MEPH collimator. The figure shows a view of the MEPH aperture collimator (from top to bottom in Figure 3B with an angle of 45°) set on the carrier trolley. Pb is the lead housing facing the targeted activity. W is the tungsten insert. (a) The floor of the room. (b) The inner side of the conical lead housing. (c) The bottom part of the lead thread in which the insert is screwed.