An apparatus for verifying the beam range in a target irradiated by a charged hadron beam, in an installation comprising: • a particle accelerator, • a nozzle configured to deliver a charged hadron beam along a beam line, • a target support system for supporting a target in a position with respect to said beam line, • a prompt gamma camera provided with a slit-shaped opening, the field of view of said camera being significantly wider than the width of the slit-shaped opening, so that the camera allows to detect prompt gammas emitted not only from the direction which is at 90° with respect to the beam line, said apparatus comprising: • a precalculation module, configured to • receive an irradiation plan, said plan comprising the following data or data which allow to define the following data: • the position of the target relative to the nozzle, • a plurality of beam energies of a charged hadron beam to be delivered from the prescribed nozzle position, • for each energy: the location of a plurality of spots situated in the target and in a plane that is perpendicular to the beam line, wherein said spots are to be irradiated with a beam having the defined energy, • optionally: the beam charge for each spot. • for each spot: receive as a part of said plan or calculate the predicted beam range and preferably the predicted dose deposited at the location of the predicted beam range, • for each of said energy levels: calculate a position of the camera wherein the slit is orthogonal with respect to the beam line and wherein the iris line of the slit is placed at a location in the direction of the beam line, in between and including the extremes of the predicted beam ranges, • a positioning module, configured to position the camera at each said position, • a verification module, configured to receive beam ranges for each spot measured by the camera at the positions, wherein the verification module is further configured to provide an output representative of the difference between the predicted beam ranges and the beam ranges as measured by the camera. • a particle accelerator, • a nozzle configured to deliver a charged hadron beam along a beam line, • a target support system for supporting a target in a position with respect to said beam line, • a prompt gamma camera provided with a slit-shaped opening, the field of view of said camera being significantly wider than the width of the slit-shaped opening, so that the camera allows to detect prompt gammas emitted not only from the direction which is at 90° with respect to the beam line, • a precalculation module, configured to • receive an irradiation plan, said plan comprising the following data or data which allow to define the following data: • the position of the target relative to the nozzle, • a plurality of beam energies of a charged hadron beam to be delivered from the prescribed nozzle position, • for each energy: the location of a plurality of spots situated in the target and in a plane that is perpendicular to the beam line, wherein said spots are to be irradiated with a beam having the defined energy, • optionally: the beam charge for each spot. • for each spot: receive as a part of said plan or calculate the predicted beam range and preferably the predicted dose deposited at the location of the predicted beam range, • receive an irradiation plan, said plan comprising the following data or data which allow to define the following data: • the position of the target relative to the nozzle, • a plurality of beam energies of a charged hadron beam to be delivered from the prescribed nozzle position, • for each energy: the location of a plurality of spots situated in the target and in a plane that is perpendicular to the beam line, wherein said spots are to be irradiated with a beam having the defined energy, • optionally: the beam charge for each spot. • the position of the target relative to the nozzle, • a plurality of beam energies of a charged hadron beam to be delivered from the prescribed nozzle position, • for each energy: the location of a plurality of spots situated in the target and in a plane that is perpendicular to the beam line, wherein said spots are to be irradiated with a beam having the defined energy, • optionally: the beam charge for each spot. • for each spot: receive as a part of said plan or calculate the predicted beam range and preferably the predicted dose deposited at the location of the predicted beam range, • for each of said energy levels: calculate a position of the camera wherein the slit is orthogonal with respect to the beam line and wherein the iris line of the slit is placed at a location in the direction of the beam line, in between and including the extremes of the predicted beam ranges, • a positioning module, configured to position the camera at each said position, • a verification module, configured to receive beam ranges for each spot measured by the camera at the positions, wherein the verification module is further configured to provide an output representative of the difference between the predicted beam ranges and the beam ranges as measured by the camera.