WO1990009667A1

WO1990009667A1 - An irradiation device and a lense

Info

Publication number: WO1990009667A1
Application number: PCT/SE1990/000085
Authority: WO
Inventors: Bengt Anderberg
Original assignee: Scanditronix Ab
Priority date: 1989-02-10
Filing date: 1990-02-08
Publication date: 1990-08-23
Also published as: SE8900460D0; SE463055B; SE8900460L

Abstract

An installation (5') for irradiating objects with an electron stream is proposed according to the invention, wherein the electron ray is caused to scan the object while this is moved forward. In order to make the irradiation more homogeneous and more efficient an extra magnetic lense (21) is located immediately beneath the region to be irradiated. This magnetic lense (21) deflects the electron ray in such a way that the different parts of the electron ray will be essentially parallel or a little convergent after said lense. The magnetic lense used consists of a two elongated permanent magnets located close to each other at a small distance. With an appropriate adjustment of the strength and distance of said magnets a suitable deflecting effect is obtained. In addition the permanent magnets are advantageously constructed of magnet elements having a plate-shape, between which a softmagnetical material is located.

Description

An irradiation device and a lense

The present invention is rel ted to irradiation of objects, more particurlary electrone irradiation, as for sterilizing medical equipment. In sterilizing by means of irradiation a possible way is to arrange in such a way that an object is transported through an electron ray. This electron ray scans the object laterally, that is in a direction perpendicular the direction of movement of said object. The scanning movement is repeated periodically in such way that said object in all its mass is subjected to the influence of the electron ray. The electron ray then scans an area in space having a fan-like shape. In irradiating thick objects, because of this fan-like shape, a lower useful ratio of the rays is obtained and also a lower irradiation of the rear side of said object, that is the side which is turned away from the source of the ray. These non-homogeneities derive partly from merely geometrical . reasons, since the lateral parts have an angle different from 90° in relation to the incident electrons and further the rear part of the object has a larger distance from the point where the electrons are, deflected to perform the scanning movement than the front part of the object.

In the published international applications WO 88/02920, W088/01731 and W087/06391 installations are disclosed for irradiation with ions, wherein the above stated problem has received a solution. Here means are used to give the incident ionic ray a constant angle of incidence to the irradiated object. In all these prior installations the_, ionic ray is deflected only to one side in relation to its position without _f deflection. This depends on the lenses used which consists of sectors- or wedgeshaped magnets or of a dipole magnet having a gap therein ov_er its length. This unsymmetrical deflection of the partical ray is not economical by the fact that high magnetic field intensities are required resulting in high costs for the correcting magnetic lens, and_,in addition this arrangement of the partical paths is space consuming. Thuls it"would be desirable to have a symmetrical deflection of the electron ray with an equal deflection on both sides of the undeflected position of the particle ray. This, however, cannot be produced with the types of magnetic lenses known from these published documents.

Such a desired deflection is provided according to the invention. According to this a correcting lense or parallelizing lense as above is provided which is located preferably close to the object to be irradiated. This correcting lense has a linearly deflecting characteristic on both sides of the undeflected position of the ray, that is the deflection is such that the magnitude of the deflection with a sign is essentially proportional to the distance from the deflecting point in the lense and the center point of that lenses. For an appropriately adjusted field intensity of the magnetic field generated by this lense the paths of the electron ray after traversing this lense will be essentially parallel or convergent contrary to the divergent or fanshaped paths obtained without the lense.

Although the configuration with parallel! ray paths is preferred in the general case in some cases it may be advantageous to provide somewhat convergent ray paths after passage through the correcting lens. This may cancel border effects with a reduced dose efficiency in the marginal regions of the object.

In the patent specification US 4661 712 a device is described by means of which a symmetrical deflection may be obtained. The parallelizing lense is in this prior device a space charge lense. This utilizes electrons enclosed by static magnetic and electrical fields in order to generate a cylindrically symmetrical, electrostatic field having a radial direction. Thus this lense can only be used for guiding positively charged particals together. An electron ray will only be unfocused in the passage through this prior lense. Thus this known method has no indication that the desired purpose stated above could be fulfilled also for electron rays.

In the patent specification US 3845312 a partical accelerator for ionic inplantation is disclosed. Here a broad divergent and stationar ionic ray is produced, which thus has a conelike shape. Next to the irradiated object there is a solenoid lense 19 consisting of a magnetic winding and an annular iron core. This lense turns the particle paths inside the ray beam and the deflection can be adjusted in such a way that all particles in the ray will hit the object with nearly parallel paths. Thus in this known device there is no scanning with a ray over an object. This known irradiating device also appears to be limited to irradiating widths of about 10 cm because the parallelizing lense generates a powerful magnetic field which heavily would disturb the particle paths and the operation of other components included in the system for guiding the particle ray. Possibly this could be prevented by the use of heavy shealding or by designing the system with particle paths of sufficient length, but these solutions are naturally disadvantageous. According to the invention a scanning width is used which is significantly larger, for instance in the order of magnitude of 80 cm and generally at least 30 cm.

The magnetic lense which is used according to the invention'may for instance be a conventionel quadrupole magnet. Such a magnet is however space consuming for the large sweeping widths for which this Invention primarily is intended to be used, and for electrical operation¹ such^" a magnet also contains electrically isolating material which can be influenced in a unfavourable way by the radiation. Quadrupole magnets are naturally well known for shaping the section of a particle ray, also scanning rays, see the patent specification US 4075488.

According to the invention a magnetic lense is proposed the specific features of which appear from the accompanying claims.

Thus the magnetic lense comprises two elongated magnets. These are magnetized in their longitudinal direction and they are placed parallel! to each other at a small distance and further they have opposite directions of magnetization. The electrically charged particle stream is intended to pass through the gap formed between the magnets. Ely appropriately adjusting the distance between the magnets such a magnetic field intensity is obtained and thus a deflecting influence on the electron rays passing therebetween can be obtainer that the paths of the electron rays can be made more or less parallel after the passage. The elongated magnets can be constructed as electrical magnets having essentially equally distributed winding turns over the length of the magnetic core. Electrical operation of the elongated magnets"included in the magnetic lense however can be unsuitable because of the risk of radiation damages mentioned above. Thus it is preferred that that these magnets are made permanent.

In addition it is preferred that the permanent magnets are formed by packages of primary magnets and that these primary magnets are separated by a material having a high susceptibility such as transformer core sheets. The primary magnets in a package all are magnetized in the same direction and by varying a little the mutual distance between these primary magnets also the desired adjustment of the resulting magnetic field can be obtained.

By means of the invention an irradiation method is achieved such that a beam of electrically charged particles is moved with mutually parallel paths in a plane over an object without the requirement that said object has to move, that is it can be standing still or displaced with a constant velocity in a displacement direction perpendicular to the plane containing the electron rays. The parallel displacement movement of the electron ray is repeated, possibly in the opposite direction. As observed above it may here be advantageous to give the ray of charged particle a little convergence, in particular in such a way, that the ray paths located at the margins of the object are somewhat convergent. In this way the marginal regions, in compensation for their otherwise normally lower radiation intensity, will be given an elevated radiation compared to the central or inner regions of the object.

The invention will now be described with reference to the appended drawings, on which Fig. 1 schematically illustrates a prior art installation for sterilizing medical equipment,

Fig. 2 schematically illustrates the installation according to the invention seen from the side,

Fig. 3 is a side view of the magnetic lense mounted beneath a flange,

Fig. 4 is an end view of the magnetic lense with retainers mounted beneath the outlet chamber of the electron ray,

Fig. 5 is a schematical view seen from above of the magnetic lense. In Fig. 1 an installation is shown for sterilizing objects such as cases 1 containing medical equipment. This may for instance have a largest dimension of 10 - 80 cm. They pass on a conveyor belt 3 above an irradiation device 5. In passing the irradiation device 5 the objects 1 are subjected to an electron ray which fanlike and repeatedly scans the article while the article is transported forward perpendicularly to the plane of the fan shape.

In Fig. 2 the installation according to the invention is schematically illustrated, wherein it must be observed that the scanning direction of the electron ray here is perpendicular to the one illustrated in Fig. 1. The irradiation installation 5' consists of an electron gun which, as in the conventional way, consists of an electron source 7, a linear accelerator having several steps 9, focusing and direction changing lenses 11 and 13 respectively. The electron ray which altogether passes in a vacuum exits upwardly and is deflected periodically by a lense 15 providing the scanning movement. In this part the electron ray is enclosed in a vacuum chamber 17 having a triangular configuration. This triangular chamber 17 is terminated at its upper part by a upper flange 19 having a thin window, through which the electron ray exits and will pass through an object to be irradiated. Before the electron ray exits from the vacuum chamber 17 it has been reflected by a lense 21. This results in the fact that the electron ray issued always is approximately parallel to the center ray, that is to the ray which is not deflected by means of the scanning lense 15. The magnetic lense 21 is constructed in such a way that it will provide an angular deflection of the electron ray which is proportionel to the distance between the passage of the central ray through the lense 21 and the deflection point of the electron ray considered inside the lense 21. By making this deflection larger or smaller various diverging, parallel! or convergent ray paths can be obtained.

In Fig. 5 is illustrated schematically seen from above how the magnetic lense 21 used is constructed. It consists of two permanent magnets 23 having a generally elongated shape and placed adjacent to each other in such a way that a gap is formed. Through this gap the electron ray is intended to pass. The two permanent magnets 23 are magnetized in their longitudinal direction, they are located in parallel to each other and at an adjustable distance from each other. The magnetizing direction of the magnets 23 are opposite. In the gap between these magnets 23 a magnetic field is formed which is the appropriate deflecting type. In the preferred embodiment each magnet 23 further is constructed as a pac age of primary magnets 25 and softmagnetical material 23 located therebetween. The primary magnets 25 may be designed as plates, for instance having a square shape, and then they are magnetized perpendicularly to the large surfaces of the plate. The primary magnets 25 are in addition distributed essentially with an equal spacing over the length of the resulting magnets 23. Small deviations from the equal distribution may be provided in special cases.

In Fig. 3 and 4 is illustrated in greater detail the magnet 23 and the securing thereof to the top of the vacuum chamber 17. The vacuum chamber 17 is on its outer side provided with various reinforcing ribs 29. In appropriate longitudinal ribs of this kind bolts 31 are screwed at their one end, having their other end connected to end pieces of channels 35. These channels 35 contain the package forming the permanent magnets 23. The magnet packages are maintained in their place by the end pieces 33 by the fact that the opposing end pieces for a magnet channel are attached by means of pull rods 37. The bolts 31 make it possible to change the distance of the magnet channels from each other in such a way that the field intensity desired can be adjusted in the gap between the magnets 23.

Claims

Claims:

1. A device for irradiating objects with electrons comprising a ray gun, which is arranged to deliver an essentially straight electron ray, and a scanning lense by means of which said electron ray is deflected and made to move in the plane scanning the objects, c h a r a c t e r i z e d in that adjacent the object, before the passage of the ray into this and after said scanning lense, seen in the direction of the ray, another magnetic lense is located generating a static magnetic fiels in the path of said ray, this magnetic field being such that the magnitude of the deflection of said ray in its passage through the lense is essentially proportional to the distance from the point, through which that ray passes through the lense, and the center point of the lense, the deflection having inverse values of the deflection on different sides of the center point of that lense, in such a way that the electron rays, which have passed that lense on different times and in different scanning positions, together form an essentially parallel or convergent ray bundle.

2. A device according to claim 1, c h a r a c t e r i z e d in that said scanning lense is arranged in such a way that the scanning displacement has a length of at least 10 cm and preferrably at least 30 cm, for instance 80 cm, taken laterally in the plane here said electron ray is intended to hit said objects.

3. A device according to one of claims 1 - 2, c h a r a c t e r i z e d in that the further lense comprises two elongated, parallel lenses located at at distance from each other and having an opposite direction of magnetization.

4. A device accordint to claim 3, c h a r a c t e r i z e d in that said elongated magnets are permanent magnets.

5. A device according to claim 4, c h a r a c t e r i z e d in that said permanent magnets comprise a number of primary magnets separated by a softmagnetical material.

6. A device according to claim 5, c h a r a c t e r i z e d in that said primary magnets are plate-shaped having their direction of magnetization perpendicular to the large surfaces of said plates.

7. A device according to one of claims 5 - 6, c h a r a c t e r i z e d in that said primary magnets are essentially equally distributed over the length of the resulting permanent magnets.

8. A magnetic lense for deflection of rays of charged particles, particularily electrons, c h a r a c t e r i z e d in that it comprises two elongated, parallel permanent magnets located at distance from each other and having an opposite direction of magnetization.

9. A magnetic lense according to claim 8, c h a r a c t e r i z e d in that said permanent magnets comprise several primary magnets separated by a softmagnetical material.

10. A magnetic lense according to claim 9, c h a r a c t e r i z e d in that said primary magnets are plate-shaped having a direction of magnetization perpendicular to the large surfaces of said plates.

11. A magnetic lense according to one of claims 9 - 10, c h a r a c t e r i z e d in that said primary magnets are essentially equally distributed over the length of the resulting permanent magnets.