U.S. patent number 3,795,832 [Application Number 05/230,053] was granted by the patent office on 1974-03-05 for target for x-ray tubes.
This patent grant is currently assigned to The Machlett Laboratories, Incorporated. Invention is credited to William P. Holland.
United States Patent |
3,795,832 |
Holland |
March 5, 1974 |
TARGET FOR X-RAY TUBES
Abstract
A target for x-ray tubes, which is comprised of separate
elements mechanically connected together whereby the element which
includes the x-ray generating focal area is of a selected material
mechanically attached to at least one element of a second material
by means which efficiently permits ready transfer of heat from the
first element to the second while permitting free thermal expansion
of the elements relative to one another.
Inventors: |
Holland; William P. (West
Redding, CT) |
Assignee: |
The Machlett Laboratories,
Incorporated (Springdale, CT)
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Family
ID: |
22863768 |
Appl.
No.: |
05/230,053 |
Filed: |
February 28, 1972 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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42375 |
Jun 1, 1970 |
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Current U.S.
Class: |
378/127;
378/144 |
Current CPC
Class: |
H01J
35/10 (20130101) |
Current International
Class: |
H01J
35/10 (20060101); H01J 35/00 (20060101); H01j
035/10 () |
Field of
Search: |
;313/55,60,330 |
Foreign Patent Documents
Primary Examiner: Saalbach; Herman Karl
Assistant Examiner: Hostetter; Darwin R.
Attorney, Agent or Firm: Murphy; Harold A. Pannone; Joseph
D. Meaney; John T.
Parent Case Text
This application is a continuation of Ser. No. 42,375 filed June 1,
1970, now abandoned.
Claims
1. An anode for X-ray tubes having a cathode which operates to
produce an electron beam of known cross-sectional dimensions, said
anode including a target assembly comprising a target member of
material having a known thermal storage capacity per unit weight
and capable of X-ray emission when impinged by said electron beam,
a pair of supporting members one on each side of the target member,
and means for engaging said supporting members to sandwich the
target member therebetween, said supporting members overlying the
target member in a manner whereby only a selected surface area of
the target member is exposed and both having substantially higher
thermal storage capacity per unit weight than the material of said
target member, said surface area being of a size in one direction
which corresponds to one cross-sectional dimension of said electron
beam impinging thereon, and said supporting members comprising
means for
2. An anode for X-ray tubes as set forth in claim 1 wherein said
supporting members are disclike in shape, said target member is
annular in shape, and
3. An X-ray tube comprising a hermetically sealed envelope, a
cathode electrode and an anode electrode located in spaced relation
within the envelope, and means for connecting said electrodes to
external sources of electrical energy, said cathode electrode
comprising means for producing an electron beam of known
cross-sectional dimensions, said anode electrode including a target
assembly comprising a target member of material having a known
thermal storage capacity per unit weight and capable of emission of
X-rays and secondary electrons when impinged by said electron beam,
a pair of supporting members one on each side of the target member,
and means for engaging said supporting members to sandwich the
target member therebetween, said supporting members overlying the
target member in a manner whereby only a selected surface area of
the target member is exposed and both having substantially higher
thermal storage capacity per unit weight than the material of said
target member, said supporting members further being of a material
incapable of substantial production of X-rays when impinged by
secondary electrons from said target, said exposed surface area
being of a size in one direction which corresponds to one
4. An X-ray tube as set forth in claim 3 wherein said supporting
members are disclike in shape, said target member is annular in
shape, and said
5. A rotating anode X-ray tube comprising a hermetically sealed
envelope, a cathode electrode and an anode electrode located in
spaced relation within the envelope, and means for connecting said
electrodes to external sources of electrical energy, said cathode
electrode comprising means for producing an electron beam of known
cross-sectional dimensions, said anode electrode comprising a
rotatable shaft and a target assembly mounted on the shaft for
rotation therewith, said target assembly comprising a target member
of material having a known thermal storage capacity per unit weight
and capable of emission of X-rays and secondary electrons when
impinged by said electron beam, a pair of supporting members one on
each side of the target member, both of which members have a
thermal storage capacity per unit weight which is substantially
higher than the target member, and means for engaging said
supporting members to sandwich the target member therebetween, said
supporting members overlying the target member in a manner whereby
only a selected surface area of the target member is exposed and
having substantially higher thermal storage capacity per unit
weight than the material of said target member, said supporting
members further being of a material incapable of substantial
production of X-rays when impinged by secondary electrons from said
target, said surface area being of a size in one direction which
corresponds to one cross-sectional
6. A target assembly for X-ray tube anodes comprising a target
member and a pair of supporting members located one on each side of
the target member, said supporting members overlying the target
member except for an elongated focal track thereon, said focal
track being of a size in the lateral direction which corresponds to
one dimension of a desired X-ray generating focal spot to be
produced thereon, and said supporting members being of material
incapable of production of substantial amounts of X-radiation when
impinged by secondary electrons from said focal track.
7. An X-ray tube comprising a hermetically sealed envelope, a
cathode electrode and an anode electrode located in spaced relation
within the envelope, and means for connecting said electrodes to
external sources of electrical energy, said anode comprising a
support, and an X-ray generating target assembly mounted on the
support, said target assembly comprising a target member having a
focal track on the side thereof facing the cathode electrode of a
material capable of emission of X-rays and secondary electrons when
impinged by electrons from said cathode electrode, and a pair of
supporting members one on each side of the target member, said
supporting members overlying the surface of the target member
facing said cathode electrode with only said focal track being
exposed, said exposed focal track being of a size in the lateral
direction which corresponds to one dimension of a desired X-ray
generating focal spot to be produced thereon, and said supporting
members being of a material incapable of substantial production of
X-radiation when impinged by
8. An X-ray tube as set forth in claim 7 wherein said supporting
members are disc-like in shape, said target member is annular in
shape, and said focal track is an annular surface portion of the
target member.
Description
BACKGROUND OF THE INVENTION
In the manufacture of targets for x-ray tubes, the portion of the
target which is to be subjected to electron bombardment for the
resultant production of x-rays is preferably made of a high atomic
number material except in cases where characteristic radiation is
required. However, it has been found that many problems exist when
making the entire target of high atomic number material, due at
least in part to the fact that during operation of the device the
target will become seriously damaged through high thermal gradients
causing severe mechanical stresses which result from bombardment by
high energy electrons. This produces cracking, warping, and focal
track disruption. For example, the temperature assumed by a
conventional tungsten target at the focal spot may approach
3,400.degree.C and such heat may create hoop stresses which produce
radial cracking resulting in mechanical failure, or warpage which
alters the target angle and thereby changes the focal spot
size.
Attempts to overcome these and other problems have been made by
forming a target of a selected refractory base material having high
termal capacity, such as molybdenum or graphite, for example. On
this base material is deposted a layer of high atomic number
material which has high melting point and low vapor pressure. This
layer, which may be vapor deposited, flame sprayed, or brazed, may
cover one side of the base or may cover the entire base surface.
Such materials as rhenium, tungsten, or suitable alloys are
deposited in the selected area or areas and are attached by a
metallurgical bond to the base material.
These coated targets, however, have also been found to be
unsatisfactory because of the extreme difficulty in obtaining good
adhesion of the deposit to the base material. Differences in
thermal expansion coefficients have caused much of the failure in
devices of this character.
SUMMARY OF THE INVENTION
The above and other objections to the prior art are overcome in the
present invention by a novel target structure which includes a
first element of high atomic number mechanically connected in
intimate relation to a base element, wherein the first element
comprises the focal area which is to be impacted by electrons
during subsequent operation of the device, and the base element
comprises a suitable material which will rigidly retain the first
element in its prelocated position but will also act as an
efficient heat sink to carry heat away from the first element and
to radiate such thermal energy away from the target.
The first element, in accordance with this invention may be any
suitable refractory metal, such as tungsten or tungsten-rhenium
alloy, for example, which has a high enough melting point and which
generates x-rays in copious supply when bombarded by electrons in
the known manner. The base element may be any suitable material
which will support the first element and act as a heat sink to
carry the heat away from the first element and dissipate it through
radiation. The base element, preferably, should be a relatively low
density material such as molybdenum, titanium or graphite, for
example, which will meet vacuum tube processing requirements.
In further accordance with this invention, a rotating anode may be
made which comprises a ring of the selected material for the
electron impinging area or focal track, and inner and outer rings
or discs of the base material which mechanically sandwich the focal
area member between them. The materials of the focal area member
and the base material do not need to possess similar expansion
coefficients since one may expand and contract relative to the
other without destroying the efficient heat transfer relationship
achieved by mechanically joining the parts together. It has been
found that heat flow across the interface between the focal track
area and the backing is surprisingly efficient with a mechanical
interconnection, while a metallurgical bond is unsatisfactory, as
pointed out above.
In one form of the invention, a focal area member of tungsten or
tungsten-rhenium alloy is mounted in intimate physical engagement
upon a backing of graphite and held thereupon by bolts. In another
form, the focal area member rests in a cavity in the face of the
backing and is firmly held in place by spring means. In a preferred
form of rotating anode, the focal area member comprises a ring of
the selected material which is sandwiched between two discs or
rings of the selected base material, and is held therein through
pressure exerted by the threaded clamping means which serves to
attach the anode in place on a central rotatable shaft.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and other advantages of the invention will become
apparent from the following description taken in connection with
the accompanying drawings, wherein:
FIG. 1 is an axial section through an x-ray tube of the rotating
anode type showing a target structured in accordance with this
invention;
FIG. 2 is an elevational view of the target in the tube of FIG.
1;
FIG. 3 is an axial section through a target having means for
preventing relative rotary motion between the target members;
FIG. 4 is an axial sectional view of a target illustrating an
alternative means of applying pressure to the parts;
FIG. 5 is an axial sectional view of a target illustrating a
further means for physically connecting together the parts of the
target;
FIG. 6 is an enlarged fragmentary sectional view of a target
illustrating flow of electrons to it from an adjacent cathode;
FIG. 7 is an enlarged elevational view of the target shown in FIG.
5 illustrating a focal spot area thereon;
FIG. 8 is an enlarged fragmentary sectional view of a slightly
modified target embodying the invention; and
FIG. 9 is a side elevational view partly in axial section of an
x-ray tube with a stationary target embodying the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to the drawings, there is shown in FIG. 1 an axial
sectional view of an x-ray tube of the rotating anode type which
embodies a dielectric envelope 10 in which is supported an anode 12
and a cathode 14. The cathode 14 includes a supporting cylinder 16
one end of which is sealed to a reentrant end portion 18 of the
envelope. On the inner end of cylinder 16 is mounted one end of a
transversely extending angled support bracket 20, in the free end
of which is located a cathode head 22. The cathode head 22 contains
an electron-emitted filament (see FIG. 6) to which a suitable
electrical potential is applied through leads 24 extending
externally of the tube through cylinder 16.
The opposite end of the envelope 10 carries the anode 12 which
includes a target 26 mounted on one end of a rotor shaft 28
extending from a rotor 30 rotatably located in a neck portion 32 of
the envelope. The rotor carries a skirt 34 bolted thereto, and the
assembly is adapted to rotate rapidly when the tube is mounted in
suitable inductive means surrounding the neck 32 and when the
inductive means is energized.
In accordance with this invention, the anode target assembly
comprises a focal track member 36 in the form of a ring made of
suitable high atomic number material, such refractory materials as
tungsten or tungsten-rhenium being particularly suitable. The focal
track member 36 produces x-rays when bombarded by electrons from
the cathode 22 in the usual manner of x-ray generators. The exposed
surface or track of the focal target member 36 is inclined so that
x-rays will pass from the surface out of the tube through the side
wall of the envelope.
The focal target of conventional x-ray tubes usually comprises the
entire target 26 or is a metallurgically deposited coating upon a
suitable backing of high thermal capacity material. For example,
the entire target 26 may be made of tungsten, or a target backing
of tungsten, graphite, molybdenum or the like may carry on its
surface a focal target of a deposited or metallurgically bonded
material such as tungsten or tungsten-rhenium alloy.
It has been found that solid targets of tungsten do not have
satisfactory thermal characteristics and, when bombarded by high
density electrons, become damaged by the resulting severe
mechanical stresses. It has also been found that a coating of
target material upon the surface of a backing will not prove
satisfactory since the metallurgical bond between the target and
backing will not withstand the stresses resulting from the thermal
shock of the impinging electron beam. Furthermore, it is difficult
to obtain a thermal expansion match with suitable backing materials
over a full operating temperature range which may extend from room
temperature to approximately 3,000.degree.C. Also, even if a
satisfactory metallurgical bond could be achieved, when graphite is
used as the backing the weak bond between atom layers will not
withstand the mechanical stresses involved.
In accordance with this invention, the focal target member 36 is
made as a completely separate element or part which is physically
located upon a separate and independent backing 38 which comprises
in itself a selected suitable high thermal capacity, high thermal
emissivity material. While the two elements 36 and 38 are entirely
separate, it is essential that they be maintained in physical
contact throughout an extended area so that heat may be efficiently
conducted from the focal target 36 into the backing 38.
As applied to a rotating anode tube as shown in FIG. 1, the focal
target member comprises a ring 36 having its lower surface
positioned upon a surface of a backing disc 38, which surface is
shaped to mate with the adjacent surface of the target ring. The
upper surface of the backing disc or ring 38 is recessed to receive
the target disc 36 as shown. However, as will be noted, the outer
periphery of the recess is of a slightly larger diameter than the
outer periphery of the target disc 36 so that disc 36 may thermally
expand without damaging the backing disc which has a lower
coefficient of expansion. Likewise the upper surface of the target
disc 36 is recessed except in the actual focal track area which is
to be exposed to the cathode. In this way a second backing ring or
dome 42 can be nested within the recess, with clearance being
provided between the outer peripheral edge of the dome and the
outer side wall of the recess in the disc 36 so as to permit
expansion of the disc 36 without damage to the dome. With a nested
structure of this type the target disc 36 engages the adjacent
surfaces of the backing disc 38 and dome 42 throughout relatively
expansive surface areas to achieve efficient conduction of heat
from disc or ring 36 to the disc 38 and the dome 42 as is
desired.
When mounting a target assembly 26 on its supporting anode
structure, it is important to insure that the two rings or discs 36
and 38 are at all times held in the required closely abutting
relationship. Therefore, there must be some means provided for this
purpose. In the example shown in FIG. 1, this is achieved by
providing the rotor shaft 28 with an enlarged portion or collar 40,
and backing ring 38 is mounted over the end of shaft and seated
upon the collar. The lower surface of the backing ring may be
suitably recessed as illustrated to receive the collar. Then the
focal track ring 36 is slid down over the shaft into intimate
engagement with the backing ring 38. As shown in FIG. 1, the second
backing ring or dome 42 is then mounted on the shaft 28 and slid
down into intimate physical contact with the opposite adjacent
surface of ring 36, and the complete assembly is compactly and
firmly pressed into an assembled unit by means such as a nut 44
which is threaded onto the end of the shaft into engagement with
the second backing ring or dome 42, preferably within a recess
provided therefor, as illustrated.
The focal track ring or target ring 36 is thus firmly sandwiched
between the two backing rings so that heat is efficiently
transmitted from the ring 36 into the relatively massive bulks of
the two backing rings.
Although the second backing ring or dome 42 is shown and described,
there may be certain instances where this ring need not be
provided, in which case the nut 44 is made to directly engage and
exert pressure upon the target ring 36. Other desirable reasons for
utilizing the second backing ring will be set forth, however, in a
later part of this description.
From the above it will be apparent that when the tube is operated a
stream or beam of electrons will be emitted by the cathode 22 in
the well-known manner and will impinge upon the adjacent inclined
surface of the target ring 36, whereupon x-radiation will be
emitted by this surface and will pass out of the tube through the
x-ray transparent wall of the envelope 10. During this operation
considerable heat is generated within the target ring 36.
Therefore, to partially aid in the distribution of heat throughout
the ring, as opposed to a localized area thereof, the target
assembly 26 is caused to rotate at a relatively high speed so that
a new surface area is constantly and continuously being presented
to the electron beam, as is well known.
It was found that with a target assembly strucutred as described,
under normal operating conditions the heat dissipation
characteristics are greatly improved, with maximum temperatures of
the three rings 36, 38 and 42 all being within about 750.degree.C
to 1,040.degree.C during the tests performed, while with
conventional target discs as presently made the temperature will
approach about 1,500.degree.C. No damage to the target results at
the achieved lower temperatures because of the improved heat
storage capacity and heat dissipation of the sandwich structure
described. Because of the high emissivity and increased overall
surface area the target assembly cools much more rapidly than known
prior art targets, and these thermal improvements reduce the amount
of heat flow to adjacent rotor bearings, thus also improving
rotational performance and extending the life of the tube.
Referring now to FIG. 3, there is shown a rotating anode target
assembly 26a which is similar to the target assembly shown in FIG.
1. However, in FIG. 3 the target ring 36a is sandwiched between
backing rings 38a and 42a and are interconnected thereto by pins 46
as shown so that slippage between the respective rings is
prevented. Furthermore, pins 46 can be adjusted by control of
weight, size, location, etc. so as to provide means for dynamically
balancing the target assembly. It will be noted that collar 40a is
also similarly interconnected to backing ring 38a by pins 48. This
will insure that the target assembly 26a will rotate with rotor
shaft 28a without slippage.
It has been found that a strong spring pressure will suitably
retain the rings in assembled and intimate physical relation. One
example of such a spring arrangement is shown in FIG. 4 wherein the
target assembly 26b includes backing rings 42b and 38b between
which is sandwiched a target ring 36b. The rotor shaft 28b is
provided with the aforementioned nut 44b which engages the second
backing ring or dome 42b. However, instead of the previously
described collar 40, this embodiment is provided with a spring
device 50 of suitable shape which extends between backing ring 38b
and the adjacent end of the rotor skirt 34b. Thus, the spring
device constantly urges the three rings of the assembly into firm
physical abutting relation so that efficient heat conduction is
provided from target ring 36b into the backing rings.
In FIG. 5 there is shown a still further modification of a rotating
anode target assembly 26c embodying the invention. In this
embodiment, the free end of the rotor shaft 28c is threaded to
receive thereon a cup-shaped retainer 52 having an outwardly
extending peripheral flange portion 54 which overlies and firmly
engages a ledge or shelf 56 provided therefor on the inner wall of
the recess in backing ring or dome 42c. In this embodiment the
collar 40c engages the backing disc 38c while the flange 54 engages
the dome 42c. A nut 44c threaded onto shaft 28c then is moved into
engagement with the base or bottom of the cup 52 as shown.
Tightening of nut 44c will urge the three rings or discs of the
target assembly into firm physical engagement with one another and
cooperates with the flange 54 in retaining the assembled parts in
such relationship.
In addition to the improved efficient heat transfer characteristics
of this invention, an additional feature of importance is achieved
by this invention. Referring to FIGS. 6 and 7, it will be seen that
control of the size of the focal spot in one direction may be
achieved by strict control of the width of the surface of the focal
track which is exposed to the cathode. The focal track of target
ring 36 is exposed throughout an annular surface area as shown and
described, and it is upon this area that electrons in the form of a
beam as indicated at 58 in FIG. 6 are directed from a filament 60
in cathode head 22. The cavity 62 within which the filament 60
resides is designed to perform some control of the size of the
focal spot on the focal track which is impinged by the electron
beam 58. However, such requires critical control and adjustment of
the configuration of the recess 62 and the potentials applied to
the filament 60 and to the head 22. In accordance with this
invention, the width of the exposed focal track 36 is of a size
which is intended to be the size desired of the focal spot length.
Such a focal spot is indicated by the shaded area 64 in FIG. 7.
It is well known in the x-ray tube industry that it is desirable in
most cases to provide a source of x-rays which emanates from a
focal spot as small as possible. A conventional rotating target
with its large surface area of relatively high atomic number is
exposed to scattered secondary and primary high field emission
electrons causing extrafocal radiation. This unwanted off focus
radiation contributes to added patient dosage and degradation of
radiographic image quality. Electron impingement, either primary or
scattered, onto the low density elements 38 or 42 produce low
energy x-rays which are absorbed in the envelope 10. This off focus
radiation is virtually eliminated except for that which occurs
adjacent to the focal spot 64 on the target ring 36. This low
energy radiation, however, is absorbed in the glass envelope of the
tube and in other filtering material which may be placed in the
x-ray beam. As shown in FIGS. 6 and 7, a focal spot as viewed from
the side of the tube desirably will appear as a substantially
square spot as indicated by numeral 66.
A definite x-ray focal spot length, therefore, can be established
by the appropriate selection of the width dimension of the exposed
focal track as shown in FIGS. 6 and 7. This feature precisely
controls the focal spot length dimension even at high tube current
levels where the electron beam tends to enlarge.
While the foregoing description has dealt with a composite target
wherein the middle element of the sandwich is primarily of x-ray
generating material, it is also possible to make the middle
element, such as disc 36 in FIG. 1, of high thermal capacity
material such as molybdenum or a mixture of about 95 percent
molybdenum and about 5 percent tungsten, for example. In this case
the focal track area will be comprised of a relatively thin layer
of efficient x-ray generating material such as a mixture of about
90 percent tungsten and about 10 percent rhenium, for example. This
is illustrated in FIG. 8 wherein disc 36d is sandwiched between
discs 38d and 42d, and the exposed focal track area is provided
with a thin layer or coating 96 of the selected x-ray generating
material which may be deposited by evaporation, flame spraying or
other selected method.
While the foregoing description relates primarily to x-ray tubes
having rotating anodes, the invention is also particularly well
suited for use in stationary anode tubes such as, for example, the
type shown in FIG. 8. The stationary anode tube shown in FIG. 8
includes an envelope 68 within one end portion of which is a
cathode head 70 housing an electron emitting filament 72 which is
intended to direct a beam of electrons toward an anode 74. Anode 74
is a body of copper, usually, which is provided with a hollow
cylindrical extension portion 76 having an open end directed toward
the cathode. An x-ray emitting target button 78 is provided in the
base of the cavity thus formed in the anode for the purpose of
receiving electrons from the cathode and directing resultant x-rays
out through an opening 80 and then through the wall of the envelope
68.
In accordance with the present invention, a block or body of
graphite or other selected backing material 82 is deposited in the
bottom or the hollow anode extension and is provided with an
inclined surface having a recess therein in which the target button
78 is positioned. A sleeve or shell 84 of graphite or other
selected high thermal capacity material is then positioned in the
extension with one end thereof engaging the target button 78.
Sleeve 84 is provided with an opening 86 which may contain a window
88 of beryllium or other material highly transmissive to
x-radiation which is suitably aligned with opening 80 in extension
76 whereby x-rays emanating from the target button 78 will pass
outwardly through the window 88 and opening 80.
It is, of course, necessary that there be efficient conduction of
heat from the target button 78 into the adjacent graphite bodies 82
and 84. Therefore, means is provided for continually urging the
sleeve 84 against the target button 78 and to thereby maintain
efficient heat conductive relationship of the button 78 with
backing 82. Such means is illustrated in exemplary form as a spring
90 which at one end engages the outer end of the shell 84 and at
its other end engages the inner side of a retaining ring or collar
92 which is attached to the inner circumference of the anode
extension 78 as by set screws 94 or the like.
Thus, there is described one type of stationary anode tube which
has most, if not all, of the advantages of the rotating anode
structure described hereinbefore. Other stationary anode tubes may
be provided with this invention, however, such as the type which
embodies a metal housing without the glass envelope, as is well
known.
In any of the aforementioned embodiments of the invention thermal
expansion between the target member and the high thermal
conductivity backing elements is permitted. This, then, opens up
the possiblity of a large number of materials which may be used to
serve the desired functions. Low density elements can serve the
function of the target backings while selected different materials
may be used for the x-ray producing elements of the structures.
From the foregoing description it will be apparent that all of the
objects and advantages of this invention have been achieved by the
various structures shown and described. It will be apparent,
however, that various modifications may be made by those skilled in
the art without departing from the spirit of the invention as
expressed in the accompanying claims. Therefore, all matter shown
and described should be interpreted as illustrative and not in a
limiting sense.
* * * * *