Modular Electron Source For Uniformly Irradiating The Surface Of A Product

Abstract

The invention relates to the irradiation of coatings and similar materials wherein a relatively large surface of complex shape is to be irradiated. The invention employs a modular construction wherein each module comprises a relatively small and independently movable electron accelerator which is complete but for the provision of the accelerating voltage. In general, apparatus is used having a common insulating housing and a common voltage source. In the housing are mounted independently movable electron accelerators. A multiplicity of these accelerators is arranged so as to provide a wide array of generally parallel electron beams which are positioned so as to overlap at the surface of the product to be irradiated. Depending upon the nature of the surface, the electron beams may or may not be actually parallel, the purpose being to insure that each electron beam impinges on the surface to be irradiated normal thereto.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to electron irradiation for achieving chemical change or for surface sterilization of medical, food or other materials.

2. Description of the Prior Art

Electron irradiation of materials for various purposes is well known. In general high-energy electrons as a source of energy are expensive, and improvements in this general field have been directed towards reducing the cost of this form of energy. Because high-energy electrons as a rule deliver most of their energy to the surface layers of the product irradiated, electron irradiation has been particularly useful in the irradiation of surface materials such as coatings. In this field it is possible to use electrons of relatively low energy, since only surface layers need be penetrated, but having selected a low-energy electron source it then becomes important to ensure that the impinging electrons have a trajectory which is substantially normal to the surface. Otherwise, the limited energy of these electrons will prevent them from adequately penetrating the surface layer to be irradiated. Electron accelerators such as the capacitron have been constructed which produced an electron beam having a naturally relatively large cross-sectional area, but such devices had a relatively uncontrolled discharge: its output was pulsed, and the beam energy and current were highly nonuniform and difficult to measure. In general, prior art devices heretofore used to irradiate relatively large areas have been electron accelerators having a scanned electron beam. In addition, various techniques have been proposed for insuring normal incidence on the product surface such as the beam-splitting, beam-inverting, and beam-reflecting inventions disclosed in U.S. Pat. Nos. 2,741,704 to Trump and Van de Graaff, 2,785,313 to Trump, 2,887,583 to Emanuelson, and 2,897,365 to Dewey and Trump.

SUMMARY

The invention comprehends the irradiation of shaped surfaces by utilizing a modular form of construction of the electron source and by building the containing vessel for the modules to conform with the contour of the surfaces to be treated. Each module produces an electron beam, initially essentially cylindrical, but finally fanning out into a cone-type envelope under the combined action of space charge forces and scattering both in the window foil and the gas surrounding the product. The complete electron source consists of an array of modules mounted in a containing vessel filled with oil or other insulating medium. The array is designed so that, at the surface of the product, the beam envelopes from adjacent modules will overlap to give essentially uniform electron current density. The containing vessel holds the necessary connecting devices to convey the high-voltage power to the individual modules. This high-voltage power is either introduced into the containing vessel by means of a cable connection to a separate high-voltage power supply or the high-voltage power supply may be built into the containing vessel itself. Suitable power supplies include those of the type known as transformer-rectifier sets, insulating core transformers, cascade rectifier sets, electrostatic belt generators, electrostatic disc generators, resonant transformers, electrogas dynamic-type generators and others.

The invention also comprehends the idea that individual modules be disposable and that they plug into the containing vessel in a simple manner comparable, in concept, to an electric light bulb.

The electrical rating of each module will depend upon the nature of the product. For dose-rate-sensitive coatings, for example, each module may be rated at voltage up to 250 kilovolts and currents up to one milliampere. Non dose-rate-sensitive products in other fields may require currents to 10 milliamperes per module at voltages higher than 250 kilovolts. The invention is of course not limited to these ratings.

BRIEF DESCRIPTION OF THE DRAWING

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 in vertical central section of an electron irradiating apparatus constructed in accordance with the principles of the invention;

FIG. 2 is a longitudinal central section of one of the modules of the apparatus of FIG. 1;

FIG. 3 is a graph in which voltage across the module shown in FIG. 2 is plotted as a function of current in the gas discharge passing through said module;

FIG. 4 is a view similar to that of FIG. 2 and showing a modification thereof for relatively high-voltage operation;

FIG. 5 is a view similar to that of FIG. 2 but showing a module utilizing thermionic emission rather than gas discharge principles; and

FIG. 6 is a plan view showing an array of modules.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawings, and first to FIG. 1 thereof, a metal tank is fitted with a cover 2, through which project a multiplicity of electron tube modules 3. Each module 3 is connected through a series resistor 4 to a metal plate 5 insulated from the tank 1 by supports 6 which are made of an electrical insulating material with sufficient mechanical strength. Power is connected to plate 5 by resistor 7 which interconnects plate 5 with high-voltage cable 8. Tank 1 is filled with a suitable insulating fluid, either gas or oil, which is also used as a heat-exchange medium to remove heat generated by the resistors 4 and the modules 3. The tank 1 is supported on mounting trunnions 9; and, to facilitate replacement of the modules 3, the complete assembly can be inverted on the mounting trunnions 9, so that a module 3 can be replaced without draining the oil. In this event, it is intended that the cable 8 does not rotate but is connected with the resistor 7 through a rotary joint 10. The cable 8 is supported on the tank 1 by means of a cable clamp 11, which is also free to rotate with respect to the tank 1, oil tight integrity being maintained by O-ring seals (not shown).

Resistor 7 is not essential, but if this is not used then resistors 4 would need to be longer in order to support the full supply voltage in the event that one module 3 should spark over. The main function of resistors 4 is to regulate the current in each module 3 so that each unit operates at about the same current by using the properties of the current/voltage characteristic in which a small change in voltage produces a large change in current.

While the tank 1 shown in FIG. 1 is fitted with a flat cover 2, it is intended that any convenient shape should be used to conform with the shape of the product to be treated.

Two distinct principles of operation of the modules 3 are comprehended within the scope of the invention. In one embodiment of this invention, each module 3 produces an electron stream by means of a self-sustained, low-pressure gas discharge (as used in early x-ray tubes). Referring now to FIG. 2, therein is shown this type of module.

The essential elements are:

i. A vacuum-tight container 12 consisting of a glass or ceramic tube -3, hollow tube cathode 14, hollow tube anode 15, and thin foil window 16. These items are brazed or welded together such that they can withstand a high "bake-out" temperature while undergoing evacuation through evacuation tube 17. Evacuation tube 17 is sealed off after evacuation.

ii. A gas replenisher 18, consisting of a piece of metal (e.g., titanium or zirconium) capable of adsorbing the required gas which can subsequently be released, as needed, when it is heated by a small electrical heater. Current for the heater is introduced by means of the vacuum feed-through 19.

iii. High-voltage connection assembly 20 consisting of: contact assembly 21, electric field control ring 22, and insulated support 23.

iv. Mounting provisions 24 consisting of: flange 25, welded to the anode tube 15, is clamped through seal 26 to the mounting vessel 27 by means of the flange 28 and screws 29. Field control ring 30 is mounted on the inside of the containing vessel 27. The containing vessel shown at 27 at FIG. 2 may comprise, for example, the cover shown at 2 in FIG. 1.

During operation, a voltage, negative with respect to the grounded anode 15, is applied to the cathode 14. Ions which exist in the low pressure gas (10.sup..sup.-4 mm. to 10.sup..sup.-3 mm. Hg) due to external stimuli (e.g., cosmic radiation) are attracted to the electrodes 14, 15. Those striking the cathode 14 produce electrons by secondary emission, which electrons in turn create more gas ions by collision. These further gas ions produce more secondary electrons at the cathode 14 and so the electrons build up in quantity by an avalanche process until there is ultimately a self-sustained discharge whose magnitude is independent of the initiating stimulus. The electrons produced emerge from the tube 12 through the window 16 and are directed at the product (not shown). The magnitude of the self-sustained discharge is a function of applied voltage, gas type, gas pressure and cathode material. Gas type and cathode material would be chosen to suit the required rating of the tube. Criteria would include: low chemical activity, high secondary emission coefficient, low negative ion production, freedom from sputtering. Aluminum and nitrogen make a good combination.

The materials used in the tube assembly, particularly the cathode 14, will tend to absorb the gas in the tube 12 so that the pressure, and consequently the electron current, will decrease with use. An electric current passed through the heater coil in the gas replenisher 18 can be used to restore normal conditions. This current control can be either automatic or manual.

The discharge current will vary by large amounts when there is a small change in the applied voltage, consequently, to minimize current fluctuations and to prevent the discharge from passing into the abnormal and arc conditions, as shown in the graph of FIG. 3, it is necessary to include a resistor in series with the device.

For the higher operation voltages, it may be necessary to include some form of gradient control to avoid locally intensified electric field which may lead to sparkover.

One method of achieving gradient control comprises a conducting film deposited on either the inside or outside of the ceramic tube, as shown for example, at 41 on the inside of the ceramic tube 13 of FIG. 2. This can be manufactured by processes common in the resistor industry. The conducting film may or may not be cut in spiral form. By intentionally varying the thickness of the film or the pitch of the spiral, it is possible to produce varying electric field configurations within the anode-cathode space thus affecting the electron-focusing properties of the interelectrode region. For gradient control at even higher operating voltages, one or more intermediate electrodes can be inserted as shown in FIG. 4. The ceramic tube 13' therein shown is divided into two sections, 31 and 32, butt brazed to an intermediate electrode 33. Field control ring 34 is mounted external to the tube 13. The potential of the intermediate electrode 33 is defined either by a conducting coating on the ceramic tube, as previously described, or by an external resistor network (not shown). Focusing of the electron beam can be controlled by varying the voltage on the intermediate electrode 33. The desirable focus condition is that which results in the electron beam being slightly less in diameter than the window 16, resulting in the lowest electron current density and consequent lowest dose rate. The electrode geometry can be varied to achieve this condition. In the event that, in the gas discharge tube, the phenomenon known as "gas focusing" makes the beam too small in cross section, then a simple permanent magnet, axial field, lens could be mounted over the anode tube. Window-cooling considerations also dictate a diffuse beam. The window would preferably be conduction cooled in which case suitable materials would be aluminum or a composite foil as described and claimed in copending patent application assigned to the assignee of the present invention.

As hereinabove noted, two distinct principles of operation of the modules 3 are comprehended within the scope of the invention. One such principle, that of the self-sustained, low-pressure gas discharge, has already been described in connection with the embodiment shown in FIG. 2. Another such principle, that of the thermionic cathode, is shown in FIG. 5. Referring thereto, in this type of module, the electron source would be a heated cathode or filament 35 which would replace the secondary emission cathode 14 of the gas discharge type shown in FIG. 2. Generally the method of construction would be as for the gas type shown in FIG. 2, the cathode assembly being similar to that used in cathode-ray tubes. No gas reservoir 18 (FIG. 2) would be needed, but as it is desired that the tube should operate with a high degree of vacuum, a suitable getter material 36 would be used.

CUrrent control in this type of module would be either by variation of the cathode temperature or by the application of a bias voltage to a control electrode. These techniques are well known by those skilled in the art.

In the event that the electron tube modules are of the thermionic emitter type, then the series resistors (as shown at 4 in FIG. 1) would not be used and provision must be made to supply the heater power to the cathode 35. In order to isolate each thermionic type module from intermittent sparking, which may occur during initial conditioning in adjacent modules, resistors 4 are replaced by a bifilar inductance coil 37 designed to withstand the very rapid rise time surges which result from vacuum kicks while simultaneously allowing heater power to be transmitted to the cathode 35.

FIG. 1 shows a single row of electron sources. To cover large areas in order to keep the dose rate low, it is intended that several rows should be used with mounting centers of each module staggered from one row to the next to give more uniform coverage of the product. Referring now to FIG. 6, therein is shown a plan view of a four-row assembly. Certain chemical systems appear to make most efficient use of the radiation if it is applied in pulses with a "dark" period between pulses. This can be arranged by row 1 from row 2 by a distance calculated to give the desired "dark" period at the requisite product conveyor speed.

In a multiple-gun installation of the type described, it is necessary to periodically check that each individual module is operating correctly. This can be done by passing two pieces of a dose-sensitive material (e.g., blue cellophane) under the assembly. The first piece would be passed under in the same direction as the conveyor movement and the second piece would be passed under in a direction at right angles. By reading the relative dose along the length of each piece of blue cellophane, x and y coordinates of areas of low dose (and hence faulty modules) can be measured and the faulty modules replaced. Criteria can be set for permissible dose variations for a given product so that, if there are many modules in the assembly, replacement of faulty units is not necessary until the measured variation is greater than is allowable.

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.

Claims

I claim:

1. A complete electron source for irradiating a product comprising a tank containing an insulating fluid, a multiplicity of electron-accelerator modules mounted within said tank, each having an electron permeable portion projecting through said tank, each module being adapted to produce an electron beam, initially essentially cylindrical, but finally fanning out into a cone-type envelope under the combined action of space charge forces and scattering both in the window foil and the gas surrounding the product, and an integral power supply adapted to furnish power for all the modules, said modules being mounted so that, at the surface of the product, the beam envelopes from adjacent modules will overlap to give essentially uniform electron density, transverse to the principal direction of any product movement, over the surface of the product to be irradiated.

2. Apparatus in accordance with claim 1, wherein each module comprises a pencil-beam electron source fitted with a thin foil window operating at a reduced gas pressure with a secondary emission cathode.

3. Apparatus according to claim 1, wherein each module comprises a high-vacuum tube having a thermionic cathode as the electron source.

4. Apparatus according to claim 1, wherein each module includes a tubular insulator having gradient control down the surface thereof by means of a deposited uniform coating of resistive material.

5. Apparatus according to claim 1, wherein each module includes an insulating tube having gradient control down the surface of the insulator by means of a deposited coating of resistive material, said coating being purposely made nonuniform to control the focusing of the electron beam.

6. Apparatus according to claim 1, wherein each module includes an insulating tube having one or more intermediate electrodes.

7. Apparatus according to claim 1, wherein the modules are mounted so as to conform to the contour of the product.

8. Apparatus according to claim 1, wherein the modules are arranged to give a nonuniform electron density either graduated or noncontinuous in the direction of product movement,

9. Apparatus according to claim 1, wherein the assembly is mounted on trunnions so that the individual modules may be changed by inverting the assembly to avoid spilling the insulating oil.

10. Apparatus according to claim 1, wherein said modules are mounted on an articulated stand so that the assembly can be moved in synchronism with the product to give uniform treatment of irregularly shaped objects.

11. Apparatus according to claim 1, wherein there is one power supply common to all modules mounted in the containing tank.

12. Apparatus according to claim 1, wherein there is one external power supply cable connected to the containing tank, the cable connection being capable of rotation to permit inversion of the tank.

13. Apparatus according to claim 1, wherein there is one external power supply cable connected to the containing tank.