An apparatus for verification of characteristics of pencil beams produced by a hadron beam irradiation installation, comprising: • a main degrader element, comprising two mutually parallel surfaces (2,3), the distance between said surfaces defining the thickness of said main degrader element, • associated with said main degrader element, a multiple thickness degrader element, comprising a plurality of degrader portions of different thicknesses, each of said portions facing a different portion of the cross-section of one of said mutually parallel surfaces (2,3), • a two-dimensional detection means, suitable for detecting a deposited dose or a signal representative thereof or proportional thereto, said detection means having a first portion associated with said main and multiple thickness degrader elements (1,5), said first portion being configured to detect pencil beams which have passed through said main degrader element and said multiple thickness degrader element, wherein: - the thickness of said main degrader element and the thickness of said degrader portions of the multiple thickness degrader element are designed with respect to a pre-defined beam energy and for a given hadron type, so that the predicted maximum of a Bragg peak appearing when a beam having said beam energy passes through said main degrader element is situated approximately at the end plane of the main degrader element, and so as to obtain - through said first portion of the detection means - a plurality of data points in the vicinity of said Bragg peak, enabling thereby to define the shape and position of the Bragg peak, - said two-dimensional detection means comprises a second portion configured to detect pencil beams that have not passed through said degrader elements. • a main degrader element, comprising two mutually parallel surfaces (2,3), the distance between said surfaces defining the thickness of said main degrader element, • associated with said main degrader element, a multiple thickness degrader element, comprising a plurality of degrader portions of different thicknesses, each of said portions facing a different portion of the cross-section of one of said mutually parallel surfaces (2,3), • a two-dimensional detection means, suitable for detecting a deposited dose or a signal representative thereof or proportional thereto, said detection means having a first portion associated with said main and multiple thickness degrader elements (1,5), said first portion being configured to detect pencil beams which have passed through said main degrader element and said multiple thickness degrader element, - the thickness of said main degrader element and the thickness of said degrader portions of the multiple thickness degrader element are designed with respect to a pre-defined beam energy and for a given hadron type, so that the predicted maximum of a Bragg peak appearing when a beam having said beam energy passes through said main degrader element is situated approximately at the end plane of the main degrader element, and so as to obtain - through said first portion of the detection means - a plurality of data points in the vicinity of said Bragg peak, enabling thereby to define the shape and position of the Bragg peak, - said two-dimensional detection means comprises a second portion configured to detect pencil beams that have not passed through said degrader elements.