U.S. patent number 3,617,740 [Application Number 04/765,846] was granted by the patent office on 1971-11-02 for modular electron source for uniformly irradiating the surface of a product.
This patent grant is currently assigned to High Voltage Engineering Corp.. Invention is credited to Brian Skillicorn.
United States Patent |
3,617,740 |
Skillicorn |
November 2, 1971 |
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.
Inventors: |
Skillicorn; Brian (Topsfield,
MA) |
Assignee: |
High Voltage Engineering Corp.
(Burlington, MA)
|
Family
ID: |
25074659 |
Appl.
No.: |
04/765,846 |
Filed: |
October 8, 1968 |
Current U.S.
Class: |
250/492.3;
250/493.1; 976/DIG.444; 250/492.1 |
Current CPC
Class: |
H01J
33/00 (20130101); G21K 5/10 (20130101) |
Current International
Class: |
G21K
5/10 (20060101); H01J 33/00 (20060101); G01n
021/00 () |
Field of
Search: |
;250/49.5 (O)/ ;250/49.5
(7)/ ;250/207 ;313/74,68,69 ;204/157.1 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Lawrence; James W.
Assistant Examiner: Birch; A. L.
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.
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.
* * * * *