Performance of parallel plate ionization chamber for medical irradiation

We have developed parallel plate ionization chambers (PPIC) to measure not only the cumulative intensity but also the time structure of slow-extracted heavy-ion beams from a medical synchrotron. The characteristics of the PPIC with 3 mm and 1 mm gap distances for 760, 200, 111, and 55 Torr air were investigated with C/sup 6+/ beam (290 MeV/u) at HIMAC. The applied voltage to start the plateau region strongly depends on the beam intensity, and pressure of counter gas. The PPIC can be also used as a useful beam monitor for the time-structure measurement of heavy-ion beams.