[en] A collimating apparatus for a radiation source from which X or Gamma radiation is emitted through an aperture means, the dimensions of said aperture means defining a boundary of the desired beam to be projected, said collimating apparatus defining means for projecting towards a remote target a beam of radiation which is substantially non-diverging from said defined boundary in at least one direction, said collimating apparatus comprising an elongated sandwich assembly of a plurality of layers of material means exhibiting relatively high radiation attenuation characteristics, said attenuating layers being spaced apart and separated from one another by interleaved layers of material means exhibiting relatively low radiation attenuation characteristics, said sandwich assembly being adapted for lengthwise disposition adjacent the aperture means in the path of and throughout the defined boundary of the radiation emitted through the aperture means, such that said attenuating layers are parallel to the desired direction of the projected beam with said interleaved spacing layers providing multiple direct paths for the radiation comprising the projected beam