US2887583A - Electron accelerator for irradiation - Google Patents

Description

y 1-959 R. M. EMANUELSON ELECTRON ACCELERATOR FOR IRRADIATION Filed Oct. s. 1956 "'llll United States Patent 2,887,583 ELECTRON ACCELERATOR FOR IRRADIATION Roy M. Emanuelson, Reading, Mass., assignor to High Voltage Engineering Corporation, Cambridge, Mass., a corporation of Massachusetts Application October 8, 1956, Serial No. 614,503

4 Claims. (Cl. 250-495) This invention relates to irradiation with high-energy electrons and in particular to irradiation with high-energy electrons from a single electron accelerator in such a manner as to utilize the ionizing energy of the high-energy electrons in a more eflicient manner and to permit greater thicknesses to be irradiated. More specifically stated, the invention comprehends an electron accelerator adapted to produce an extended beam of electrons (i.e. a beam of electrons whose cross-section is extended in at least one plane), said extended beam emerging from the evacuated region of the electron accelerator so as to irradiate an object to be irradiated laterally from opposite sides. Differently stated, the invention comprehends an electron accelerator from the evacuated region of which most of the high-energy electrons emerge in the form of two opposed and laterally directed streams. In the following detailed description the invention will be described with particular reference to an electron accelerator in which an extended beamof electrons is produced by imparting a scanning movement to an electron beam; but the invention is not limited to any particular means for producing the extended beam of electrons, and includes other means for accomplishing this result; such as focusing an electron beam by an electron-optical system in accordance with the teachings of U.S. Patent No. 2,737,593 to Robinson.

The invention may best be understood from the following detailed description thereof, having reference to the accompanying drawing, in which: q

Fig. 1 is a somewhat diagrammatic side view of apparatus embodying the invention and including an electron accelerator adapted to produce an extended beam of electrons which emerges from the evacuated region of the electron accelerator principally in the form of two opposed and laterally directed streams;

Fig. 2 is a view along the line 2-2 of Fig. 1;

' Fig. 3 is a side view of the apparatus of Fig. 1 taken at right angles to the view in Fig. 1; and

Fig. 4 is a diagrammatic view of a portion of the apparatus of Fig. 1 and shows the electron trajectories in the apparatus of Fig. 1.

It is now becoming established that all types of living organisms are aifected by gamma rays and high energy electrons and that lethal effects can be produced on unwanted organisms by doses which will raise the temperature of water only a few degrees centigrade. The grow ing availability of streams of high energy electrons makes possible the practical application of this knowledge to the sterilization of many important products, such as pharmaceuticals, surgical instruments, animal tissues for transplant purposes, as well as for the preservation of cer- 3 tain foods. Only high energy electron sources, as distinct Moreover, the possibility of using various forms of 2,887,583 Patented. May 19, 1959 ice ionizing radiation to promote chemical reactions has recently been explored, including highly endothermic chemical reactions which require large quantities of energy in concentrated form and exothermic chemical reactions which are triggered by the initial application of concentrated energy. Among available sources of ionizing radia tion, high energy electrons seem to be the best medium for delivering ionizing energy in an efficient and controlled manner to a substance or substances for the purpose of promoting chemical reactions.

Measurements of the properties of high energy electrons have disclosed that their range of typical materials is small comparedto that of gamma rays. A 2-millionvolt electron has a maximum range in water of 1 cm. Beyond this limiting distance there is no ionizing eflfect, while the maximum ionizing eifect occurs at one-third this range. Although practical high energy electron sources may be constructed for many millions of volts,

such higher energy apparatus becomes progressively more expensive and also often has a lower output electron current capacity.

A common method of, in effect, doubling the range of penetration of an available stream of electrons is to irradiate the object from both sides as by reversing the object and irradiating again, or by irradiating the object simultaneously from two electron sources. However, the former is limited to the irradiation of material in rigid form, and the latter entails the additional expense and space requirements of a second electron source. Furthermore, an interruption or modulation of electron intensity would not affect both aspects simultaneously, unless the product is irradiated simultaneously, from two electron sources with special electronic coupling being introduced between the two sources; consequently, it would be difficult to reirradiate the partially irradiated material to bring its dose up to the proper level.

Various proposals have been made to irradiate an object directly with only a portion of the available electron stream from a single electron source, and to redirect the rest of'the available electron stream on to the object from an essentially opposing aspect, in order to obtain the increased penetration of crossfiring with a single electron source. However, the proposals heretofore made are subject to many objections, some of which will now be enumerated.

The portion of the electron stream which is redirected may be affected by scattering and other influences which will result in loss of beam power. This power loss may run as high as 30%. If an object is also irradiated by other portions of the electron stream which are not subjected to this power loss, an objectionable asymmetry of dose distribution will result in the object irradiated.

Moreover, in any system in which high-energy electrons are redirected back towards the electron accelerator these back-streaming electrons constitute a source of intense radiation towhich many sensitive parts of the electron accelerator are exposed, since not only the highenergy electrons but also the principal beam of X-rays generated by these electrons is directed back up the electron-accelerating device. Unless elaborate and costly .radiation protection measures are taken, this radiation could damage glass portions of acceleration tubes, magnet coils, and other similar electrical equipment using insulating materials sensitive to radiation. Moreover, in high-voltage electron accelerators, such radiation could cause ionization in the insulating'gas surrounding the high-voltage parts of the apparatus, thereby impairing Operation of the electron accelerator.

In accordance with the invention, the two portions of the electron stream which irradiate the object are both redirected in the same manner, so that asymmetry of dose distribution is avoided. Moreover, the electrons travel in vacuo throughout the redirecting process and therefore are subjected to negligible scattering prior to impinging on the object to be irradiated. In addition, all redirected electrons are redirected laterally, so that damage to the electron accelerator as a result of radiation is minimized.

Referring -more particularly tothe drawings, one embodiment of the invention is shown in Figs, 1, 2 and 3, wherein an electrostatic accelerator for the acceleration of electrons to'high energy is indicated at 1r The electron accelerator 1 may comprise an electrostatic generator of the type disclosed in U.S. Patent No. 2,252,668 to Trump in conjunction with an acceleration tube of the type disclosed in U.S. Patent No. 2,517,260 to Van'de Graalf and Buechner. Alternatively, the electron accelerator 1 may comprise a microwave linear accelerator of the type described by Walkinshaw at volume 61, pages 246-254,by R. Shcrsby Harvie at volume 61, pages 255-270, and by Mullett and Loach at volume 61, pages 271-283 of The Proceedings of the Physical Society (1948), or any other suitable electron accelerator, such as a resonant transformer.

Electrons are accelerated by the electron accelerator '1 in a manner not necessary to explain herein in detail and enter an evacuated tube extension 2 as a beam of'highenergy electrons. Said tube extension2 terminates in a flared portion 3 the lower extremity of which includes twoelectron window assemblies 4, 5 each of which terminates in an electron window 6, 7, extendingparallel to and flanking the axis ofsaid beam of high energy electrons. Thus a recess is formed between the windows 6, 7. The tube extension 2, the flared portion 3, and the twoelectron window assemblies .4, 5 all form part of the evacuated region of the electron accelerator 1, so that the electrons accelerated by the electron accelerator 1 remain in vacuo until they-emerge through the electron windows 6, 7. A beam-scanning device 8 imparts a scanning movement to the electron beam in the plane of the drawing in Fig. 1 in accordance with the teachings of U.S. Patent Nos. 2,602,751 and 2,729,748toRobinson.

A product, material or substance 9 which is to be irradiated is positioned between the electron windows 6, 7 by a support 10, which may be movable or stationary, and which is shown in Figs. 1, 2 and 3 as, a traveling belt. Electrons accelerated by the electron accelerator 1 are redirected by two magnets 11, 12 so as to emerge through the electron windows 6, 7 onto the object 9 in opposed lateral directions. Throughout the specification and claims hereof, it is to be understood that the word lateral is used with reference to the primary beam of electrons, and means a direction which is generally transverse to the direction in which the primary beam of electrons is accelerated. The polarity of the magnets 11, 12 is such that, as shown in Figs. 1 and 4, electrons traveling between the pole faces 13, 14 of the left-hand magnet 11 are deflected to the right and electrons traveling between the pole faces 15, 16 of the right-hand magnet 12 are deflected to the left. The pole faces 13, 14, 15, 16 are spaced apart by a greater distance at their outer extremities than at their inner extremities as shown in Fig. 2, with the result that the intensity of the magnetic field therebetween decreases with increasing distance from the electron windows 6, 7. The shape of the pole faces 13, 14, 15, 16 is so chosen that the electron trajectories assume the form shown by the broken lines 17 in Fig. 4, and the electrons issue through the electron windows 6, 7 in a direction which is substantiallynormal to the electron windows 6, 7 and with an intensity which is-.distributed substantially uniformly over the irradiated surfaces of the object 9.

By avoiding damage due to radiation produced by back-streaming electrons, the invention increases the time during which an electron accelerator may be operated without having to undergo repairs. Thus, for example, a ten percent reduction in energy of radiation dissipated in sensitive parts of the electron accelerator would increase the life of the electron accelerator from, say, 1000 hours to 10,000 hours.

Moreover, the invention provides the advantages of crossfiring from a single electron accelerator without using costly and intricate beam-bending or beam-splitting devices and circuits.

Having thus described the principles of the invention, together with'an illustrative embodiment thereof, it is to be understood that although specific terms are employed they are used in a generic and descriptive sense and not for purposes of limitation, the scope of the invention being set forth in the following claims.

I claim:

'1. An electron acceleratorfor electron irradiation comprising in combination: means for producing a beam of high-energy electrons along an axis in an evacuated region, said evacuated region terminating in a flared portion having a recesswhose boundary includes at least two electron-windows extending parallel to and flanking-said axis and adapted to receive an object to be irradiated therebetween, and means for directing a substantial portion .of said beam through each of said windows.

2. An electron accelerator for electron irradiation comprising in combination: means for producing a beam of high-energy electrons along-an axis in an evacuated region, said evacuated region terminating in a flaredportion having a recess whose boundary includes at least two electronwindows extending parallel to and flanking said axisrand adapted to receive an object to be irradiated therebetween, means for directing a substantial portion of said beam through each of said windows, and means for conveying objects to irradiated between said electron windows.

3. An electron accelerator for electron irradiation comprising in combination: means for producing a beam of high-energy electrons along an axis in an evacuated region, ,said evacuated region terminating in a flared portion having a recess whose boundary includes at least two electron windows extending parallel to and flanking said-axis'and adapted to receive an object to be irradiated therebetween, means for directing a portion of said beam through one of said electron windows laterally onto said object from one aspect, and means for directing another portion of said beam through the other electron window laterally onto said object from an essentially opposing aspect.

4. An electron accelerator for electron irradiation comprising in combination: means for producing an extended beam' of high-energy electrons along an axis in an evacuated region, said evacuated region terminatingin a flared portion having a recess whose boundary includes at least two electron windows extending parallel to and flanking said axis and adapted to receive an object to be irradiated therebetween, and means for creating a magnetic field pattern in the path of said extended beam so oriented and distributed that a substantial portion of said extended beam is directed through each of said windows.

References Cited in the file of this patent UNITED STATES PATENTS 12,741,704 Trump et al. -.Apr. 10, 19,56

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