US2993120A - Electron irradiation - Google Patents

Description

y 1961 R. M. EMANUELSON 2,993,120

ELECTRON IRRADIATION Filed Jan. 14, 1959 United States Patent 2,993,120 ELECT-RON IRRADIATION Roy M. Emanuelson, Reading, Mass, assignor to High Voltage Engineering Corporation, Burlington, Mass., a corporation of Massachusetts Filed Jan. 14, 1959, Ser. No. 786,780 3 Claims. (Cl. 250-495) This invention relates to the irradiation of materials with high energy electrons and in particular to the irradiation of thin layers of material. In the electron irradiation of materials, a high energy electron beam is produced in an evacuated region and then issues from the evacuated region through an electron permeable window and onto the product to be irradiated. In the process of traveling through the electron window a certain proportion of the energy in the electron beam is lost so that it is necessary that fairly high energies of the order of one million volts or more be used if the process is not to be prohibitively ineificient. This creates problems in the irradiation of thin sheets of material such as polyethylene film and other thin products. Because of their thinness, such products absorb only a small amount of energy from the beam. In order to avoid wasting the remainder various proposals have been made for causing the film to follow a serpentine path through the electron beam or by irradiating the film while wound on a roll and other methods. These methods may become impractical with certain products such as brittle material which cannot be wound over a drum of small circumference. Accordingly my invention provides a device by means of which a thin film may be passed through an electron beam and nevertheless absorb substantially all of the available energy. In accordance with the invention I provide a magnetic field in the vicinity of the intersection of the electron beam and the product which field is of sufiicient strength to confine the electrons to the general vicinity of the product with the result that all the energy of the electrons are absorbed in the product.

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

FIG. 1 is a view along the line l1 of FIG. 2;

FIG. 2 is a somewhat diagrammatic view of an electron accelerator for the production of an electron beam to irradiate a thin product in accordance with the invention;

FIG. 3 is a view along the line 33 of FIG. 4;

FIG. 4 is a view similar to FIG. 1 but showing a modification of the invention;

FIG. 5 is a view showing another embodiment of the invention;

FIG. 6 is a view along the line 66 of FIG. 5 and is similar to FIG. 2;

FIG. 7 is a view in perspective showing still another embodiment of the invention;

FIG. 8 is a diagrammatic view showing still another embodiment of the invention;

FIG. 9 is a view taken at right angles to that of FIG. 8; and

FIG. 10 is a diagram showing a view similar to that of FIG. 1.

Referring to the drawings and first to FIGS. 1 and 2 thereof therein is shown the lower extremity of an electron accelerator I which is equipped with a scanning device 2 which serves to spread the beam 3 out in the plane of the drawing in FIG. 1 in accordance with the teachings of US. Patent No. 2,602,751 to Robinson. A thin product 4 to be irradiated is conveyed through the beam 3 transverse to the plane thereof. The electron accelerator 1 is fully disclosed in the aforementioned patent to ice Robinson and in other literature well known in the art and need not be referred to herein in any greater detail. In accordance with the invention, I provide a permanent magnet 5 suported in such a manner that one pole N lies adjacent the emergent electron beam 3 and extends parallel to but spaced from the plane of the beam, and the other pole S of the permanent magnet 5 is similarly situated on the opposite side of the plane of the beam. Since there is no such thing as a permanent magnet having a single pole, it is necessary to provide a magnetic path between the two poles N, S and in FIGS. 1 and 2 this path is shown as a connecting piece 6 extending the length of the permanent magnet and crossing between the two poles below the product. The permanent magnet produces a magnetic field perpendicular to the plane of the scanned electron beam and serves to deflect the electrons in the plane of the electron beam in a manner shown by the dotted lines in FIG. 1. The trajectory of each electron in the magnetic field is a circle whose radius is proportional to the energy of the electrons. Consequently, as each electron traverses the thin sheet of product, it loses some energy and hence travels in a continuously smaller circle. Thus no electron can escape from the region of the product so that all its energy is expended in the product and no energy is lost despite the extremely thin nature of the product. Appropriate apertures 7 are provided in the poles of the magnet for passage of the conveyor belt and the product. In the embodiment of the invention shown in FIGS. 1 and 2 the product is extremely thin material and is supported in such a manner that a conveyor belt is unnecessary. This arrangement is, of course, preferable to that in which a conveyor belt is employed since the belt itself would absorb energy from the electron beam.

Ordinarily a scanning device such as that shown in FIGS. 1 and 2 spreads out the beam in such a manner that the electron trajectories emerging from the electron window 8 are straight lines emanating from the same point. This, of course, means that the electrons enter the magnetic field at a different angle of incidence depending upon the phase in the scanning cycle. In the case of very wide angle scans, the electrons at the extremities of the scan will enter the magnetic field at an angle of incidence which differs substantially from that at the center of the scan. At both ends this means that the electrons will be in the magnetic field for a longer time that at the center of the scan, and therefore they will be deflected through a greater angle of deflection. At one end of the scan this will be partially compensated by the fact that the angle of incidence into the magnetic field is not normal to the boundary of the field. But at the other end of the scan, the two effects will be cumulative. In order to eliminate this effect entirely in accordance with the invention, additional magnets may be provided on either side of the scanning bucket which should be made of a non-magnetic material, these mag-nets being so shaped as to give a non-uniform field or a field with non-parallel boundaries such that all the electrons emerge through the electron window in a direction normal to the boundary of the principal magnetic field. The use of such auxiliary magnets is, of course, not limited to use with the embodiment of my invention which is related to the irradiation of thin sheets, but is also useful wherever wide scans are employed, since a non-uniform angle of incident on the product tends to produce a. non-uniform dose distribtuion since the electrons at the extremities of the scan will have less penetration than those at the center of the scan.

In accordance, therefore, with one feature of the invention I provide auxiliary magnet shims 9 as shown in FIGS. 1 and 2 for the purpose of further deflecting the electrons emerging from the acceleration tube so that they all enter the magnetic field at the same angle of incidence, preferably perpendicular to the plane of the product.

Referring now to FIGS. 3 and 4 therein is shown a modification of the embodiment shown in FIGS. 1 and 2. In the embodiment shown in FIGS. 3 and 4 the permanent magnet of FIGS. 1 and 2 has been replaced by a pair of coils 10. These coils flank the plane of the electron beam and each encircles the product. A strong magnetic field is thus created between the two coils across the plane of the electron beam perpendicular thereto. In order to confine the necessary return flux path, magnetic material 11 is provided about the coils as shown. This magnetic shield will, of course, have suitable apertures 12 therein for the passage of the product. Except for the manner of creation of the magnetic field, the device shown in FIGS. 3 and 4 operates on the same principles as that shown in FIGS. 1 and 2.

Referring now to FIGS. and 6, therein is shown still another embodiment of the invention. In this embodiment, the product does not pass directly between the poles of the magnet being employed to create the necessary magnetic field, but rather the product is caused to pass very close to the gap between the poles so that the fringing fields between poles is present in the product. In the device shown in FIGS. 5 and 6, an electromagnet is employed, as shown, wherein the pole faces 13 form part of a magnetic circuit 14 which is energized by the coils 15. It will readily be understood that the device shown in FIGS. 5 and 6 may be modified by the provision of permanent magnets or simple coils. The sheet product may travel either above or below the pole faces or both and in the device shown two layers 16, 17 of product are simultaneously being irradiated, one 16 being located above the pole faces, and the other 17 being located below them. It will be appreciated that the effect of the fringing fields is to initiate the deflection of electrons somewhat before they impinge upon the product surface so that it may be necessary to offset the product somewhat with respect to its normal position were the magnet of the invention not employed.

In the devices or embodiments of the invention described hereinbefore, the magnetic field is produced perpendicular to the plane of the electron beam. It is also possible to construct a device in which the direction of the magnetic field is parallel to the plane of the electron beam. In all cases, of course, the magnetic field will be parallel to the plane of the product which in turn is transverse to the direction of travel of the electrons in the beam. This, of course, is because a magnetic field has no effect on the electrons path except to the extent that it has a component perpendicular to the direction of travel of the electrons. In principle it would be possible to create a magnetic field across the product and parallel to the plane of the electron beam by means of permanent magnets. However, for any appreciable product width, this becomes impracticable since it necessitates that the pole faces of the magnets be separated so widely that the field strength in the mid point is either not sufliciently strong or else the fringing fields are prohibitively large. Accordingly, the preferred embodiment of this form of the invention is through the use of coils. For the same reason that prohibits the use of a permanent magnet, it is not practicable to use two coils, one at each end of the product. Obviously it is not possible to distribute the length of the coil across the product in the normal sense, since it would be necessary to cut through the coil and thus interrupt the current path. However, it is possible to approximate the effect of a coil extending across the width of the product by means of the arrangement shown in FIG. 7.

Referring now to FIG. 7, therein are shown three coils 18, 19, each of which occupies one of the three spaces defined by the incident electron beam and the sheet product. The upper two coils 18, 19 should be placed as closely as possible to the product as shown. The vertical 7 height of these upper two coils should be minimized and the size thereof is somewhat exaggerated in FIG. 7 merely in order to make the principles of this embodiment of the invention more clear. The lowermost coil 20 has an indentation in it as shown, and it is the space within this indentation that corresponds to the interior of the desired coil. It will be observed that this space is surrounded by current paths which have the effect of a regular coil. As noted, the vertical height of the upper coils 18, 19 should be minimized in order to prevent premature deflection of the electrons. The three coils 18, 19, 20 are shown in FIG. 7 as being energized by separate power supplies 21, 22, 23 respectively, but, of course, it would be possible to run them all from a common power supply. The embodiment of the invention shown in FIG. 7 tends to reduce loss of electrons near the edges of the product.

Still another embodiment of the invention is shown in FIGS. 8 and 9. The purpose of this embodiment is to eliminate the need for a scanning or other beam expanding device. Referring to FIGS. 8 and 9 a single electron beam 24 of small cross section is injected between the pole faces 25, 26 of a magnet which may either be a permanent magnet as shown or an electromagnet. The pole faces 25, 26 are provided with apertures 27 for passage of the product. As previously noted, the path of an electron in a magnetic field is a circle whose radius is proportional to the energy of the electron. The size of the magnet is determined so that at the energy which the electrons possess on being injected into the space between magnet poles the size of the magnet will be sufiicient to bent the electrons in a circular path which will strike the product, as shown. After traversing the product, however, the electrons lose energy and so they then travel in a circle of smaller radius. With each passage through the product, the electrons continue to lose energy and hence travel in smaller and smaller circles. The electrons are thus confined to the vicinity of the product in such a manner that they continue to bombard it so that all the energy of the electron beam is absorbed by the product.

Since the electrons strike the product surface at a greater angle of incidence after several revolutions of the electrons, the invention tends to compensate for the otherwise relatively low dose at the surface of the product.

Another feature of the invention is the fact that the magnetic system reduces electron radiation hazard.

It is also possible to shape the magnetic poles to compensate for the usual lateral dose distribution which tends to result, for example, from. scanning a beam having energy variations such as are present in a beam produced by a linear accelerator or similar device.

Such lateral dose distribution usually includes dose peaks spaced somewhat from the center of the product and from the extremities of the scan. Accordingly, the magnetic field would be strongest near the center of the product so as to confine the electrons there and increase the dose. At the extremities of the scan a similar strengthening of the magnetic field can be provided not only to compensate for the lesser dose resulting from the above-mentioned lateral dose distribution, but also to prevent losses by electrons which would otherwise miss the product entirely.

In general, successful operation of the invention requires that the electrons lose sufficient energy in their first passage through the product so that they can be confined within the region of the magnetic field. This means that the product to be irradiated cannot be too thin since in that event insufficient energy absorption takes place in the product. It is desirable to maximize the energy absorption in the initial path through the product by placing it in such a position that the angle of incidence of the electrons is large, that is to say, the angle beween the electron trajectory and the normal to the product surface approximates Referring now to FIG. 10 the incident electrons travel in the magnetic field in a circle of radius R The product 4 is placed, as shown, tangentially to the electron trajectory. After the electron has passed through the product the first time, it has lost some energy and therefore travels on a circle of smaller radius R By placing the product near the bottom of the electron trajectory, as shown in FIG. 10, one ensures that the electron will re-enter the product a second time. In traveling through the product the second time the election again loses energy so that upon emerging from the product, it travels in a circle of radius R which is still smaller than R and R It is essential to the operation of the invention that this third radius R be sufiiciently small so that the electron is retained within the magnetic field and returns to the product as shown in FIG. 10. This requirement imposes a lower limit on the amount of material through which the electrons pass in their first two traversals of the product. This amount of material traversed by the electrons is proportional to the thickness of the product and also to the angle of incidence. It will be appreciated that if the product is too thin, this elfect cannot be compensated for by increasing the strength of the magnetic field since increasing the magnetic field strength, although it will reduce the radius R will also at the same time reduce the radius R and it is obvious from an inspection of FIG. that R represents the maximum distance which the product may be placed below the boundary of the magnetic field.

The discussion in the preceding paragraph is, of course, only an approximation, and various factors have been neglected such as, for example, fringing fields.

Having thus described the principles of the invention, together with several illustrative embodiments 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. The method of irradiating thin solids with high energy electrons, which method comprises producing a stream of high energy electrons, directing said stream onto a lateral surface of a layer of a thin solid to be irradiated, and producing a magnetic field in the path of said stream in the vicinity of said solid transverse to the direction of travel of said electrons as they strike said surface initially, whereby said electrons are caused to repeatedly traverse the same layer in such a manner as to dissipate substantially all their ionizing energy in said layer.

2. Apparatus for irradiating thin solids with high energy electrons comprising in combination means for producing a sheet of high energy electrons, means for directing said sheet onto a lateral surface of a thin solid to be irradiated, and at least three current-carrying coils arranged in close juxtaposition but nutually separated by said sheet and said solid and with their axes mutually parallel, so that a portion of the outer periphery of each of said coils together form the boundary of a T-shaped zone the column of which is adapted to receive said sheet and the head of which is adapted to receive said solid, whereby said solid in passing through the head of said T-shaped zone is repeatedly traversed by said electrons in such a manner that substantially all their energy is dissipated in said solid.

3. The method of irradiating thin solids with high energy electrons, which method comprises producing a beam of high energy electrons, directing said beam onto a one edge of a lateral surface of a layer of a thin solid to be irradiated, and producing a magnetic field in the path of said beam in the vicinity of said solid transverse to the direction of travel of said electrons as they strike said surface initially, said magnetic field having sufiicient intensity and extent to cause said electrons to repeatedly traverse the same layer in such a manner as to dissipate substantially all their energy in a region which includes but extends away from said edge across said layer.

References Cited in the file of this patent UNITED STATES PATENTS 2,741,704 Trump Apr. 10, 1956 2,824,969 Crowley-Milling Feb. 25, 1958 2,887,583 Emanuelson May 17, 1959 2,897,365 Dewey July 28, 1959

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