A high-sensitivity air ionization chamber with extremely thin walls and a narrow gap has been designed for monitoring therapeutic beams of protons and other particles. The new components of the chamber are 1.5-μm-thick polyimide films created using special technology on quartz and stainless-steel rings. These films are coated with nanolayers of copper and gold and serve as the chamber electrodes. The chamber has high transparency and is hence able to detect protons and α particles with energies of >200 and >500 keV, respectively. The signal-to-noise ratio is high owing to a reduced leakage current (<40 pA). The beam profiles and the two-dimensional dose distributions are detected in 60 cathode strips grown up on thin films in a sensitive area of 8 × 8 cm2. The chamber is also suitable for the detection of low-energy nuclei.