A thin-walled air-ionization chamber for proton therapy

A thin-walled air-ionization chamber with narrow gap was built for monitoring the proton therapy beam at the Moscow linac. The key chamber elements are 1.5 μ m thick polyimide films mounted on quartz and stainless steel rings by means of a special technology. Gold-coated films were used as chamber electrodes. Consisting of only a small amount of material, this chamber allows the detection of low-energy protons and nuclei. The 64 cm 2 sensitive area contains 60 strips for measuring two-dimensional dose profiles. The low noise as well as the 2 mm gap between the electrodes allow to reach a sensitivity of 1 pA for protons and electrons. No ageing effects were observed after about 10 17 protons had passed through the chambers.