An intensity modulator for controlling the intensity of ions, such as protons, controllably block a portion of sub-areas of an area beam to control the average intensity within that sub-area. The resilient sheet member has a foot-contact surface, and a tack adhesive layer is provided on at least a portion of the foot-contact surface. There is further provided a seal 20 for producing a pressure-tight transition between two mutually superposed segments 4, 6, wherein the height of the seal 20 substantially corresponds to the intended spacing of the segments.