Abstract: A variable neutron collimator comprising at least three neutron absorbing blocks spaced from the position of a source of neutrons, each block having a surface remote from said position which is curved and concentric with said position, each block also having a beam-defining face adjacent the direction of the required beam, said faces together defining a polygonal aperture divergent from said position, the thickness of the blocks immediately adjacent each beam-defining face and in the general direction of the beam being sufficient to absorb substantially all neutrons emitted by the source in that direction, and the blocks being rotatable about said position so as to vary the angle of divergence of the aperture.