IL33138A - Irradiation apparatus - Google Patents

Description

Irradiation apparatus na' p |pna COMMISSARIAT A L'ENERGIE ATOMIQUB Cf 3H02 This invention, relates to irradiation apparatus in which substances, e.g. fluids or solids are exposed to radiation as they pass in front of a radioactive source.

Different methods or special devices for improving the distribution and homogeneity of radiation doses delivered to the interior of a body of the substance to be irradiated are already known# Thus, it is possible to increase the number of transfers of the body o each side of the radioactive source which is usually provided in the form of a vertioally disposed "plaque" and which contains a series of suitably distributed radioactive elements. This is achieved by means of a conveyor which is designed to move the body in front of the source a number of times, in parallel relation to the faces of the source and" t a distance from this latter which varies from one pass to the next.

A method of the type just mentioned makes it possible in particular to attenuate the heterogeneity of doses received in the transverse direction at right angles to the plane of the sources however, this improvement is limited inasmuch as the most homogeneous doses delivered during passes which are carried ou at a greater distance from the source are also the lowest in value and can correct only to a very partial extent the principal heterogeneity which results from passes carried out within close proximity to the source. If the initial passes are carried out at an even greater distance away in order to enhance, the homogeneity of radiation doses, the efficiency of the installation falls off rapidly and limits the value of the improvement whioh it was intended to achieve. Whatever procedure may be adopted, it is nevertheless sought in the majority of cases to limit the thickness of the body while retaining the same density of constituent substances in order to improve the homogeneity in the transverse direction.

I order that the dose received by tbe body should also be made uniform in the vertical direction, a transfer is also carried out in front of the source but at different levels; the bodies themselves should preferably have dimensions such that the extend to an appreciable distance beyond the apparent contour of the source. Finally, another method consists in determining in the plane of the source the best distribution of radioactive elements for the purpose of regularizing the doses which are delivered. In particular, provision can be made or a design in which the source projects beyond i e top and bottom of the bodies to be processed and comprises in this case a central zone which is relieved of radioactive elements. However, in all the designs which have been proposed, homogeneity of "absorbed doses is not wholly satisfactory, especially i the case of bodies of appreciable thickness. ihe aim of this invention is to circumvent the disadvantages which have been discussed in the foregoing by means of an improved irradiation apparatus having a collimation device which is capable of being placed in proximity to the source so as to limit radiations which are delivered in certain directions while leaving a free passage for thete radiations in other direc ions. ϊο this end, according to the invention there is provided irradiation apparatus comprising a generally planar radio* . active source comprising a plurality of radioactive elements disposed in spaced relation and a mask having apertures therein related to the radioactive elements and extending adjacent and parallel to a face Of the source, the mask comprising screen portions at least in part defining the .. a-pertures, each s oreen portion having a transverse croos^section ^hich decreases in a direction away from the source.

Preferably a mask as above extends in front of each of the faces of the source.

By virtue of the foregoing arrangements, the screen portions of the mask are accordingly interposed between the radioactive elements of the source and predetermined points of a body to be irradiated. In particular, as a result of the pre* settee of the screen portions said radioactive elements can only partially be "seen" or in other words are in direct line of access to the nearest face of the body considered only to a partial extent whereas the same elements can be "seen" directly by points which are located at the centre of said body or* at an even greater distance from the plane of the source. The disparity between the doses received at these different points, especially between points of the body face which is located nearest the : source in parallel relation to this latter and points which have a different location, is thus considerably reduced under these conditions. Thus, the dose applied to those parts of the body which would otherwise have received an excessive dose in accordingly ^reduced in suitable and especially greater proportions than those parts of the same body which would have received lower total doses without the collimation mask. Accordingly, a substantia 33138/2 . 5 ^ Improvement In homogeneity of the radiation doses which are delivered is thus achieved simply at the price of a slight reduction in output of the unit, this improvement being more marked as the bodie are located closer to the plane of the source.

According to one embodiment, the radioactive elements are elongate rods disposed in parallel relation in a source holder frame and the screen portions are elongate metallic members which extend parallel to the rods, and are arranged at levels intermediate the levels of the radioactive elements, each screen portion having a triangular transverse cross-section and being arranged with one lateral face parallel to the plane of the source.

According to another embodiment, the radioactive elements are point elements disposed in a uniform lattice in a source holder frame and the or each maek has the form of a grid with the apertures positioned in front of isspective radioactive elements, the apertures being located in the bases of recesses flared outwardly in the said direction.

Advantageously the screen members and the radioactive elements In said source are oriented in such a manner as to have a given inclination to the direction of motion of a body in front of said source* It Is thus possible to contemplate a horizontal array of elements and transfer of the body at a predetermined angle of slope or, conversely, to give an orientation to the ' elements and the direction of transfer which have two angles of inclination as desired but of different value * A number of alternative forms can be used in the assembly of the mask and of the source; thus, the mask can be directly secured to the source-holder frame or be separate from this latter so as to constitute a container in which the frame is slidably fitted.

Further features of the invention will become apparent from the following description of. a number of exemplified embodiments which are given by way of example only, reference being made to the accompanying drawings, in which: Pigs. 1 and 2 are views taken in cross-section and in perspective showing an irradiation apparatus in which bodies are subjected to irradiation as they move in front of a radioactive source; Figs. 3 and 4 are schematic diagrams which serve to set forth the manner of ensuring that uniform radiation doses are received at the different points of any body respectively in the transverse direction and in the vertical direction; Fig. 5 shows diagrammatlcally a modification of the embodiment of Figs. 1 and 2 to limit the effects arising from inherent heterogeneity of the source; and Figs. 6 and 7 and 6 are respectively a sectional view and top views of other embodiments of irradiation apparatus of which the source comprises a series of point radioactive elements* There is shown at 1 in Figs. 1 and 2 a radioactive source which is constituted in this example by cylindrical rods 2 of suitable radioactive elements, said rods being set in parallel relation at a suitable distance from each other and mounted horizontally within the interio of a source-holder frame 3, the source being designed under these conditions in the form of a flat plaque which is placed vertically. by two masks 4 wjhich are placed parallel to the two faces of the frame 3 and in proximity to these latter. These masks can be separate from each other or joined along their edges so as to form in this case a kind of narrow container in which the source 1 can readily be inserted by sliding. Each mask comprises support 5 for a series of screen portions.6 each having the shape of an elongate metallic prism with a triangular transverse cross-section and mounted in the support in parallel relation both to each other and to the cylindrical rods 2, said prisms being uniformly spaced over a distance such that they are located at intermediate levels with respect to the levels occupied by the rods 2 in the source-holder frame 3 and partly define apertures in the mask. There is also shown in Pigs. 1 and 2 a body 7 to be irradiated which is carried by a conveyor (not shown in the drawings) for the purpose of transferring said body in a given number of passes in front of both faces of the source 1 which is fitted with the masks 4.

Figs. 3 and 4 serve to obtain a better understanding of the mode of distribution of doses within the body 7 in the case of a radioactive source and a collimation mask according to the arrangement which is contemplated in Figs. 1 and 2, and especially of the manner in which doses are distributed at points of said body which are separated from each other either transversely (in the case of Figure 3) or vertically (in the case of Figure 4). In order to simplify the drawings, there are shown in cross-section in these figures only three cylindrical rods 2a, 2b, 2c and two prismatic screen portions 6a and 6b which are placed between the source and the body.

If one considers any two points A and B of the body 7 which r, ·■·- Θ · r defined by the solid angle through which said points "see all the rods of the source are different at A and at 8· In the case of the point A whic is located nearest the source, it is thus observed that this point sees the whole rod 2a, approximately two-thirds of the rod 2b and only a very small portion of the rod 2c by reason of the presence of the two portions 6a and 6b. Similarly, the point B sees approximately three-quarters of the rod 2a, the whole of the rod 2b and a portion of the rod 2c which is approximately equal to two-thirds, namely an overall source volume which is greater than that seen b the point A but at a greater distance from the source.

It is therefore apparent that the points A and B of the body are not subjected to uniform radiation doses inasmuch as these doses depend in particular on the shape of the screen , portions which are placed in front of the source and more especially on the angl ά at the base of these prisms. Moreover, it has been shown in FigVl by means of a dashed-line representation that a point located on that face of the body which is located nearest fe source sees a smaller number of rods than another point whloh is located at a greater distance within the interior of the body.

It can therefore be appreciated that, by virtue of a suitable choice of the distance at which the body is placed with respect to the source, and of the shape of the screen portions of the correspondin collimation mask, the total dose received at each point of the body can thus be made more homeeeneous, said dose being a function of the solid angle. The prismatic screen portions must- accordingly mask the radioactive elements essentially 3313 9 - in respect of those points of the body which are located nearest the source whereas 4ft- the same prisms must be interposed only to a very slight extent between said elements and the centre of the body and even less so between said elements and points located beyond the centre.

Referring now to Figure 4, it is seen that the collima-tion device which is contemplated also makes it possible to improve the homogeneity of doses received in the vertical directio by a body which moves in front of the source. In fact, any two points C and D which are spaced apart but located on the same vertical generator-line of that face of the body which is located nearest the radioactive source, for example, do not see the radioactive elements 2a, 2b and 2c in the same manner inasmuch as each point is in direct line of access to radiation from only a limited number of elements which are either located opposite to said point or intercepted by a solid angle having its vertex at this point and being a function of the collimation angle. Broadly speaking and in the case of a radioactive source which is constituted by linear elements such as the parallel and horisontal cylindrical rods of the embodiment of Figures 1 and 2, the intensity of radiation received at any one point of a vertical generator-line located close to the source will be determined solely by the radioactive elements which are located in a horizontal band. The vertical width of the bands must be just sufficient to permit the intensities of radiations received along the vertical generator line to be substantially equalized (i.e. without overlap of radiation f Om two adjacent bands) taking into account the inherent heterogeneity However, the distribution of radio elements within the interior of each rod cannot be perfectly uniform.

In order to overcome the disadvantage which has just been referred to and which arises from inherent heterogeneity of the source as constituted by an assembly of separate radioactive elements, an equal number of which is consequently not necessarily seen by two points of the body which are located at a distance from each other in the vertical direction, the structure of the mask 4 can advantageously be modified by providing the screen portions 6 with an orientation which is slightly inclined to the horizontal plane (as shown in Pig. 5) whilst the rod. 2 remain parallel to the prisms which form said screens. On the other hand, the orientatio of the body 7 is maintained and, i particular, there is no modification to the direction of motion of said body in front of the collimation mask as shown diagrammat-icall by the arrow 8, said direction being horizontal in this case.

It is readily apparent that, by way of alternative, the screen portions and radioactive rods could be maintained horizontal whilst the bodies are transferred at·,-._?¾,,fgiVen; angle of inclination to the horizontal$ it would even be possible to choose two different ' angles of inclination for the screen portions and for the direction of transfer respectively.

Whatever alternative embodiment is adopted, the following results may be put forward: in the case of a body which has a height of 20 cm, a thickness of 30 cm, a length of 50 cm,' a density of 1 g/cm , which is -subjected to two passes on each side of a vertically placed source and at a distance of 0 cm from said source, it is observed that the total heterogeneity of substantiall equal to the product of partial heterogeneities in the transverse, vertical and longitudinal directions can be evaluated at R ■» 1.40. By means of a collimation device as hereinabove described, the total heterogeneity of the dose is reduced under the same conditions from 1.40 to 1.20 while the efficiency of the installation is reduced by only 10$.

Figures 6 to 8 illustrate alternative embodiments of the irradiation apparatus in which the radioactive elements 10, mounted in a source-holder frame 11, are of the point type and are provided preferably in the form of spherical beads of small diameter which are disposed on ^uniform lattice having a square pitch. In these alternative embodiments, masks 12 are associated with the source-holder frame in proximity to and on each side of the flat vertical faces of said frame, said masks being so arranged as to carry out a double collimation both in the vert* leal and horizontal directions. To this end, each mask is con-stitituted by a grid 13 of screen portions defining a series of apertures 14 placed opposite to the single-point radioactive elements of the source and formed In the bases of flared recesses 15 extending outwards from said orifices in order to permit the passage of radiations. In the embodiment of Figure 7, said recesses 15 are designed in the form of truncated pyramids whereas in the case of the embodiment of Figure 8, said recesses have the shape of cones which are also truncated. The operation of the masks otherwise remains similar to that which was described in connection with the first example given above* In consequence, and irrespective of the form of construction adopted the judicious determination of the different parameters of the masks an le of collimation nature of the con 33138/2 ^ ■ - 12 -forth) results In substantial homogenization of the doses received by the bodies as these latter pass I front of the source without thereby imposing any undue limitation on overall efficiency. The advantage of this device is an industrial irradiation plant can thus take many different forms; i particular, by virtue of the use of collima-tion masks, the device makes it possible to achieve identical homogeneity of doses within bodies which have the same dimensions but a different bulk density. This effect can result in particular in enhanced efficiency of the plant inasmuch as the increase in density substantially compensates for the relative reduction in efficiency which results from the interposition of masks. The versatility of an existing plant can therefore be substantially enhanced by thus increasing the range of densities of the products which can be processed.

The device considered also makes it possible to construct plants for the irradiation of unitary bodies having a greater thickness than in conventional plants without thereby entailing any " danger Of substantial heterogeneities in absorbed doses and this.; also results in enhanced efficiency of the plant in respect of a given number of passes.

It must be understood that this invention is not limited in any sense to the forms of construction which have been described in the foregoing with reference to the accompanying drawings and which have been given solely by way of example; on the contrary, the invention extends to all alternative forms and can be employed in particular for the irradiation of liquid substances.

Claims (1)

1. 33158/2 _ ' - 13 - » ■ ■ ■ - ■ · ■ 1. Irradiation apparatus comprising a generally planar radioactive source comprising a plurality of radioactive elements disposed in spaced relation and a mask having apertures therei related to the radioactive elements and extending adjacent and parallel to a face of the source, the mask comprising screen portions at least in part defining the apertures, each screen portion having a transverse cross-section which decreases in a direction away from the source, 2· Apparatus in accordance with claim 1 wherein a mask extends adjacent and parallel to each face of the source, each defined; : mask having apertures therein cfafiftrmod- at least in part by screen portions, each screen portion having a transverse cross-seotion Which decreases in a direction away from the source. · Apparatus as claimed in either claim 1 'or claim 2 wherein the radioactive elements are elongate rods disposed in parallel relation in a source holder. frame and the screen portions are elongate metallic members which extend parallel to the rods and are arranged at levels intermediate the levels of the radioactive elements, each screen portion having a triangular transverse cross-section and being arranged with one lateral face parallel to the plane of the source* 4. Apparatus as claimed in either claim 1 or claim 2 wherein the radioactive elements are point elements disposed in a uniform lattice in a source holder frame and the or each mask has the form of a grid with the apertures positioned in front of respective radioactive elements, the apertures being located in the bases of recesses flared outwardly in the said direction. ...14 - shape o a truncated pyramid* 6. Apparatus as claimed in claim 4 wherein each recess has the shape of a truncated cone. 7. Apparatus as claimed in an of the preceding claims wherein the screen portions and radioactive elements are orientated to be at a given angle of inclination to the direction of movement of a substance to be irradiated pass^ ing the source* 8i Irradiation apparatus substantially as herein described with reference to the accompanying drawings. For the Applicants ERS IS/DD