Patent Number: 054689708
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 2 diagrammatically shows a section of a collimation device conforming to the invention and able to collimate an incident beam of radiation 4 and obtain at the outlet of this collimation device a collimated beam 6, the maximum divergence of this outgoing beam 6 being equal to 2.phi.. The collimation device or collimator shown in FIG. 2 includes a plurality of parallel plys 8 of wires 10 which are made of or coated with a material able to absorb the radiation. In the example shown in FIG. 2, the plys 8 are equidistant from one another and in each ply 8 the wires 10 are round wires parallel to one another and the wires of a given row, that is with the same order number in the plys, are in planes P parallel to one another and perpendicular to the planes of the plys 8. FIG. 3 shows a perspective view of a collimator conforming to the invention. As can be seen in FIG. 3, the wires 10 are individually stretched between two parallel plates 12 which are rendered strictly integral with each another, for example by means of braces 14 (placed outside the beam to be collimated 4). In order to collimate a beam of neutrons, boron wires which are able to be stretched between the plates 12 are preferably used. As a variant, it is possible to use tungsten wires coated with boron. In the case where one wishes to collimate a beam of X rays, use is made of wires which are made of or coated with a material able to absorb the X rays, preferably tungsten wires. FIG. 4 diagrammatically shows the way on how the wires are placed in relation to one another in a collimator conforming to the invention so as to use a minimum number of wires. The collimator diagrammatically and partly shown in FIG. 4 includes a plurality of parallel plys of round wires, such as the adjacent plys A and B. Having selected the maximum divergence 2.phi. it is desired to obtain with the collimator of FIG. 4, the distance D12 is selected between the first two wires A1 and A2 of the ply A (which is equal to the distance between the first two wires B1 and B2 of ply B). Then the position of the third wire A3 of the ply A is determined and the position of the third wire B3 of the ply B is determined as indicated hereafter. The wire A3 is tangent to the plane B1 A2 which passes between the wires B1 and A2 and which is tangent to these wires B1 and A2. Similarly, the wire B3 is tangent to the plane A1 B2 which passes between the wires A1 and B2 and which is tangent to these wires A1 and B2. Then the position of the wires A4 and B4 is determined as follows. The wire A4 is tangent to the plane B1 A3 which passes between the wires B1 and A3 and which is tangent to these wires B1 and A3. Similarly, the wire B4 is tangent to the plane A1 B3 which passes between the wires A1 and B3 and which is tangent to these wires A1 and B3. Thus, FIG. 4 shows how the collimator is gradually formed. The construction of this collimator is completed with the wires of row n, such as the wires An and Bn, making it possible to obtain the initially fixed maximum angle of divergence 2.phi.. The distance Li between the wires Ai and Ai+1 (equal to the distance between the wires Bi and Bi+1) depends on the distance d of the wires and the local angular divergence 2.times..phi.i of the beam of radiation at the level of the wires of row i (FIG. 4 shows the parameters L4 and 2.times..phi.4 which relate to the wires of row i=4). The distance Li (maximum distance between the wires Ai and Ai+1) is such that: EQU Li=d/tg.phi.i. As .phi.i reduces when i increases, that is gradually when the radiation extends into the collimator (in other words when going away from the inlet E of this collimator), the spacing Li of the wires is an increasing function of i. The construction explained above thus makes it possible to embody a collimator conforming to the invention with a minimum number of wires. By way of non-restrictive illustration, in order to embody a collimator conforming to the invention for collimating a beam of neutrons and obtain at the outlet of this collimator a maximum angular divergence beam equal to 0.5.degree., the following parameters are used: boron wire, diameter 0.1 mm PA1 length of collimator: 250 mm PA1 distance between two adjacent plys: 2.2 mm. The present invention is able to significantly reduce and, in certain preferred embodiments, completely eliminate the total reflection of the radiation. Furthermore, the fact of stretching the wires individually makes it possible to clearly define the ply constituted by these wires, contrary to the case with stretched strips which are used in known types of Soller collimators, these strips from a mechanical point of view having a poorly determined position, especially at the inlet and outlet of this Soller collimator (it never being possible to ensure that a strip is not warped). In addition, the wires are less sensitive than these strips to thermic variations and degradation by the radiation.