US2866902A

US2866902A - Method of and apparatus for irradiating matter with high energy electrons

Info

Publication number: US2866902A
Application number: US519774A
Authority: US
Inventors: John C Nygard
Original assignee: High Voltage Engineering Corp
Current assignee: High Voltage Engineering Corp
Priority date: 1955-07-05
Filing date: 1955-07-05
Publication date: 1958-12-30
Anticipated expiration: 1975-12-30

Description

Dec. 30, 1958 J. c. NYGARD 2,866,902

METHOD OF AND APPARATUS FOR IRRADIATING MATTER WITH HIGH ENERGY ELECTRONS Filed July 5, 1955 4 Sheets-Sheet 1 FIG-l FIG. IA

f LINEAR DIVERGENT 6 CYLINDRICAL LENS y fw FIG.IB f 5 x NON-LINEAR 7 4 convenesm AM/L Ill LINEAR DIVERGENT /8 'CYLINDRICAL LENS Dec. 30, 1958 7 METHOD OF AND APPARATUS FOR IRRADIATING MATTER 7 Filed July 5, 1955 FIG 2 J. C. NYGARD WITH HIGH ENERGY ELECTRONS LINEAR SCAN-SMALL AMPLITUDE LINEAR DIVERGENT CYLINDRICAL LENS FIG.2A

FIG. 2C

4 Sheets-Sheet 2 Dec. 30, 1958 J c. NYGARD 2 866,902

.' METHOD OF AND APPARATUS FOR IRRADIATING MATTER WITH HIGH ENERGY ELECTRONS Filed July 5, 1955 4 Sheets-Sheet 3 FIG. 3

A! A FIG. 3A

SINUSOIDAL 1 SCAN- SMALL AMPLITUDE I NON-LINEAR ,12 DIVERGENT CYLINDRICAL LENS Dec. 30, 1958 v J. c. NYGARD 2,866,902

METHOD OF AND APPARATUS FOR IRRADIATING MATTER WITH HIGH ENERGY ELECTRONS Filed July 5, 1955 4 S heets-Sheet 4 FIG.5

FIG, 4A

NON-LINEAR DIVERGENT CYLINDRICAL FIG.4B

United Sttes Patent METHOD OF AND APPARATUS FOR IRRADIAT- ING MATTER WITH HIGH ENERGY ELECTRONS John C. Nygard, Lexington, Mass, assignor to High Voltage Engineering Corporation, Cambridge, Mass, a corporation of Massachusetts Application July 5, 1955, Serial No. 519,774

Claims. (Cl. 250-495) This invention relates to the irradiation of matter with high energy electrons, and in particular to a method of and apparatus for delivering the ionizing energy of a beam of high energy electrons to the matter being irradiated, with maximum energy efficiency and minimum side effects due to excess dosage, by appropriate focusing of the electron beam through an electron-optical lens system.

High-energy electrons are an important form of ionizing energy, and high-energy electron sources are finding increasing'application in radiation chemistry, sterilization and preservation of food and drugs, and similar fields. A high-energy electron source may be provided by accelerating electrons to high energy in an evacuated tube, and permitting the high-energy electrons to issue from the tube through an appropriate electron window onto the matter to be irradiated.

When a high-energy electron beam issues from an evacuated acceleration tube into the atmosphere, the electrons are scattered by the gas molecules. Hence, although the diameter of the electron beam may be maintained quite small within the vacuum, the diameter of the same beam may be about four or five inches at a distance of about two feet from the electron window. The most natural way in which to irradiate a substance with such an electron beam is to place the substance at about two feet from the electron window, where.-the*be"am:diameter is adequate to cover the surface of most substances which one might wish to irradiate; and, in fact, this technique has been and still is frequently employed.

However, the transverse distribution of electrons in the beam is normally characterized by a central region of relatively high intensity which attenuates gradually as the distance from the axis of the electron beam is increased. This uneven distribution of electron current density .means a loss of efficiency, since the excess power delivered near the axis of the beam is wasted. Such efiiciency loss is highly objectionable where the power delivered is costly, as is the case in electron irradiation. Moreover, frequently the dose required to obtain a desired result by electron irradiation is only slightly less than a dose which may produce undesired side effects. Thus the overdosage near the beam axis not only reduces efiiciency, but may even be harmful.

My invention saves the use of ionizing energy which would otherwise be wasted, avoids the damaging effects of excess dosage, and provides a source of high energy electrons which can deliver an accurate, predetermined dosage which can be adjusted merely by controlling the current and voltage of the electron beam.

In order thatthe principle of the invention may be readily understood, 1 have disclosed several embodiments thereof in the accompanying drawings, wherein:

Fig. l is a diagram illustrating one embodiment of the invention, wherein an electron-optical system comprising three cylindrical lenses focuses an electron beam of small diameter into an elongated pattern;

Figs. 1A, 1B, 1C and ID are graphs illustrating the cur- "ice rent density, as a function of distance from the beam axis, of the electron beam in Fig. l at the points A, B, C, andv D, respectively;

Fig. 2 is a diagram illustrating another embodiment of V the invention, wherein an electron-optical system com-- prising a single cylindrical lens acts upon an electron beam of small diameter, to which a linear scanning move-- ment of small amplitude has been imparted, so as to magnify the amplitude of the scanning movement of the beam;

Figs. 2A, 2B, and 2C are graphs illustrating the current density, as a function of distance from the beam axis, of the electron beam in Fig. 2 at the points A, B, and C, respectively;

Fig. 3 is a diagram illustrating another embodiment of the invention, wherein an electron-optical system com-- prising a single cylindrical lens acts upon an electron beam of small diameter, to which a sinusoidal scanning move-v ment of small amplitude has been imparted, so as to magnify the amplitude of scanning movement of the beam and also to modify the distribution of current density along the length of such scanning movement;

Figs. 3A, 3B and 3C are graphs illustrating the current density,as a function of distance from the beam axis, of the electron beam in Fig. 3 at the points A, B, and C, respectively;

Fig. 4is a diagram illustrating another embodiment of the invention, wherein an electron-optical system comprising a single cylindrical lens focuses an electron beam. of small diameter into an elongated pattern;

Figs. 4A and 4B are graphs illustrating the current den sity, as a function of distance from the beam axis, of

the electron beam in Fig. 4 at the points A and B, respec-- tively;

Fig. 5 is a diagram illustrating one form of cylindrical lens which may be used in the electron-optical systems of f Figs. 1-4; and

Fig. 6 is a diagram illustrating another form of cy-- lindrical lens which may be used in the electron-optical systems of Figs. 1-4.

Referring to the drawings, and first to Fig. 1 thereof, an electron accelecator 1 produces a beam 2 of high energy electrons which is directed onto the matter 3 to be irradiated. In accordance with my invention, the electron beam 2, which is of small diameter when it issues from the electron accelerator 1, is focused into an elongated pattern by an electron-optical system 4, so that the electron beam 2 impinges on the matter 3 more or less in sheet form. The matter 3 is supported by a conveyor belt 5, which travels in the direction indicated transformer-rectifiers in any of their modifications, im-' pulse or Marx generators or capacitrons, betatrons and synchro-trons. Moreover, the invention herein disclosed can be usedeither with a continuous electron beam, such as that produced by an electrostatic belt-type accelerator, or with a pulsed electron beam, such as that produced by a microwave linear accelerator. Consequently, the electron accelerator 1 is shown merely diagrammatically in the drawings.

When the electron beam 2 issues from the electron accelerator 1, the electrons in the beam are distributed about the beam axis in an approximate Gaussian pattern, as in dicated by-the graph of Fig. 1A, wherein the beam current density i is plotted as a function of the distance x from p 2,866,902 Patented Dec. 30', 1958] thebeam axis, in the plane of the drawing. In accordan'ce with the embodir'nentof my invention shown in Fig. l, the electron beam 2 is successively focused by a first linear divergent cylindrical lens 6, a non-linear cylindrical lens 7, and a second linear divergent cylindrical lens 8. By the term cylindrical lens Imean a lens whose strength in one plane is negligible compared with its strength action in the plane perpendicularthereto. In all the cylindricallcnses shown in the drawings, the strength in the plane perpendicular to that of the drawing is negligible compared with thestrength in the plane of the drawing.

The etfect of the first linear divergent cylindrical lens 6 is to spread the beam 2 in the plane of the drawing, so that at the point B in Fig. 1 the current density 1' varies with the distance x in the manner shown by the graph of Fig. 18. Since the lens 6 is linear, the electrons still form a Gaussian distribution about the beam axis in the plane of the drawing.

The efiect of. the non-linear cylindrical lens 7 is to convert the Gaussian distribution shown in the graph of Fig. 18 into the uniform distribution shown in the graph of Fig. 1C. In order toaccomplish this result, the lens 7 may be constructed so as to have a convergent focusing effect which increases with increasing distance x; or, alternatively, the lens 7 may be constructed so as to have a divergent focusing effect which decreases with increasing distance x. In Fig. 1, the lens 7 is shown as having a convergent focusing effect.

The effect of the second linear divergent cylindrical lens 8 is to spread the beam 2 in the plane of the drawing, so that at the surface of the matter being irradiated, shown as point D in Fig. 1, the current density i has the uniform distribution shown by the graph of Fig. 1D.

A second embodiment of the invention is shown in Fig. 2, wherein the electron beam 2 which issues from the electron accelerator 1 is given a linear scanning movement of small amplitude in the plane of the drawing by means of a suitable electron-deflection device 9. Suitable apparatus for imparting such a scanning movement to the electron beam 2 is disclosed and claimed in a co-pending application, Ser. No. 236,652, filed July 13, 1951, now U. S. Patent No. 2,729,748, and assigned to the assignee of the present invention. Since the scanning movement imparted to the electron beam 2 is linear, the electron distribution at the point B in Fig. 2 is uniform in the plane of the drawing, as shown by the graph of Fig. 213. However, the electron-deflection device which is required for theproduction of a linear scan is somewhat complex, and the expense and complexity involved in constructing such adevice increases markedly with the amplitude of the scanning movement. In accordance with the invention, the electron-deflection device 9 imparts a scanning movement of small amplitude to the electron beam 2, and the resulting scanned beam is then magnified, as it were, by a linear divergent cylindrical lens 10, so that at the surface of the matter being irradiated} shown as point C in Fig. 2, the current density i has the extended uniform distribution shown by the graph of Fig. 2C.

The difficulties attendant upon imparting a linear scanningrnovernent to the electron beam may be avoided through use of the embodiment of the invention which is shown in Fig. 3. In said Fig. 3, a sinusoidal scanning movement of small amplitude is imparted to the electron beam 2 by a suitable electron-deflection device 11. Since the scanning movement is sinusoidal, the electron-deflecting device 11 may be of simple construction; however, the resultant electron distribution at the point B in Fig. 3 is non-uniform in the plane of the drawing, as shown by the graph of Fig. 3B. In accordance with the inven tion, the sinusoidally scanned electron beam is spread out in a non-uniform manner in the plane of the drawing by a non-linear divergent lens 12, so that'at the surface of the matterbeing irradiated, shown as point C in Fig. 3, the current density 1' has the uniform distribution shown by the graph. of Fig. 3C. The non-linear divergent lens 12 is constructed so as to have a divergent focusing effect which increases with increasing distance x, so that the regions of high current density shown in the graph of Fig. 3B are spread outward, as shown by the graph of. Fig. 3C.

By appropriate electron lens construction, the desired focusing of the electron beam 2 which issues from the electron accelerator 1 may be accomplished in accordance withthe invention by means of an electron-optical system which comprises a single non-linear divergent cylindrical lens 13, as shown in Fig. 4. Such a lens 13 should be so constructed that its divergent focusing effect decreases somewhat with increasing distance x from the beam axis, so that the axial region of high current density 1' shown in the graph ofFig. 4A is diffused to a greater extent than the peripheral regions of low current density, whereby the electron distribution at the surface of the matter being irradiated, shown as point B in Fig. 4, has the uniformity shown by the graph of Fig. 4B.

In the foregoing description, various types of cylindrical lenses have been referred to which may be summarized as follows:

(1) Linear divergent (lenses 6 and 8 in Fig. 1 and lens 10 in Fig. 2)

(2) Non-linear divergent (the alternate form of lens 7 in Fig. 1, lens 12 in Fig. 3, and lens 13 in Fig. 4)

(3) Non-linear convergent (the illustrated form of lens 7 in Fig. 1)

Many forms ofelectron lenses are of course well-known in the art of electron optics, and my invention is not hmited to any particular form of electron lens. However, in general, I prefer to use electron lenses of the type shown in Figs. 5 and 6, which are simple to construct and which are particularly suited to cylindrical electron-optical systems.

Referring now to Fig. 5, the electron lens therein shown comprises a pair of

permanent horseshoe magnets

14, 15 disposed in the plane of the drawing and perpendicular to the axis of the electron beam 2, which travels into the plane of the drawing. The poles of the

magnets

14, 15 are arranged as shown, with the north pole of each magnet placed opposite the south pole of the other magnet. The magnetic field is zero at the beam axis 16, which is the location of the origin of the x and y axes shown in Fig. 5 to simplify the description. The magnetic field to the right of the beam axis 16 in Fig. 5 is directed downward, and the magnetic field to the left of the beam axis 16 is directed upward, so that all electrons which are not on the y-axis (i. e. x:0) are deflected away from the beam axis 16 either to the right or to the left, so that the electron beam is spread out along the x-axis as shown by the broken line 2a. By spacing the

magnets

14, 15 from one another by a distance large compared with the distance between opposite poles of each magnet, the magnetic field is caused'to be substantially parallel to the y-axis over the entire cross-sectional area of the electron beam, so that there is substantially no deflection of electrons in the y direction. That is to say, there is substantially no deflection of electrons either upward or downward in Fig. 5. The

magnets

14, 15 thus constitute a divergent cylindrical lens.

The lens formed by the

magnets

14, 15 may be rendered linear or non-linear, and the nature of its deviation from linearity may be adjusted, byproper shaping of the pole faces of the

magnets

14, 15. The appropriate shape of these pole faces depends on so many variables that its exact determination is largely accomplished empirically. It depends, among other things, on the energy of the electrons, the strength of the magnets, and the size of the pole faces. However, since the magnetic field created by the

magnets

14, 15 will in general tend to increase in a linear fashion along the x-axis over the crosssectional asea of theelectron beam 2, and since only a.

small deviation from linearity is 'requiredby the invention, the proper shaping of the pole faces ofthe

magnets

14, 15 is a matter which is well within the capabilities of those skilled in the art of electron optics.

In order to convert the lens shown in Fig. 5 into a convergent cylindrical lens, it is necessary merely to reverse the polarity of the

magnets

14, 15, so that the magnetic field to the right of the beam axis 16 in Fig. 5 is directed upward, and the magnetic field to the left of the beam axis 16 is directed downward. All electrons which are not on the y-axis will then be deflected toward the beam axis 16. Such a lens may be used as the illustrated form of lens 7 in Fig. l, the necessary degree of nonlinearity being provided by appropriate shaping of the pole faces of the magnets, as in'the case of the divergent lens.

An alternative form of electron lens suitable for the purposes of the invention is shown in Fig. 6, wherein four

conductive plates

17, 18, 19, 20 are disposed about the electron beam 2 in a manner similar to the disposition of the

magnets

14, 15 of Fig. 5. Plates 17, 18 are placed to the left and right, respectively, of the electron beam 2, and are spaced relatively close to each other. 19, 20 are placed respectively above and below the elec+ tron beam 2, and are spaced apart a distance which is large relative to the spacing between plates 17, 18. For a divergent lens, a positive potential is impressed on plates 17, 18 with respect to

plates

19, 20; for a convergent lens, a negative potential is impressed on plates 17, 18 with respect to

plates

19, 28. In Fig. 6, which illustrates a divergent lens, the voltage source 21 impresses a positive potential on plates 17, 18 with respect to

plates

19, 20.

The effect upon the electron beam 2 of the electrostatic field created by the

plates

17, 18, 18, 20 of Fig. 6 is readily seen to be precisely the same as the effect of the magnetic field created by the

magnets

14, 15 of Fig. 5, so that in Fig. 6, as in Fig. 5, the electron beam 2 is spread out as shown by the broken line 2a. Deviation from linearity is accomplished by appropriate shaping of the

plates

17, 18, 19, 20, the requisite'shaping being determined 1n an empirical manner as in the case of the shaping of the pole faces of the

magnets

14, 15 of Fig. 5. I

Having thus described the method of my invention together with several embodiments of apparatus for carrying out the method, it is to be understood that although specific terms are employed, they are used in a generlc and descriptive sense and not for purposes of limitation, the scope of the invention being set forth by the following claims.

I claim:

Plates in an evacuated region; directing said beam of high-energy electrons out of said evacuatedfr'egion and ontothe surface of the matter to be irradiated by said high-energy electrons; and distributing during the irradiation the intensity of the action of the high-energy electrons on the matter being irradiated by imparting a scanning movement of small amplitude to said beam, subjecting the scanned beam to the focusing action of a divergent cylindrical electronoptical system, the plane in which said focusing action occurs being the same as the plane in which said beam is scanned, whereby the area of the intersection of said beam and the incident surface of the matter being irradiated is elongated, and. introducing a non-linearity into said focusing action -such ,thatthe electron current, impinging on said surface, perunit length of said elongated area is rendered substantially uniform over the length of said elongated area.

3. A method in accordance with claim 2, wherein said scanning movement is sinusoidal.

4. The method of delivering the ionizing energy of a beam of high-energy electrons to matter to be irradiated, with maximum energy efliciency and minimum side effects due to excess dosage, which method comprising thefollowing steps; creating a beam of high-energy electrons in an evacuated region; directing said beam of high-energy electrons out of said evacuated region and onto the sur- 1. The method of delivering the ionizing energy of a 1 beam of high-energy electrons to matter to be irradiated, with maximum energy efificiency and minimum s de effects due to excess dosage, which method comprising the following steps: creating a beam of high-energy electrons in an evacuated region; directing said beam of high-energy electrons out of said evacuated region and onto the surface of the matter to be irradiated by said high-energy electrons; and distributing during the irradiation the intensity of the action of the high-energy electrons on the matter being irradiated by subjecting said beam to the focusing action of a divergent cylindrical electron-optical system,

whereby the area of the intersection of said beam and the incident surface of the matter being irradiated 1s elongated, and introducing a non-linearity into said focus ing action such that the electron current, lmpinging on said surface, per unit length of said elongated area is rendered substantially uniform over the length of said elongated area.

2. The method of delivering the ionizing energy of a beam of high-energy electrons to matter to be irradiated, with maximum energy efficiency and minimum side effects due to excess dosage, which method comprising the following steps: creating a beam of high-energy electrons face of the matter to be irradiated by said high-energy electrons; and distributing during the irradiation the intensity of the action of the high-energy electrons on the matter being irradiated by imparting a linear scanning movement of small amplitude to said beam and subjecting the scanned beam to the focusing action of a linear divergent cylindrical electron-optical system, the plane in which said focusing-action occurs being the same as the plane in which said beam is scanned.

5. Apparatus for delivering the ionizing energy of a beam of high-energy. electrons to matter to be irradiated,

with maximum energy efliciency and minimum side effects due to excess dosage, comprising in combination: means for creating a beam of high-energy electrons in an evacuated region; means for directing said beam out of said evacuated region and onto matter to be irradiated by said 1 high-energy electrons; and a divergent cylindrical electronoptical system so constructed and arranged that its focusing action on said beam is divergent but non-linear, whereby the area of the intersection of said beam and the incident surface of the matter being irradiated is elongated, the non-linearity of said focusing action being such that the electron current, impinging on said surface, per unit length of said elongated area is rendered substantially uniform over the length of said elongated area.

6. Apparatus in accordance with claim 5, wherein said divergent cylindrical electron-optical system comprises a first linear divergent cylindrical lens, a second linear divergent cylindrical lens, and a cylindrical lens, between said first linear divergent cylindrical lens and said second linear divergent cylindrical lens, into which the required non-linearity is introduced. 1

7. Apparatus in accordance with claim 5, wherein said divergent cylindrical electron-optical system comprises a single non-linear divergent cylindrical lens.

8. Apparatus for delivering the ionizing energy of a beam of high-energy electrons to matter to be irradiated, with maximum energy efficiency and minimum side effects due to excess dosage, comprising in combination: means for creating a beam of high-energy electrons in an evacuated region; means for directing said beam out of said evacuated region and onto matter to be irradiated by said high-energy electrons; means for imparting a scanning, movement of small amplitude to said beam; and a divergent cylindrical electron-optical system so constructed and arranged that its focusing action on the scanned beam is divergent, in the plane in which the beam is scanned, but non-linear, whereby the area of the intersection of said beam and the incident surface ofth e matter being irradiated .is elongated, the non-linearity of said focusing action being such :that :the electroncurrent, impinging on said surface, per unit length of said elongated area is rendered substantially uniform over the length of said elongated area, t

9. Apparatus in accordance with claim 8, wherein said scanning movement is sinusoidal. t

10. Apparatus for delivering the ionizing energy of a beam of high-energy electrons to matter to be irradiated, with maximum energy eflicieney and minimum side effects due to excess dosage, comprising ineombination: means for creatinga beamofhigh-energy electrons in an evacuated region; means for directingsaid beam out of said evacuated region and onto matter to be irradiated bysai'd high-energy electrons "means 'for imparting a linear scanning" movement oi small amplitude to said beam;and a linear divergent cylindrical electron-optical system so constructed and arranged that its focusing action on the scanned beam is divergent, in the plane in which the beam is scanned.

References Cited in the file of this patent UNITED STATES PATENTS Crowley-Milling Feb. 25, 1958