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 absolute instrument for fluence measurements of proton beams. As the FC is easily manufactured it can be considered an 'in-house' calibration system. Moreover, at the moment no national standards for proton dosimetry are available. Up to now the experimental tests of these instruments show that much study still has to be done to better understand their use in reference dosimetry. To investigate the possibility of using an FC as a secondary standard, an FC was jointly designed by the 'TERA Collaboration' and 'Centre Antoine-Lacassagne' (Nice, France) to evaluate the main parameters of the instrument. A comparison between two FCs of different designs--the 'TERA FC' and the 'Nice FC'--and an ionization chamber (IC) used for routine proton dosimetry was carried out. Results show that the two FCs agree to within 1.5-3.6%. While the differences between FC and IC are larger--6% for the 'TERA FC' and 8.2% for the 'Nice FC', the FC giving a lower dose evaluation--they follow the same trend shown by the calorimetric measurements. The data show that once the beam characteristics are defined, the fluence measurements are reproducible and show a good accuracy.