A parallel plate ionization chamber for a proton beam monitoring

A Parallel Plate Ionization Chamber (PPIC) was developed not only to measure the proton beam intensity but also to derive relevant parameters with other reference ionization chambers aimed at assisting in the quality control of a proton accelerator. Before installing a proton beam line, a fabricated PPIC must be experimentally certified for its linearity, saturation characteristics, etc. in a standard radiation field. The main design feature is that the active area of a PPIC must be large enough to monitor the whole proton beam intensity and not attenuate the proton beam energy because the proton beams must pass through it. A guard electrode was inserted within the PPIC to increase the sensitivity. Although there are many factors affecting the operation of an ionization chamber, we focused on the polarity of the applied voltage, the distance between the two electrodes, and the filling gases to roll rapidly over the plateau region. These operational characteristics were measured at high dose rates by using the X-ray generator located at the Korea Research Institute of Standards and Science (KRISS) and at a low dose rate by using a 25-mCi Am-241 source. The collection efficiency derived from the experimental two-voltage method and the root-mean-square linearity against variations in the dose rates were 99.9 % and 1, respectively, in both potential polarities. The operational characteristics derived from the experimental two-voltage method were also compared with the results obtained from the theoretical model.