COMPARATIVE N-GAS MEASUREMENTS FOR A PARALLEL PLATE CHAMBER IN PROTON, ELECTRON, AND CO-60 BEAMS

The TG21 protocol introduced the N-gas calibration method for parallel plate chambers in high- energy electron beams. This calibration metho d was performed for a Markus parallel plate chamber in proton and elec tron beams of various energies as well as a Co-60 beam. For an individ ual chamber, the N-gas value in proton beams differs from the N-gas va lue in cobalt and electron beams by the ratio of ((W) over bar le) for proton beams to that of a Co-60 beam. While the replacement correctio n factor is essential for Markus chambers in low-energy electron beams , the results of our N-gas(PP) measurements in proton beams showed tha t the Markus chamber does not need a replacement correction factor for therapeutic proton beams of energy 20-170 MeV. These results indicate that the 0.7-mm guard ring of the Markus chamber is adequate to preve nt the in-scattering of secondary electrons produced by proton irradia tion of the chamber wall or medium.