U.S. patent number RE30,082 [Application Number 05/645,835] was granted by the patent office on 1979-08-21 for x-ray tube having focusing cup with non-emitting coating.
This patent grant is currently assigned to Picker Corporation. Invention is credited to Zed J. Atlee, Roy F. Kasten, Jr..
United States Patent |
RE30,082 |
Atlee , et al. |
August 21, 1979 |
X-ray tube having focusing cup with non-emitting coating
Abstract
An x-ray tube is described including a focusing cup electrode
coated with a high work function material, such as platinum or
gold, to prevent the field emission of electrons from such cup. The
method of applying the non-emitting coating is preferably
sputtering or ion plating, but may also be electroplating followed
by vacuum fusion in the case of gold or other low melting point
metals.
Inventors: |
Atlee; Zed J. (Tigard, OR),
Kasten, Jr.; Roy F. (Elmhurst, IL) |
Assignee: |
Picker Corporation (Cleveland,
OH)
|
Family
ID: |
26962773 |
Appl.
No.: |
05/645,835 |
Filed: |
January 27, 1977 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
284735 |
Aug 30, 1972 |
03783323 |
Jan 1, 1974 |
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Current U.S.
Class: |
378/138;
378/125 |
Current CPC
Class: |
H05G
1/12 (20130101); H01J 35/066 (20190501); H01J
35/14 (20130101); H01J 35/064 (20190501) |
Current International
Class: |
H01J
35/14 (20060101); H01J 35/00 (20060101); H01J
35/06 (20060101); H05G 1/00 (20060101); H05G
1/12 (20060101); H01J 035/00 () |
Field of
Search: |
;313/57 ;204/37R,192
;29/25.14,25.17 ;117/230 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Rolinec; Rudolph V.
Assistant Examiner: Hostetter; Darwin R.
Attorney, Agent or Firm: Watts, Hoffmann, Fisher &
Heinke
Claims
We claim:
1. An x-ray tube in which the improvement comprises:
an anode;
a focusing electrode .Iadd.including a body having a focusing
recess therein which defines a focusing surface.Iaddend.;
a thermionic cathode mounted .Iadd.in said recess .Iaddend.adjacent
.Iadd.said focusing surface of .Iaddend.said focusing electrode so
that said focusing .[.electrode.]. .Iadd.surface .Iaddend.focuses
the electrons emitted by said cathode onto said anode to cause
x-rays to be emitted from said anode;
an evacuated envelope containing said cathode, anode and focusing
electrode; and
a layer of non-electron emissive material provided on at least a
portion of .Iadd.said focusing surface .Iaddend.of the focusing
electrode and having a higher work function than the underlying
base material of said focusing electrode .[.substantially.]. to
prevent the .Iadd.field .Iaddend.emission of electrons from said
focusing electrode.
2. A tube in accordance with claim 1 in which the non-emissive
material is taken from the group consisting of gold and
platinum.
3. A tube in accordance with claim 2 in which the cathode is a
coiled filament.
4. A tube in accordance with claim 3 in which the filament contains
thoriated tungsten.
5. A tube in accordance with claim 1 in which the layer of
non-emissive material is a sputtered layer.
6. A tube in accordance with claim 5 in which the sputtered layer
contains platinum.
7. A tube in accordance with claim 1 in which the .[.forming.].
.Iadd.focusing .Iaddend.electrode includes a focusing cup within
which a filament cathode is mounted, and the layer of non-emissive
material is provided on the surface of said focusing cup. .Iadd.8.
The x-ray tube of claim 1 wherein said focusing surface is a curved
surface. .Iaddend. .Iadd.9. An x-ray tube comprising:
an evacuated envelope;
an anode mounted in the envelope and including an x-ray generating
target area;
a cathode mounted in the envelope in spaced relationship with the
anode and adapted to emit electrons toward the anode;
an electron focusing electrode mounted in the envelope and
including a focusing surface shaped and positioned to focus
electrons emitted by the cathode onto said target area;
conductor means connected to the anode, the cathode, and the
focusing electrode for connecting each of them to sources of
electric energy whereby to cause the focused flow of electrons from
the cathode to said target area; and,
said focusing electrode being comprised of a base material and a
coating of a nonelectron emissive inhibiting material having a
higher work function than the base material, said coating forming
at least a portion of said focusing surface whereby to prevent
electron emission by the focusing
electrode. .Iaddend. .Iadd.10. The X-ray tube of claim 9 wherein
the nonelectron emissive material is sufficiently smooth that
arcing between said cathode and said focusing surface is minimized.
.Iaddend. .Iadd.11. The X-ray tube of claim 10 wherein said
nonelectron emissive material is taken from the group consisting of
gold and platinum. .Iaddend.
Description
BACKGROUND OF THE INVENTION
The subject matter of the present invention relates within a to
x-ray tubes employing heated filament cathodes contained within
focusing cup electrode, and in particular to such x-ray tubes in
which a layer of high work function material is coated on the
focusing cup to prevent the field emission of electrons
therefrom.
Thermionic x-ray tubes having rotating anodes are operated at
extremely high voltages, typically on the order of 100 kilovolts,
so that there is a tendency for the focusing cup to emit electrons
by field emission to the anode or to the cathode filament when the
cup is negatively biased relative to such filament. This is a
particular problem in condenser discharge x-ray systems and other
systems employing coaxial cables or transformers of high secondary
capacitance because the high voltage is stored in the capacitance
across the x-ray tube so that any field emission from the focusing
cup causes a high current discharge. The field emission of
electrons from the focusing cup to the filament cathode can destroy
such cathode, particularly if it is a thoriated tungsten filament
cathode which is easily damaged by evaporation of thorium from the
filament or contamination of the filament by the deposit of
evaporated material from the focusing cup. In addition, during
manufacture the unactivated filament cathode may be damaged by
field emission from the focusing cup during "seasoning" as
described in copending U.S. patent application Ser. No. 228,951,
filed Feb. 24, 1972, by Z. J. Atlee et al. .Iadd., now U.S. Pat.
No. 3,846,006.Iaddend..
These problems are avoided in the x-ray tube of the present
invention by employing a non-emitting coating of high work function
materials, such as platinum or gold, on the surface of the focusing
cup electrode including the inner surface portions within the cup
which are immediately adjacent to the cathode filament. In the
preferred embodiment, a platinum coating is employed because of its
higher work function and higher permissible operation temperature.
The platinum is applied to the focusing cup by sputtering or ion
plating which avoids melting the underlying focusing cup metal
which would happen if a fusion coating method were employed due to
the high melting point of platinum. Previously, it has been
suggested .[.tht.]. .Iadd.that .Iaddend.the anodes of high voltage
rectifier tubes can be coated with a thin layer of gold over a
thicker intermediate nickel layer provided on such anodes by
electroplating and subsequent heating below 780.degree. Centigrade,
as discussed in U.S. Pat. No. 3,611,523 of E. S. Den Dulk, patented
Oct. 12, 1971. However, heating above this temperature causes a low
melting temperature alloy of gold and nickel to form which no
longer has a high work function. However, this is impractical for
coating the focusing cup of an x-ray tube because frequently such
focusing cup is processed and operated at higher temperatures.
It is, therefore, one object of the present invention to provide an
improved x-ray tube of longer useful lifetime in which the focusing
cup electrode is coated with a non-emissive layer of high work
function material.
Another object of the invention is to provide such an x-ray tube in
which the coating of non-emissive material is applied to the
focusing cup by a method which maintains the high work function of
the material, results in good adherence and provides a smooth
surface on such focusing cup.
Still another object of the present invention is to provide such a
method in which the low emissive material is applied to the
focusing cup base material by sputtering.
A further object of the invention is to provide such a method using
ion plating.
A still further object of the invention is to provide such a method
in which a low emissive material is applied to the focusing cup by
electroplating followed by fusion.
An additional object of the present invention is to provide the
focusing cup electrode of an x-ray tube with such non-emissive
coating of a high melting point material, such as platinum, without
melting the underlying base material.
BRIEF DESCRIPTION OF THE DRAWINGS
Other objects and advantages of the present invention will be
apparent from the following detailed description of preferred
embodiments thereof and from the attached drawings of which:
FIG. 1 is a plan view of an x-ray tube having a focusing electrode
made in accordance with the present invention, with parts broken
away for clarity;
FIG. 2 is a partial horizontal section view taken along the line
2--2 of FIG. 1 on an enlarged scale; and
FIG. 3 is a schematic diagram of the electrical circuit of a
capacitor discharge x-ray apparatus employing the tube of the
present invention.
DESCRIPTION OF PREFERRED EMBODIMENTS
As shown in FIG. 1, one embodiment of an x-ray tube made in
accordance with the present invention includes an evacuated
envelope 10 of glass containing a rotary anode 12 and a thermionic
cathode assembly 14 supported at the opposite ends of such envelope
in a conventional manner. Thus, the rotary anode 12 is attached by
a rod 16 to a bearing sleeve 18 of magnetic material which is
rotationally mounted on an inner shaft 20 for rotation by field
coils (not shown) external to the envelope. The bearing sleeve 18
and anode 12 are supported on anode support shaft 20 which extends
through a glass-to-metal seal in the left end of the envelope 10
for applying positive voltage to such anode.
The cathode assembly 14 includes a filament cathode 22 in the form
of a coil of tungsten, thoriated tungsten, or other suitable
electron emissive material. The filament cathode 22 is supported
within a notch 24 in the focusing cup electrode 26. The focusing
cup electrode 26 is provided with a cup-shaped focusing aperture 28
.Iadd.having shaped focusing surfaces including a curved inner
surface adjacent the cathode .Iaddend.flaring outwardly from the
notch 24. As a result, the electrons emitted by the filament 22 are
focused by .Iadd.the shaped focusing surfaces of the focusing
.Iaddend.cup 28 onto a target surface 30 of the rotary anode 12 to
cause x-rays to be emitted therefrom and transmitted through the
side of the envelope 10.
The focusing cup electrode 26 of the present invention includes a
base member 32 of steel, nickel, molybdenum, or other suitable
refractory material including ceramic, such as alumina, having a
non-emissive coating 34 of high work function material, such as
platinum or gold. The non-emissive coating 34 is provided on the
upper surface of the focusing cup electrode 26, including the inner
.Iadd.shaped focusing .Iaddend.surface of the focusing cup aperture
28 and notch 24. The purpose of this non-emissive coating 34 is to
prevent the field emission of electrons from the focusing cup
electrode to either the cathode filament 22 or the anode 12, which
results in damage to these elements. Thus, the cathode filament 22
includes end leads 36 which may be insulated from the focusing cup
electrode 26 to enable a negative bias voltage of about -4
kilovolts to be applied between such filament and such focusing
electrode so that the x-ray tube is quiescently biased
nonconducting.
The x-ray tube of FIGS. 1 and 2 may be connected in a capacitive
discharge x-ray circuit like that shown in FIG. 3, or such tube may
be connected to coaxial cables or to a transformer of high
secondary winding capacitance, so that a high capacitance on the
order of one microfarad is connected across such tube. This
capacitance acts as a voltage storage element which tends to cause
an undesirable field emission or "cold cathode" type of an electron
discharge from the focusing cup electrode 26 to the anode 12 or to
the filament cathode 22. Thus, the anode 12 is connected through a
first capacitance 38 to ground, while the cathode 22 is connected
through a second capacitance 40 to ground. Capacitances 38 and 40
are charged slowly to about +60 kilovolts and -60 kilovolts,
respectively, through charging resistances 42 and 44, respectively,
and apply a total of 120 kilovolts D.C. across the anode and
cathode of the x-ray tube while the focusing cup is quiescently
biased at -64 kilovolts D.C. or -4 kilovolts relative to the
cathode to cut off such tube. These capacitances are discharged
rapidly through the x-ray tube 10 to produce an x-ray pulse when an
exposure pulse 46 of about +4 kilovolts, relative to the quiescent
voltage of the focusing cup, is applied by an exposure pulser
circuit 48 to the focusing cup 26 to remove the quiescent reverse
bias of -4 kilovolts applied to such focusing cup. Thus, the x-ray
tube is rendered conducting for a period of time determined by the
duration of the exposure pulse 46.
The storage capacitors 38 and 40 are connected through a rectifier
bridge formed by four diodes 50, 52 54 and 56, across the secondary
windings 58 and 60 of a high voltage transformer 62. The
transformer has its primary windings 64 connected across the usual
source of A. C. line voltage 66. The transformer may also be
provided with a low voltage filament heater secondary winding 68
connected across the end terminals of the cathode filament 22, as
well as the filaments of the rectifier diodes. It should be noted
that a spark gap type of discharge switch may be connected in
series between the x-ray tube anode 12 and the capacitor 38 when
the focusing cup is connected to the cathode potential so that
exposure pulser circuit 48 is not employed to switch the x-ray tube
into a conducting condition. Such a spark gap discharge switch can
be triggered or can be of the self-fire type which automatically
breaks down after the voltage on the capacitor reaches the desired
value.
The following is a description of three different methods for
applying the non-emissive coating 34 to the focusing cup electrode.
In the preferred embodiment of the invention, a non-emissive
coating 34 of platinum or gold is deposited upon a focusing cup
electrode base member 32 of steel by sputtering. Any suitable
sputtering method can be employed, such as the triode sputtering
method described in the article "Low-energy Sputtering" by J. W.
Nickerson and R. Moseson, in Research/Development, March 1965,
pages 52 to 56. Before sputtering, the focusing cup is machined to
the desired shape, cleaned by electrolytic polishing and then
dipped in a solution of trichloroethylene and placed in an
ultrasonic cleaner containing liquid dichlorodifluoromethane, sold
under the trademark Freon. Next, the cleaned focusing cup members
are inserted into the sputtering apparatus which is evacuated to
less than 3.times.10.sup.-6 torr, and are sputter etched for about
ten minutes at 300 watts of radio frequency power in an inert gas
atmosphere of argon back filled to 7 microns pressure. In sputter
etching the ions of inert gas are caused to bombard the surface of
the focusing cup directly to remove any oxide or other foreign
material and to thoroughly clean the surface for better adherence
to the platinum. Next, the etched focusing cup members are indexed
over a target of platinum or gold and spaced about one-half inch
therefrom without removing them from the vacuum chamber. The target
is then sputter deposited onto the surface of the focusing cup for
about 5 minutes at about 1 kilowatt of radio frequency power in an
argon atmosphere of 6.5 to 7.0 microns pressure to form the
non-emissive coating 34.
In triode sputtering, electrons are emitted from a cathode and
transmitted through argon or other inert gas to a separate anode,
thereby ionizing the inert gas. The positive ions of inert gas are
attracted to a target of the material to be sputter deposited which
is at a more positive potential of about +50 volts so that the ions
strike the target with sufficient energy to cause platinum or gold
atoms to be sputtered from the target upon impact of such ions.
These sputtered atoms of platinum or gold are transmitted in
straight line paths to the focusing electrode substrate member to
form the sputtered coating 34. It should be noted that this sputter
deposition technique has the advantage that the platinum is not
heated above its melting point in order to cause the coating 34 to
adhere to the base material 32 which would cause melting of the
base material due to the high melting point of platinum. However,
once the sputtered platinum layer 34 is deposited, it may, if
desired, be heated in a vacuum at about 1,000.degree. Centrigrade
for about 60 seconds to enhance the surface condition of the layer
and improve its fusion with the base material 32. The filament
cathode 22 is then assembled in the coated focusing cup electrode
26, and this assembly is mounted within the x-ray tube for further
processing including evacuation and degassing at any suitable
temperature.
Another method of depositing the non-emissive coating 34 is by ion
plating, such as by the methods described in the article "Film
Deposition Using Accelerated Ions" by D. M. Mattox in
Electrochemical Technology, Sept.-Oct., 1964, pages 295 to 298, and
in the article "Gas-Scattering and Ion-Plating Deposition Methods"
by Curt D. Kennedy et al. in Research/Development, November, 1971,
pages 40 to 44. In ion plating, the platinum, gold or other high
work function material to be deposited is first vaporized by
heating and the metal vapor is ionized. Heating and ionization may
both be accomplished by electron beam bombardment of about 5
kilovolts of a target of the material to be deposited. The metal
vapor ions are then accelerated through a high potential gradient
of approximately 5 kilovolts to bombard the focusing cup electrode
member being coated. The positive ions of platinum or gold are
imbedded into the surface of the focusing electrode base member 32
to form the non-emissive layer 34. In order to increase the number
of metal vapor ions generated, the secondary electrons emitted by
the target may be utilized by placing the target in a magnetic
field so that the secondary electrons travel in spiral paths
through the metal vapor. In addition, a plasma or glow discharge
can be produced between the substrate and the ion source by
supplying a small amount of inert gas and increasing the voltage
gradient. The ions of inert gas, as well as the ions of metal
vapor, plasma-etch the surface of the substrate so that the
non-emissive coating 34 penetrates and adheres better to the base
material 32 of the focusing cup substrate. After ion plating, the
coated substrate can, if desired, also be heated in a vacuum at
about 1,000.degree. Centrigrade for about 30 seconds to enhance the
surface condition of the layer and improve its fusion with the base
material as with sputtering. In addition, when gold is the sputter
or ion plated material, the surface of the gold seems to be made
smoother and more continuous, apparently due to surface
melting.
A third method of applying gold as the non-emissive coating 34
involves electroplating and subsequent fusion in a vacuum. Prior to
electroplating the focusing cup electrode member 32 is machined,
cleaned, and electrolytically polished. Then a thin intermediate
nickel layer about 0.0001 inch thick may be "flashed" onto the
steel to provide better adherence of the gold non-emissive layer
34. However, this is optional. Next the focusing electrode member
32 is electroplated with gold to a thickness on the order of 0.001
inch. After electroplating, the gold layer 34 is then fused to the
focusing electrode member 32 by heating it above its melting point
to approximately 1,070.degree. Centrigrade for about 30 seconds in
a vacuum by radio frequency heating or other suitable heating
techniques. This vacuum fusion also smooths the surface of the gold
coating to provide a smooth continuous layer which further reduces
the possibility of the field emission of electrons. After this, the
filament cathode 22 is assembled in the coated focusing cup
electrode and such assembly is mounted in the x-ray tube, which is
then evacuated and baked for outgassing purposes.
It will be obvious to those having ordinary skill in the art that
many changes may be made in the above-described preferred
embodiments of the invention without departing from the spirit of
the invention. For example, other high work function materials than
platinum and gold can be employed as the non-emissive coating 34.
Therefore, the scope of the present invention should only be
determined by the following claims.
* * * * *