Comparative N_gas Measurements for a Parallel Plate Chamber in Proton, Electron, and ⁶⁰Co Beams

The TG21 protocol introduced the N_gas 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 N_gas value in proton beams differs from the N_gas 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 Npp_gasmeasurements 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 oroton irradiation of the chamber wall or medium.