Variable neutron collimator

Claim 1

A variable neutron collimator comprising two pairs of neutron-absorbing blocks spaced from the position of a source of neutrons, the first pair of blocks being of two-component form, each component having a surface remote from the position of the neutron source which is curved and concentric with said position, each component of each block having a beam-defining face adjacent the direction of the required beam, the faces together defining first opposite sides of a quadrilateral aperture divergent from the position of the source, and a pair of asscoiated extensions of the block spaced from the first pair of blocks in the direction of the beam, the extensions each having a beam-defining face which together also define the same two opposite sides of the divergent aperture; the total thickness of each two-component block and associated extension immediately adjacent the 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; the first pair of blocks and the associated extensions being rotatable in a first plane orthogonal to the axis of the divergent aperture so as to vary the angle of divergence of the aperture; the second pair of blocks each having a surface remote from the position of the neutron source which is curved and concentric with said position, each block having an integral extension beyond the curved surface, each block and integral extension having a continuous beam-defining face adjacent the direction of the required beam, the faces together defining second opposite sides of a quadrilateral aperture divergent from the position of the source; the thickness of the block and extension immediately adjacent the 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; the second pair of blocks and integral extensions being rotatable in a second plane orthogonal to the first plane and to the axis of the divergent aperture so as to vary the angle of divergence of the aperture; the second pair of blocks being between the first pair of blocks and the associated extensions; and a fixed shield of neutron-absorbing material arranged to absorb neutrons travelling in directions radial from the neutron source which pass through only the curved surfaces of any block, the sum of the thicknesses of the block and the shield in directions radial from the source being sufficient to absorb all neutrons emitted by the source in that direction.