A fast, non-destructive beam-profile monitor was developed using an oxygen gas-sheet target in the HIMAC synchrotron. The size of the gas-sheet target was 85 mm in width and 1.3 mm in thickness, of which the density was 1×10−4 Pa. The base vacuum in the ring was not deteriorated by this gas-sheet target, and was on the order of 10−8 Pa. In typical cases, carbon ion beams are accelerated from 6 to 290–430 MeV/n for cancer therapy, in which the intensity is 2.5×108 particles/bunch. For these beams, two-dimensional beam profiles were successfully measured within the time (～100 ns) of the bunch separation. This article describes both the technical and physical aspects of the monitor together with some results.
