Collimator for Measuring Radioactive Radiation

The invention relates to a plate-shaped collimator with a plurality of through-bores for increasing locality, or positional, sensitivity of a measuring device during measurement of the radiation of radioactive substances, in particular .beta. radiations, by means of a detector, for example a location-sensitive, one- or two-dimensional proportional counting tube.

Measuring of radioactive radiation emanating from an active ingredient applied to a carrier has attained increased importance, for example in medical laboratory technology.

A basic disadvantage of such measurements is found in that the radiation emanating from the carrier in general extends over a spatial angle of 2.pi., which corresponds to the surface of a hemisphere. Corresponding conditions prevail with respect to secondary radiation. The accuracy of the locality-sensitive measurement is inevitably harmed by this effect.

In known devices an attempt is therefore made to bring the entrance window of the locality-sensitive counting tube as close as possible to the carrier surface to which the radioactive substance has been applied to keep this effect as small as possible. However, this method inevitably has its limits, for example because of contamination of the underside of the detector or the danger of damage to the detector interior by the radioactive sample.

A known alternative to this is the use of a plate-shaped collimator with a plurality of through-bores or -slits extending perpendicular to its surface; such a collimator may be disposed between the carrier and the detector. The detector in such a device may be, for example, a two-dimensional proportional counting tube, as disclosed, for example, in German Published, Non-examined Patent Application DE-OS 37 35 296. Depending on the bore diameter or the width of the slits and the thickness of the collimator, only a very small fraction of the isotropic radiation is selected from the "available" spatial angle .OMEGA., which extends by the amount of .DELTA..OMEGA. at right angles to the carrier surface. By means of this, the particles/rays which extend "too obliquely" are eliminated and local resolution is increased. However, unfortunately, connected with the elimination of the particles/rays not desired for local resolution is the disadvantage that these "undesired" particles/rays cannot make a contribution to the counting rate of the detector and that because of this the percentage measuring sensitivity of the detector is reduced to the same extent as the ratio of the spatial angle area .DELTA..OMEGA. "selected" by the collimator to the total spatial angle .OMEGA.. This can be briefly expressed as:

where .THETA. is the angle of divergence of the radiation. Typical values for the ratio I/I.sub.O are approximately 1%, which indicates a corresponding unsatisfactory reduction of the detector sensitivity.

It therefore is an object of the invention to provide a collimator of the above-described type in which, with unchanged collimator effect, the measuring sensitivity of the detector used is reduced by a dramatically lesser amount.

This and other objects are achieved, according to the present invention, by providing such a collimator with an insulator core which is equipped on both sides with electrically conductive layers, between which a voltage exists. For example, this collimator is disposed between a carrier of a radioactive substance to be measured and the entrance window of a locality-sensitive detector and the voltage applied between the conductive layers exerts a "suction effect" on the emitted or ionized particles so that the major part of the emitted particles is directed into the entrance window of the detector.

Such a collimator thus practically operates as an "amplifier" with an "amplification factor" of 10 to 50. Apparently the greater portion of the primary ions, electrons in this case, of the totality of the radiation located below the respective collimator bore is pulled upwardly through the bore or slit by the applied electrical field. In other words, particles which, without this suction effect, would just miss the lower entrance cross section of a bore of the collimator plate or would be absorbed inside the through-bore, are directed by means of the suction field to the entrance window of the locality-sensitive detector, and thus contribute to the counting rate and increase the measuring sensitivity.

By proper selection of the thickness of the collimator plate, the number of through-bores and of their cross section, such a collimator can furthermore be adapted in a simple manner to the locality-sensitive detector to be used. For example, for the "cooperation" of the collimator in accordance with the invention with a two-dimensional, locality-sensitive counting tube in accordance with German Published, Non-examined patent application DE-OS 37 35 296, a collimator plate in which the insulator core is approximately 5 mm thick, the diameter of the through-bores is approximately 0.5 to 1 mm and a voltage of approximately 1,000 V is applied between the two electrically conductive layers, has proven to be practical. By use of these parameters it is possible to increase the measuring sensitivity, i.e. to increase the counting rate by a factor of approximately 50, in comparison with a collimator plate without the field applied. For example, beta rays of 14C, 35S, 32P can be measured, but other radiation sources can also be detected without difficulty, for example tritium or 125 J.

Another possible use of the collimator in accordance with the invention is by "integration" in a detector, for example in a flow counting tube (locality-sensitive, if required). In this case the lower conductive layer of the collimator plate forms the entrance plane of the counting tube and the upper conductive plate is used as a cathode plane. Low-energy, ionizing radiation, such as tritium-.beta.-radiation or 125-J .beta./.delta.-radiation, can be particularly easily detected.

In this connection it is possible to realize open counting tubes with a very large, open entrance window for the direct detection of, for example, tritium contamination.

An exemplary embodiment of the collimator according to the invention will be explained with reference to two examples shown in the drawings.

FIG. 1 is a side view, partly in cross section, of a collimator according to the invention constructed for use outside of a detector.

FIG. 2 is a view similar to that of FIG. 1 of a collimator integrated with a detector.

FIG. 3 is a cross-sectional detail view showing the structure of a collimator according to the invention.

FIG. 4 is a diagram illustrating the improvement presented by the invention.

The collimator 10 shown in FIG. 1 is in the form of a plate and comprises an insulator core 10A made, for example, of epoxy fiberglass G-10, and electrically conductive layers 10B and 10C applied to the top and bottom, respectively, of core 10A. A voltage source is connected to apply a voltage U between layers 10B and 10C. Collimator plate 10 is located at a distance x above a carrier 30 carrying a source of radioactive radiation, for example a plate with radioactively marked biological substances applied to it.

At a distance y above collimator 10, a locality-sensitive detector 20 is located. Detector 20 may be, for example, a two-dimensionally operating counting tube of the type disclosed in German Published, Non-examined patent application DE-OS 37 35 296 Detector 20 has an entrance plane 20A.

In a known manner, through-bores 11 provided in collimator 10 extend perpendicular to electrically conductive layers 10B and 10C. The length, which corresponds to the thickness of the collimator plate, and diameter of each bore 11, together with the distances x and y, define the fraction .DELTA..OMEGA. of the spatial angle .OMEGA. which is sensed by the entrance plane 20A of the locality-sensitive counting tube 20.

In this first example of use, collimator 10 is a separate component which, so to speak, serves as a "base" for a suitable detector.

In contrast to this, in the second example shown in FIG. 2, collimator 10 is integrated into a counting tube which contains a total of four planes A, B, C, and D.

The lowest plane A is formed by the lower conductive layer 10C of collimator 10 and, together with the housing of the counting tube, is connected to zero potential.

The next higher plane B is formed by upper conductive layer 10B of collimator 10 and, for example, is connected to a potential of +100 to +2000 volts, and preferably +1,000 volts, by means of which the suction field between plane A and plane B is generated. Plane B simultaneously forms the lower cathode plane.

The third plane C is the anode plane, made of gold-plated tungsten wires with a diameter of 30.mu. and at a distance of approximately 2 mm from each other. Plane C is connected to a potential of +2,000 Volts.

The topmost plane D forms the upper cathode plane and is connected to a potential of +1,000 Volts.

The distance between the planes themselves is less than 10 mm, and is for example 2 mm.

The detector is a flow counting tube with a suitable counting gas, for example 90% argon, 10% methane. The mode of operation and the effect of the "suction field collimator", when used in accordance with FIG. 1, is shown by the example of a through-bore 11 in FIGS. 3 and 4.

It can be seen from FIG. 3 that the total radiation I.sub.O, emitted from a point P, releases during its passage inside the through-bore 11 the secondary electrons indicated by points. Without a suction field, only a part of these secondary electrons reaches the entrance plane 20A, the counting rate I indicated in the detector therefore is mainly determined by the value of spatial angle .DELTA..OMEGA. in that

This function is qualitatively shown in FIG. 4 by the lower curve.

With the application of the suction field, not only are primary electrons pulled towards the detector by the suction field, but the major portion of the secondary electrons formed inside the through-bore 11 almost totally reaches the entrance plane 20A of detector 20 because of the effect of this suction field. This results in a counting rate I.sub.2 which is considerably higher than I.sub.1 and which is also shown in FIG. 4 qualitatively by the upper, rectangular, curve. This counting rate I.sub.2 no longer is determined by the spatial angle section .OMEGA., but only by the ratio of the open surface of the collimator (sum of the diameters of the through-bores) to the total surface of the collimator and can be formally expressed by

with .OMEGA..sub.eff .tbd.open collimator surface/total collimator surface.

With an actual value of .OMEGA..sub.eff =0.5, the result is

Under actual conditions and with collimators without a suction field the result is a value of I.sub.1 =(app) 0.01 I.sub.O. From this it follows that I.sub.2 /I.sub.1 (app) 50, thus representing an increase in the counting rate by a factor of 50 with the use of the "suction field collimator".

Each electrically conductive layer 10B, 10C may consist of copper or aluminum and/or may be gold-plated or covered with graphite. In addition, each layer 10B, 10C may have the form of a foil and may be connected to core 10A. Each layer 10B, 10C may be applied to core 10A by vacuum evaporation.

The number of through-bores 11 per unit of surface area of collimator 10 is preferably approximately 50-300/cm.sub.2 and the ratio of the sum of the areas of through-bores 11 to the total surface of collimator 10 is preferably approximately 50-80%.

Insulating core 10A preferably has a thickness of approximately 1-10 mm, more preferably approximately 3-5 mm.

The magnitude of each distance x and y is preferably 0.1-2 mm.

While the description above refers to particular embodiments of the present invention, it will be understood that many modifications may be made without departing from the spirit thereof. The accompanying claims are intended to cover such modifications as would fall within the true scope and spirit of the present invention.

The presently disclosed embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims, rather than the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.