An apparatus for charged hadron therapy verification by detecting and/or quantifying prompt gammas produced when irradiating a target with a charged hadron beam, the beam being directed along a beam line, said apparatus comprising: - a collimator formed of a first material and provided with a slit-shaped portion which is open or which comprises a material of a lower thickness and/or density than said first material, said slit-shaped portion being configured to be arranged perpendicularly to the beam line and facing the target, - a detection means (3,4) suitable for detecting said prompt gammas and - a calculation and representation means, wherein: - the slit-shaped portion is configured to allow the passage of prompt gammas emitted at least from a zone of the target corresponding to a portion of the beam line within said target, said portion of the beam line being larger than the width of the slit-shaped portion, - said detection means (3,4) is configured to detect prompt gammas passing through the slit-shaped portion, said prompt gammas being emitted from a plurality of locations within said zone of the target, - said calculation and representation means is configured to derive from a detected prompt gamma a value representative of the dose at the location from where said prompt gamma is emitted, and to represent a dose-related distribution for said plurality of locations, thereby allowing to estimate the beam range within said target, - said detection means (3,4) and said calculation and representation means are configured to calculate and represent said value in a plurality of portions of said zone arranged along the beam line, to thereby obtain a one-dimensional view of the dose-related distribution within said zone, characterized in that: - the apparatus comprises a means configured to approximate said one-dimensional view by a fit curve composed of 3 line segments defined by 4 points. - a collimator formed of a first material and provided with a slit-shaped portion which is open or which comprises a material of a lower thickness and/or density than said first material, said slit-shaped portion being configured to be arranged perpendicularly to the beam line and facing the target, - a detection means (3,4) suitable for detecting said prompt gammas and - a calculation and representation means, wherein: - the slit-shaped portion is configured to allow the passage of prompt gammas emitted at least from a zone of the target corresponding to a portion of the beam line within said target, said portion of the beam line being larger than the width of the slit-shaped portion, - said detection means (3,4) is configured to detect prompt gammas passing through the slit-shaped portion, said prompt gammas being emitted from a plurality of locations within said zone of the target, - said calculation and representation means is configured to derive from a detected prompt gamma a value representative of the dose at the location from where said prompt gamma is emitted, and to represent a dose-related distribution for said plurality of locations, thereby allowing to estimate the beam range within said target, - said detection means (3,4) and said calculation and representation means are configured to calculate and represent said value in a plurality of portions of said zone arranged along the beam line, to thereby obtain a one-dimensional view of the dose-related distribution within said zone, - the apparatus comprises a means configured to approximate said one-dimensional view by a fit curve composed of 3 line segments defined by 4 points.