Patent Number: 056446152
Section: summary

The invention relates to an X-ray analysis apparatus comprising an X-ray collimator which comprises a plurality of plates of a radiation absorbing material which are provided with openings and which are arranged so as to extend parallel and offset relative to one another in the propagation direction of the radiation, each plate comprising a pattern of holes with a given period p.sub.1 in a direction perpendicular to one of the sides of the holes, said period having a given opening fraction t.sub.1. The invention also relates to a collimator for use in such an X-ray analysis apparatus. A collimator of the described kind is known from U.S. patent specification Ser. No. 4,465,540. The collimator described therein, notably with reference to FIG. 5, consists of a number of collimator plates which are arranged consecutively in parallel (in the direction of the X-rays to be collimated). The plates shown in the cited document comprise a pattern of square holes arranged in mutually parallel rows. The rows are situated at equal distances from one another, so that they occur with a period p.sub.1, being the distance between, for example the upper sides of the holes in two successive rows. This period thus consists of two parts, i.e. a part which is formed by the hole and which amounts to the fraction t.sub.1 (the opening fraction), so that the dimension of the hole in this direction equals t.sub.1 p.sub.1, and a pan which is formed by the intermediate absorbing material and which amounts to the fraction (1-t.sub.1), so that the dimension of the intermediate absorbing material in this direction equals (1-t.sub.1)p.sub.1. The plates in this known collimator are arranged at equal distances from one another. The distance between these plates is determined by thin plate-shaped spacers which are clamped between the collimator plates and which are made of a material which transmits the relevant X-rays. Collimators of this kind are comparatively heavy because a substantial fraction of their volume is filled with plates of an absorbing material. Moreover, this material, for example lead, tin or molybdenum, is heavy. The intermediate spacers also absorb a given amount of X-rays which is undesirable for some applications, notably in analysis equipment. It is an object of the invention to provide a collimator of the kind set forth which has a lower weight and a negligibly low absorption of X-rays in the desired transmission direction. To this end, the collimator in accordance with the invention is characterized in that the holes have a rectangular shape and that the collimator is provided with a first series of plates in which the ratio of two successive distances (d.sub.i, d.sub.i+1) between the plates of the series is equal to the given opening fraction t.sub.1 of the period p.sub.1. It can be geometrically demonstrated that said arrangement of collimator plates suffices for all X-rays which do not extend in the desired transmission direction to be intercepted by at least one plate. Moreover, this results in a collimator whose weight is much lower weight and in which a large clearance exists between the collimator plates, which clearance can be used to accommodate a variety of elements for influencing or manipulating the X-ray beam. In an attractive embodiment of the invention, the X-ray analysis apparatus is characterized in that the holes furthermore have a given second period p.sub.2 in a second direction in the plane of the plates, perpendicular to the first direction, said second period having a given second opening fraction t.sub.2, the collimator being provided with a second series of plates in which the ratio of two successive distances between the plates of the second series equals the given second opening fraction t.sub.2 of the second period p.sub.2. These steps result in a collimator whereby collimation can be performed in two mutually perpendicular directions, the degree of collimation in one direction being independent of that in the other direction. This is achieved in that the length/width ratio of the rectangular holes is decisive in this respect. These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.