Comparative Ngas measurements for a parallel plate chamber in proton, electron, and 60Co beams

The TG21 protocol introduced the Ngas calibration method for parallel plate chambers in high-energy electron beams. This calibration method was performed for a Markus parallel plate chamber in proton and electron beams of various energies as well as a 60Co beam. For an individual chamber, the Ngas value in proton beams differs from the Ngas value in cobalt and electron beams by the ratio of (W/e) for proton beams to that of a 60Co beam. While the replacement correction factor is essential for Markus chambers in low-energy electron beams, the results of our Nppgas measurements in proton beams showed that 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 prevent the in-scattering of secondary electrons produced by proton irradiation of the chamber wall or medium.