US3056024A - Apparatus for irradiating matter with high energy electrons - Google Patents

Description

Sept. 25, 1962 A. J. GALE 3,056,024

APPARATUS FOR IRRADIATING MATTER WITH HIGH ENERGY ELECTRONS Filed Dec. 2, 1959 2 ELECTRON CURRENT DENSITY AT 4 DPODUCT SURFACE IDISTANCE FROM BEAM AXiS 3,056,024 IRRADIATING MATTER WITH HIGH ENERGY ELECTRONS APPARATUS FOR This invention relates to the irradiation of matter with high energy electrons, and in particular to apparatus for delivering the ionizing energy of a beam of high energy electrons to the matter being irradiated with maximum energy efiiciency and minimum side effects due to excess dosage, simply by the interposition of conductive members between the electron source and the matter to be irradiated.

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 evac uated 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 beam 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 a gradually at increasing distances from the axis of the electron beam. This uneven distribution of electron current density means a loss of efliciency since the excess power delivered near the axis of the beam is wasted. Such efliciency 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 the dose which may produce undesirable side eiiects. Thus the overdosage near the beam axis not only reduces efliciency 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 accurately predetermined dosage which can be adjusted merely by controlling the current and voltage of the electron beam. Although various means of accomplishing this same result have been proposed such as, for example, beam scanning and beam focusing, my invention has the advantage of much greater simplicity over either of these means.

In accordance with my invention, conductive material of non-uniform thickness is interposed in the path of the electron beam, the thickness being approximately proportional to the electron current density at that point. This may be accomplished either by utilizing special electron windows of varying thickness, or by superimposing layers of aluminum foil in the path of the electron beam in an appropriate manner, or by interposing a grate in the path of the electron beam.

r 3,056,024 Patented Sept. 25, 1962 ice The invention may best be understood from the following detailed description thereof having reference to the accompanying drawing in which:

FIG. 1 is a diagram illustrating the electron current density distribution of a conventional electron beam;

FIG. 2 is a diagram illustrating the effect upon such electron current density distribution of the interposition of a conductive member which does not extend the entire width of the beam;

FIG. 3 is a diagram illustrating the effect on the electron current density distribution of the interposition of several conductive members;

FIG. 4 is a diagram indicating a possible irregular electron current distri'bution obtained with a scanned electron beam; and

FIG. 5 is a plan view of a grate that might be employed to compensate for the current distribution shown in FIG. 4.

Referring to the drawing and first to FIG. 1 thereof, a conventional electron beam 1 has a Gaussian current density distribution as shown therein. Such an electron beam would be produced by an electron accelerator 2 such as that shown in U.S. Patent No. 2,729,748 in the absence of the scanning device disclosed and claimed in said patent. If a conductive absorber such as a strip of aluminum foil 3 (FIG. 2) is superimposed between the electron window *4 and the product 5, then the electron current density distribution produced by the electrons which traverse this strip of aluminum 3 will be as shown 'by the dotted line, and the electron current density distribution produced by the electrons that pass outside the aluminum foil 3 will be as shown by the dashed line, and the combined effect is shown by the solid line in the diagram of FIG. 2. The absorber 3 should preferably be of conductive material which is grounded since it will constantly be accumulating charge from the electrons travelling therethrough, and an insulator would accumulate too much charge. It will be observed that the single piece of aluminum 3 has flattened the electron current density across the entire width of the beam as is shown in the diagram of FIG. 3.

In this manner a substantially uniform irradiation over a large area may be obtained in a relatively short distance from the electron window 4 without scanning or the use of magnetic devices. In addition to its simplicity, the invention avoids such phenomena as the undesirable pulsed effects in the product produced by scanning.

The invention may be used not only as a substitute for scanning and similar devices, but also as a corrective device. For example, in the case of extremely wide scans, it may be difiicult to provide a uniform current density distribution without the use of extremely complex electronic equipment in the scanning circuit, and a thrifty type of scanning circuit may produce a current distribution of irregular form such as that shown in the diagram of FIG. 4. Such a current distribution may be corrected by the use of aluminum foil strips such as have already been described in connection with FIGS. 1, 2 and 3. Alternatively, either the irregular scan or the conventional Gaussian distribution may be corrected by means of a grate such as is shown in FIG. 5. Such a device may com prise a frame 8 equipped with sockets 9 adapted to receive bars 10. The sockets 9, may, for example, be spaced about .050 inch apart and the bars 10 may comprise tantalum rods .050 inch in diameter. In lieu of varying the thickness of the interposed conductive material, as in FIGS. 1, 2 and 3, its lateral density, so to speak, may be varied by the grate shown in FIG. 5. Thus, at those portions of the beam where the electron current density is highest, a rod 1% would be placed in each socket 9. At regions of decreased electron current density, the rod spacing might be reduced to a rod 10 every other socket 9 or pairs of rods 10 separated by single spacings. At the low current density portions of the beam only an occasional rod 10 might be employed. In such a device, the electron current density is affected partly by absorption in the rods 10 and partly by scattering produced by the rods 10, although the latter effect is less important.

Uniformity may be obtained in two dimensions, of course, either by having a true grid efiect (by placing rods transverse to the rods 10 shown in FIG. 5) or by causing the aluminum strips 3, 6, 7 shown in FIGS. 1 through 3 to be of circular shape instead of rectangular shape. If, however, the irradiated product 5 is being conveyed through the beam in the manner shown in the above mentioned Patent No. 2,729,748, it is only necessary to correct for electron beam current density in one dimension, namely, the dimension transverse to the direction of travel of the product 5.

Having thus described the principles of the invention together with 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 claim.

I claim:

Apparatus for irradiating matter with high energy electrons comprising in combination a source of high energy electrons, means for supporting the matter to be irradiated in the path of the electron beam, an absorber of conductive material supported between said electron source and said product, said absorber comprising a plurality of conductive members so arranged in the path of the electron beam that the spacing between adjacent members is approximately inversely proportional to the electron current density at the point in question.

References Cited in the file of this patent UNITED STATES PATENTS Grossmann Dec. 24, Moreau et al. Aug. 6,

Marks Dec. 30, Graham Dec. 9,

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