US2785313A - Method and apparatus for sterilizing by electron bombardment - Google Patents

Description

March 1957 J. G. TRUMP 2,785,313

METHOD AND APPARATUS FOR STERILIZING BY ELECTRON BOMBARDMENT Filed July 26, 1952 Unite rates lVIETl-IOD AND APPABLATUS FQR STERELEZWG BY ELECTRQN EQMBARBMENT Application July 26, 1952, Serial No. 361,111

8 illairns. (Cl. 259-495) This invention relates to electron sterilization, and particularly to a simple method of and several substantially different apparatus for increasing the amount of ionizing energy absorbed by an object to be sterilized from an available source of high-energy electrons. Still more specifically and particularly stated, my invention relates to the more uniform administration of ionizing energy to object irradiated by high-energy electrons which I preferably accomplish by substantially surrounding or enclosing said object with a metal of high atomic number, so as to redirect towards said object both those electrons which have passed alongside it and also those electrons which have been scattered from its volume.

it is now becoming established that all types of living organisms are affected 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 growing 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 tissue for transplant purposes, as well as for the preservation of certain foods. Only high energy electron sources, as distinct from gamma ray sources, appear to possess enough total power output to handle economically the considerable amounts of material which may require sterilization.

Measurements of the properties of high energy electrons have disclosed that their range in typical materials is small compared to of gamma rays. A Z-millionvolt electron has a maximum range in water of 1 cm. Beyond this limiting distance there is no biological effect, while the maximum biological effect 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 efiect, doubling the range of penetration of an available stream of electrons is to irradiate the object from both sides. This can be done in principle by reversing the object and irradiating again, or by irradiating the object simultaneously from two electron sources.

In many cases such methods of securing the doubling of the electron treatment range are uneconomic and excessively complex. One obiect of my invention herein disclosed and claimed is to double the effective range of penetration of an available stream of electrons in a simple manner. I have discovered that if a primary stream of electrons is directed from one aspect or source or direction against an object to be sterilized, but some of said electrons are nevertheless allowed to pass said object and to strike a high atomic number material positioned immediately beyond or adjacent to said object, then a proportion of electrons will be electrostatically scattered backward by said high atomic numbermaterial and serve are 1:

the purpose of simultaneous irradiation of said object rom the opposite aspect.

Although it is known that the effective range of penetration of an available strearn of electrons may be doubled by irradiating an object from opposite sides, my he ein disclosed invention has the advantage of simplicity over other proposed methods. My invention has the further advan age of simultaneity. It is often not practical to irradiate an object from one aspect and then a few moments later to complete the irradiation from a second aspect. This is particularly true in the case of liquids.

Another object of my invention herein disclosed is to irradiate complex shapes with a stream of electrons in a simple manner, since the electrons which pass the object to be sterilized and are reflected backward by the high atomic number material are scattered at many angles. My invention would, for example, be particularly useful in the irradiation of a tray of surgical instruments where complete electron sterilization of even apparently inaccessible surfaces is required.

Still another object of my invention is to render the dose distribution more uniform near the surface of an object to be sterilized by high energy electrons. Most materials to be sterilized are of relatively low/atomic number, but even so, five or ten percent of the incident energy may be reflected backward from its upper surface. By placing a high-atomic number material above an object to be sterilized and alongside of the incident beam of high energy electrons, l am able to reflect back toward the object some of those electrons which have been scattered out of it. The reflected electrons have the efiect of adding dosage to the low dosed region which always exists near the surface of the object to be sterilized, so that by my invention 1 may reduce or avoid the necessity of other methods of rendering the dose distribution more uniform near the surface.

These and other objects of the invention, both as to method and to apparatus, will be best understood by reference to the following description when taken in connection with the accompanying drawing, while its scope will be more particularly pointed out in the appende claims.

in the drawing:

Fig. 1 is a diagrammatic view in vertical cross-section of one form of apparatus for doubling the effective range of an available stream of electrons;

Fig. 2 is a similar view illustrating one'forrn of ap paratus for rendering the dose distribution more uniform near the surface of an object to be sterilized by high'energy electrons;

Fig. 2A is a side elevation of the apparatus shown in Fig. 2;

Fig. 3 is a similar view illustrating a modification of the apparatus illustrated inFigs. l and 2;

Fig. 3A is a side elevation of the apparatus shown in Fi 3; and

Fig. 4 shows in vertical section a portion of an acceleration tube for the passage of a beam of high energy electrons.

Referring to the drawing, and first to Fig. 1, the arrow 1 represents a stream of high energy electrons; My invention may be practiced to advantage with any source of high energy electrons. However, in the embodiments of my invention which are illustrated in Figs. 2 and 3, 1 preferably employ a well-donned beam of electrons such as 1, in order that the reflector 4a in Fig. 2 or 412 in Fig. 3 (to be described in detail hereinafter) may not interfere with the stream of high energy electrons .1, Such a well defined beam of electrons, may be created by and discharged, from apparatus: such as disclosed in patents issued in my name as inventor or (to-inventor,

for-example, and/or apparatus manufactured by High Voltage Engineering Corp. of Cambridge, Massachusetts, whereby the electron beam may be accelerated and strike the object with the full energy, as shown, for example, in Fig. 4 at A. Referring again to Fig. 1, an object 2 to be sterilized is placed upon any suitable support 3, so that said object 2 is in the path of the stream of electrons 1 issuing as a well-defined beam from a suitable source. Immediately below the support 3 there is shown in Fig. 1 a reflector 4 which may be a plate of any material, but which is preferably a metal of high atomic number.

I have discovered from experiments made by me that a fairly high proportion of primary electrons bombarding a material of high atomic number are reflected with most or all of their initial energy. Many of these'high energy but now secondary electrons are reflected directly backward, but statistically they are reflected over a wide band of angles, depending primarily upon the atomic number of the reflecting material and the energy of the primary electrons. It can thus be seen that if, as shown in Fig. 1, the stream of electrons 1 is directed against the object 2, but some of said electrons are allowed to pass the object 2 and to strike the reflector 4, then a proportion of said electrons will be electrostatically scattered backward, as indicated by the arrow 5, and serve the purpose of simultaneous irradiation of the object 2 from the opposite aspect or direction.

Suitable high-atomic-number materials for the reflector 4 are tungsten, gold, lead, and uranium. Lower atomic number metals may be used, but the proportion of reflected high energy electrons will be less, as determined from my experiments.

, In general, the reflector 4 is stationary, although it may be in motion, as shown at 4c in Fig. 3 yet to be described. If the reflector 4 is stationary, the constant bombardment thereof by high energy electrons with the consequent absorption of large amounts of energy may result in excessive heating of such reflector 4. In order to carry away the energy absorbed by the reflector 4 in this process, I may water or air cool as by a flowing system, the reflector 4 by any appropriate apparatus indicated at 4.

The mechanism or property 'or force of matter whereby a high energy electron may be reflected with little or no loss of energy from a target material is known as elastic nuclear scattering. This type of scattering results from an interaction between an arriving high energy electron and the field of force of the nucleus of the target atoms. comet arriving in the solar system and the gravitational .field of the .sun. Both the arriving electron and the arriving comet experience a force of attraction which increases inversely with the square of the distance from the nucleus of the target atoms and the sun, respectively. Both the electron and the comet travel around the nucleus and the sun, respectively, so that they are in effect reflected back towards the direction from which they came. The electron-nucleus interaction is the more vigorous the higher the atomic number. of the target atoms, and it is primarily for the highest atomic number metals that a sufficient reflection of primary energy in the opposite direction can be obtained to justify economically the herein disclosed and claimed procedure. In principle, however, even lower atomic numbered materials, such as aluminum, may eflectively be used.

The back-reflected electrons are scattered at ,many angles, so that my invention herein disclosed is particularly advantageous in the irradiation of objects of complex shape as, for example, a tray of surgical instruments.

In another embodiment of the apparatus of my invention, and by which my method may be practiced, I may use metals of high atomic number to reflect back toward an object to be sterilized, those electrons which have been scattered out of it. 1 1

This interaction is similar to that between a Referring now to Fig. 2, therein is shown an object 2 to be sterilized, which is substantially surrounded and wholly enclosed by a high atomic number metal reflector 4a, except for an aperture 7 through which the stream of electrons 1 is directed from a suitable source. The stream of electrons lis preferably well-defined in order to avoid bombardment of the upper exterior surface of the reflector 4a. The object 2 may rest upon any convenient support 3, and as indicated, the upper part of the enclosing reflector 4a is formed as a removable cover 4x to permit the object to be positioned. Any electrons which are scattered out of the object 2 are reflected back towards said object 2 by the reflector 4a, as indicated, merely in part, by the few arrows 6. In particular, those electrons in the primary stream 1 which are scattered backward from the upper surface of the object 2 are reflected down towards said upper surface of the object 2 by the reflector 4a. Even though most objects to be sterilized are of relatively low atomic number, five or ten percent of the energy of the incident beam 1 may be scattered backward from the upper surface of the object 2. When these scattered electrons are redirected down onto the upper surface of the object 2, by the reflector 4a, the region near the upper surface which, owing to the normally occurring dose distribution in the object 2, would otherwise receive a lower dose than that received by the rest of the object 2, now receive supplemental irradiation. Consequently, the dose distribution is rendered more uniform near the said upper surface of the object 2.

In addition, any electrons in the primary stream 1 which pass the object 2 will be reflected back towards the object 2 by the reflector 4a. This is illustrated more clearly by the modified form of apparatus shown in Figs. 3 and 3A, by which the method herein disclosed may be practiced, also, and wherein the object 2 to be sterilized is supported upon a moving belt 40 by means of any suitable support, such as indicated at 3. Alternatively the object 2 may rest directly upon the moving belt 40. The upper surface 9 of the belt 40 is a. material of high atomic number, and serves to refiect back towards the object 2, as indicated by one arrow 5, those electrons in the primary stream 1 which pass the object 2 as well as some of the electrons which are scattered out of the object '2. In addition, two stationary reflectors 4b are positioned over the object and alongside the incident beam 1, and serve to reflect back towards the object 2 those electrons which are scattered out of the upper surface thereof. Such stationary reflectors 4a are desirably of the length or" the object 2, as indicated in Fig. 3A.

Having thus described several embodiments of the apparatus of my invention, and by each of which the method of my invention may be practiced, 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. That method of increasing the treatment range of an available beam of high energy electrons in electron sterilization, which comprises bombarding an object to be sterilized from one aspect with a beam of high energy electrons, permitting some of the electrons of such beam to travel beyond said object, and reflecting back by elastic nuclear scattering from a material of high atomic number onto such object a substantial amount of those electrons that traveled in the bombarding action beyond the object being bombarded.

2. That method of increasing the treatment range of an available stream of electrons in electron sterilization, which method comprises bombarding an object to be sterilized from one aspect with a beam of high energy electrons, directing said beam towards said object in such a manner that some of said electrons travel beyond said object, and reflecting said electrons which travel beyond said object, by electrostatic interaction with the nuclei of atoms of a material of high atomic number, whereby said object is simultaneously bombarded by electrons from opposing aspects.

3. That method of sterilizing an object of irregular shape by irradiation with high energy electrons, which method comprises bombarding the object from one aspect with a beam of high energy electrons, directing said beam towards said object in such a manner that some of said electrons travel beyond said object, and reflecting said electrons which travel beyond said object by electrostatic interaction with the nuclei of atoms of a material of high atomic number, whereby said object is simultaneously bombarded from opposing aspects by electrons traveling over a wide band of angles.

4. Apparatus for irradiating an isolated object with high energy electrons, comprising means for creating a well-defined beam of high energy electrons, means for directing from one aspect said beam onto an isolated object to be irradiated, and a reflector, of material of high atomic number, substantially enclosing said object, but having an aperture positioned to provide an unobstructed path for said beam directly to said object, whereby there are reflected back onto said object a proportion of those high energy electrons of said beam, that in the irradiating action passed said object without touching it, or were reflected from said object by nuclear scattering.

5. That method of securing the more uniform administration of ionizing energy with respect to an isolated object being irradiated by high energy electrons, and of increasing the amount of ionizing energy absorbed by such object, and of increasing the effective range of penetration of such high energy electrons; which method comprises creating a beam of high energy electrons and directing the same onto the object to be irradiated, and reflecting back toward such object by elastic nuclear scattering from a material of high atomic number those high energy electrons which in the irradiating operation passed alongside said object without striking it.

6. That method of securing the more uniform administration of ionizing energy with respect to an object being irradiated by high energy electrons, and of increasing the amount of ionizing energy absorbed by such object, and of increasing the etlective range of penetration of such high energy electrons; which method comprises creating a beam of high energy electrons and directing the same onto the object to be irradiated, and reflecting back toward such object by elastic nuclear scattering from a material of high atomic number at least a substantial amount of those high energy electrons which have in the irradiating operation been scattered from its volume.

7. That method of securing the more uniform administration of ionizing energy with respect to an isolated object being irradiated by high energy electrons, and of increasing the amount of ionizing energy absorbed by such object, and or" increasing the eliective range of penetration of such high energy electrons; which method comprises creating a beam of high energy electrons and directing the same onto the object to be irradiated, reflecting back toward such object by elastic nuclear scattering from a material of high atomic number those high energy electrons which in the irradiating operation passed alongside said object without striking it, and reflecting back toward such object by elastic nuclear scattering from a material of high atomic number at least a substantial amount of those high energy electrons which have in the irradiating operation been scattered from its volume.

8. That method of securing the more uniform administration of ionizing energy with respect to an isolated object being irradiated by high energy electrons, and of increasing the amount of ionizing energy absorbed by such object, and of increasing the eflective range of penetration of such high energy electrons; which method comprises creating a beam of high energy electrons and directing the same onto the object to be irradiated, causing to be redirected toward such object those high energy electrons which in the irradiating operation passed alongside said object without striking it, and causing to be redirected toward such object at least a substantial amount of those high energy electrons which have in the irradiating operation been scattered from its volume, by substantially enclosing said object by a reflector of high atomic number material so as to redirect toward such object both those high energy electrons which have, in the irradiating operation, passed alongside it without striking it, and also those electrons which have been, in the irradiating operation, scattered from its volume.

References Cited in the file of this patent UNITED STATES PATENTS 1,625,426 Rentschler Apr. 19, 1927 1,704,173 Chesney Mar. 5, 1929 1,907,507 Coolidge May 9, 1933 2,333,842 Cascio July 9, 1942 2,414,496 Varian Jan. 21, 1947 2,429,217 Brasch Oct. 21, 1947 2,602,751 Robinson July 8, 1952 2,629,831 Atchley Feb. 24, 1953

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