A device and method for on line dosimetry monitoring of a hadron beam generated from a source of radiation and delivered to a target, the device comprising a plurality of support plates arranged in parallel in a face-to-face relation, separated from each other by gas filled gaps and perpendicularly to the central axis of said hadron beam, and forming a plurality of ionization chambers, each support plate having on a first side one or more collecting electrodes and on a second side one or more high voltage electrode, arranged in such a way that each support plate has said first side substantially opposed to said second side of another support plate. Each support plate has an opening so as to form an inner cavity for allowing the undisturbed passage of a central portion of the hadron beam delivered to said target and a peripheral region for intercepting and measuring, by means of said plurality of ionization chambers, a peripheral portion of said hadron beam.
