PROTON-BEAM DOSIMETRY - A COMPARISON BETWEEN THE FARADAY CUP AND AN IONIZATION-CHAMBER

From the theoretical point of view, the Faraday cup (FC) is an absolut e instrument for fluence measurements of proton beams. As the FC is ea sily manufactured it can be considered an 'in-house' calibration syste m. Moreover, at the moment no national standards for proton dosimetry are available. Up to now the experimental tests of these instruments s how that much study still has to be done to better understand their us e in reference dosimetry. To investigate the possibility of using an F C as a secondary standard, an FC was jointly designed by the 'TERA Col laboration' and 'Centre Antoine-Lacassagne' (Nice, France) to evaluate the main parameters of the instrument. A comparison between two FCs o f different designs-the 'TERA FC' and the 'Nice FC'-and an ionization chamber (IC) used for routine proton dosimetry was carried out. Result s show that the two FCs agree to within 1.5-3.6%. While the difference s between FC and IC are larger-6% for the 'TERA FC' and 8.2% for the ' Nice FC', the FG giving a lower dose evaluation-they follow the same t rend shown by the calorimetric measurements. The data show that once t he beam characteristics are defined, the fluence measurements are repr oducible and show a good accuracy.