Patent Number: 054689708
Section: summary

FIELD OF THE INVENTION The present invention concerns a device for collimating beams of a radiation. BACKGROUND OF THE INVENTION It is applicable more particularly for collimating beams of neutrons and beams of X rays. Frequently, it may be desired to limit the angular distribution of a radiation beam without actually losing its luminosity. In order to achieve this, a Soller collimator is used. An example of this type of collimator is diagrammatically shown on FIG. 1. It includes a plurality of fine parallel thin strips 2 which are able to absorb the incident radiation 4 it is desired to collimate, or strips which are coated with a material able to absorb this radiation. The ratio Do/Lo of the distance between the strips 2 to the length Lo of these strips is equal to the tangent of half the desired maximum divergence 2.phi. for the radiation beam 6 coming out of the collimator. So as to avoid wasteful losses, the finest thin strips are used. A Soller collimator does, however, have drawbacks. In fact, if the radiation to be collimated arrives below a certain critical angle of incidence on the thin strips, it is reflected (total reflection effect), despite the presence of the absorbing material. Accordingly, it is not possible to collimate the incident radiation beam below the critical angle. In addition, this total reflection risks contaminating (this not being desired) the radiation beam diffused by a sample (not shown on FIG. 1) which is illuminated by the collimated beam. SUMMARY OF THE INVENTION The object of the present invention is to resolve these drawbacks. In order to achieve this, the device of the present invention for collimating beams of a radiation is characterized in that it includes a plurality of parallel plys of wires which are made of or coated with a material able to absorb the radiation. The use of such wires makes it possible to significantly reduce the surface which partly causes the total reflection of the radiation. According to a preferred embodiment of the device of the invention, the wires used are round. In this case, the surface partly causing total reflection is almost nil. Preferably, in each ply, each wire of row n for any whole number greater than or equal to 3 is tangent to the plane which passes between the wire of row n-1 of this ply and the wire of row 1 of an adjacent ply and which is tangent to this wire of row 1 and the wire of row n-1. This makes it possible to have wires placed at maximum distances from one another and thus use a minimum number of wires, this allowing for a further reduction of the surface partly causing total reflection. According to a particular embodiment of the device of the invention, said material is able to absorb neutrons, the device thus being able to collimate beams of neutrons. In that case, as an absorbant material, it is possible to use cadmium or gadolinium but boron is preferably used. With boron wires and a neutronic radiation, the irradiation dose due to the absorbtion of the neutrons is reduced by a factor 10 with respect to an absorbant material made of cadmium or gadolinium.