GB1409263A - Neutron beam collimators - Google Patents

Abstract

1409263 Collimating neutron beams; moderating neutrons; neutron shielding ELLIOTT BROS (LONDON) Ltd 23 Feb 1973 [16 March 1972] 12373/72 Headings G6P and G6R A neutron beam collimator comprises a body containing tungsten, for moderating neutrons by inelastic collisions, a flow neutron absorber in intimate admixture with the tungsten to reduce collisions of slow neutrons therewith, and hydrogenous material for further moderating the neutrons by elastic collisions to reduce them to thermal energies and for capturing thermal neutrons, and a material having a density of at least 10 gm./cm.<SP>3</SP> for attenuating γ-radiation arising from the neutron capture, the body including a bore to define the cross-section of the emergent beam. In one embodiment (as shown), the collimator 24 comprises a matrix 40 in three layers 34, 36, 38. Layer 34 comprises tungsten powder mixed with boron carbide powder; layer 36 comprises polyethylene granules and may also contain boron carbide mixed with the matrix bonding agent, e.g. epoxy resin, or forming a coating on the granules; and layer 38 comprises lead shot or powder. In a modification, layer 38 comprises solid lead. In another modification, lead may form the bonding agent of layer 34. In a further modification, nickel powder is added to the layer 36. In a second embodiment (Fig. 3, not shown), the matrix 40 contains a uniform dispersion of the loading materials previously mentioned in relation to Fig. 2. The collimator is intended for use with a neutron radiation therapy apparatus (Fig. 1, not shown) in which a neutron source (10) is surrounded by a shield (12) comprising an inner layer (14) of iron which slows down primary (15 MeV) neutrons by inelastic collisions, an intermediate hydrogenous layer (16) of polyethylene and boron carbide which, by elastic collisions, moderates the neutrons to thermal energies, the latter then being captured, and an outer layer (20) of lead to attenuate γ-radiation produced in the neutron capture processes. The outer layer (20) overlies a stainless steel casing (18) which contains the intermediate layer.