Reference dosimetry in a scanned pulsed proton beam using ionisation chambers and a Faraday cup.

In order to give the correct dose to a patient, the monitor chamber for a proton scanning system has to be calibrated. As recombination of ion pairs occurs in the monitor chamber, the relation between the number of particles traversing it per time unit and the ionization chamber signal is not linear. A method developed for a scanned pulsed proton beam taking the nonlinear monitor signal into account is described. A vital part of the reference dosimetry procedure is to determine the absorbed dose under reference conditions, which is recommended to be done with an ionization chamber. For a scanned pulsed proton beam, the recombination in the ionization chamber is not negligible and the signal from the ionization chamber has to be corrected. In this work, it is shown that although the pulse length is comparable to the ion transit time the beam can be considered as continuously scanned if the applied high voltage is not too small. Also shown is that the two-voltage formula for a continuous beam is under some conditions applicable for a continuous scanned beam as well.