U.S. patent number 3,609,432 [Application Number 04/774,396] was granted by the patent office on 1971-09-28 for thin target x-ray tube with means for protecting the target.
This patent grant is currently assigned to Rigaku Denki Company Limited. Invention is credited to Yoshihiro Shimula.
United States Patent |
3,609,432 |
Shimula |
September 28, 1971 |
THIN TARGET X-RAY TUBE WITH MEANS FOR PROTECTING THE TARGET
Abstract
In X-ray tubes having thin targets such as foils, a problem is
encountered in the deterioration of the foils, due to heat caused
by bombardment by an electron beam. The invention avoids this
deterioration in accordance with the one embodiment by providing
water-cooled blocks in contact with the target and provided with
openings through which the electron beam and/or emitted X-rays can
pass and in accordance with another embodiment by providing an
antioxidizing coating on the target.
Inventors: |
Shimula; Yoshihiro (Tokyo,
JA) |
Assignee: |
Rigaku Denki Company Limited
(Tokyo, JA)
|
Family
ID: |
25101110 |
Appl.
No.: |
04/774,396 |
Filed: |
November 8, 1968 |
Current U.S.
Class: |
378/141; 313/32;
378/143; 378/140; 378/161 |
Current CPC
Class: |
H01J
35/32 (20130101) |
Current International
Class: |
H01J
35/00 (20060101); H01J 35/32 (20060101); H01j
035/08 (); H01j 035/16 (); H01j 035/33 () |
Field of
Search: |
;313/32,55,59,330 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Lake; Roy
Assistant Examiner: Campbell; C. R.
Claims
I claim:
1. An X-ray tube comprising a source is an electron beam, first
means to accelerate said beam, a target arranged to be bombarded by
the accelerated beam, and second means associated with the target
to enhance the ability thereof to withstand deterioration which
would result from bombardment by said beam, said second means
including cooling means for cooling said target, said first means
including an elongated tube extending between said source and
target and wherein said cooling means includes at least one cooling
block having an opening aligned with said beam and being provided
with a passage for the passing of a cooling fluid, said block being
supported by said tube and being in contact with the target over a
major portion of the latter, said target being exposed to said beam
in correspondence with said opening.
2. A tube as claimed in claim 1 wherein said second means includes
antioxidizing means to prevent the target from oxidizing.
3. A tube as claimed in claim 1 wherein the block is arranged
between the target and source and the beam passes through said
hole.
4. A tube as claimed in claim 1 wherein the target is arranged
between the block and source and at least part of the X-rays
emitted by the target pass through the hole.
5. A tube as claimed in claim 1 comprising a second block
corresponding to the first, said blocks sandwiching the target
therebetween, said blocks having said holes in coaxial conical form
tapering towards each other to corresponding apex end openings,
said passageway being constituted by corresponding annular
semicircular grooves in the blocks, said grooves being juxtaposed
to form a passage of toroidal shape, said tube further comprising
means to supply and remove a cooling fluid to and from said
passage.
6. A tube as claimed in claim 4 wherein said tube includes window
means between the target and source for the passing of another part
of the X-rays emitted by the target.
7. A tube as claimed in claim 1 wherein the passage is positioned
internally of said tube.
8. A tube as claimed in claim 1 wherein the passage is positioned
externally of said tube.
9. A tube as claimed in claim 1 wherein the target is a thin
foil.
10. A tube as claimed in claim 2 wherein the antioxidizing means is
a layer adhering to the target.
11. A tube as claimed in claim 1 wherein the block is of a metal
having good thermal conductivity.
Description
DRAWING
FIG. 1 diagrammatically illustrates in cross section a part of a
preferred embodiment of the present invention;
FIG. 2 diagrammatically illustrates in cross section the target
section of the embodiment of FIG. 1;
FIG. 3 is a cross section taken along line III--III in FIG. 2;
FIG. 4 diagrammatically illustrates in cross section a second
embodiment of the present invention;
FIG. 5 is a cross section taken along line V--V in FIG. 4;
FIG. 6 diagrammatically illustrates in cross section another
modified form of the X-ray tube of the present invention;
FIG. 7 diagrammatically illustrates in cross section, on enlarged
scale, the target portion of FIG. 6;
FIG. 8 is a cross section taken along line VIII--VIII in FIG.
7;
FIG. 9 is a schematic illustration of the principle of the present
invention;
FIG. 10 is a diagram showing a heat radiation curve used to
describe the present invention; and
FIG. 11 shows an enlarged section of the target portion of another
embodiment of the present invention.
DETAILED DESCRIPTION
This invention generally relates to X-ray tube devices and more
particularly to cooling systems for thin target X-ray tube
devices.
In various types of X-ray measurement or observation equipment
including X-ray microscopes and X-ray diffraction devices, and
particularly for research on X-ray diffraction figures by the
Kossel-pattern technique or in the microscopic photographing of
X-ray figures, an X-ray tube having a focal point of sufficiently
small size and capable of generating a strong X-ray beam is
required. In such an X-ray tube, a minute focal point of the ray
having a size of, for example, several tens of microns is formed on
a thin metal target having a thickness of, for example, several
tens of microns.
In such an X-ray tube, the focal point area on the thin target is
heated locally to a high temperature and consequently is fused.
Therefore, an electron current higher than about 100 .mu.A. cannot
pass. Even when the back surface of a thin target is cooled
directly by passing cooling water thereover, the cooling water
boils at the back side of the focal point on the back surface of
the target, undesirably resulting in the formation and building up
of "fur."
A primary object of the present invention is to provide means for
effectively cooling a thin target of an X-ray tube and avoiding the
above problems.
Another object of the present invention is to provide a water
cooling system for cooling the thin target of an X-ray tube, which
system is simple in construction but excellent in effect.
Still another object of the present invention is to provide an
X-ray tube which is capable of generating strong X-rays to be
passed through a minute focal point formed on a thin foillike
target.
It is a further object of the present invention to provide an X-ray
tube which is arranged to prevent a minute focal point on a thin
foillike target from oxidizing due to heating.
The present invention will next be described in detail with
reference to the preferred embodiments illustrated in the
accompanying drawings.
In FIG. 1, an elongated metal tube 1 provided at one end thereof
with a Wehnelt electrode 2 and a filament 3 is grounded. A negative
high voltage is applied to the electrode 2 and the filament 3, and
an electron-beam-focusing coil 4 is fitted over and encircles the
tube 1. The other end of the tube 1 is provided with a pair of
cooling water supplying members and a target foil as will be
explained with reference to FIG. 2.
More particularly, the other end of the tube 1 is provided with a
suitable target foil 7 of a metal such as copper, aluminum or gold
interposed between a pair of cooling-water-supplying discs or
blocks 5 and 6 fitted into the tube 1. The target foil 7 has a
thickness within the range from several tens of microns to several
hundreds of microns.
The discs 5 and 6 are formed with central conical openings 8 and 9,
respectively, the diameter 2R of the openings at their apices being
formed to provide the desired diameter for the focal point. A pair
of cooling water passages 10 and 11 are formed in the discs 5 and
6, respectively, through the portions of discs 5 and 6 nearest to
the apices of the conical openings 8 and 9. The passages 10 and 11
are shaped in discs 5 and 6 to form halves of a complete circle and
the target foil 7 is centrally disposed to pass through the center
of the circle. In other words, the foil 7 divides the passage into
two halves. The passages 10 and 11 are respectively extended toward
the edges of the discs 5 and 6 and are connected to inlet and
outlet pipes P1 and P2 respectively connected to the elongated tube
1 (see FIG. 3).
In an X-ray tube arranged as described above, the electron current
discharged from the filament 3 is intensified by the elongated tube
1 to a voltage of several tens of thousands. The resultant electron
current e is focused at the apex of the conical opening 9 by the
coil 4. A portion of the focused current hits the portion of the
target foil 7 exposed in the opening at the apex. Since the foil is
very thin, a portion of the X-rays generated by the electron beam
bombardment passes through the foil and is emitted through the
opposite opening 8 to the outside as indicated by arrows x. The
electron beam e thus hits a relatively large area of the inner
surface of the conical opening 9, but the portion permitted to pass
through the opening 9, foil 7 and opening 8, to become the source
of X-rays, is defined by the diameter of the openings in the apices
2R which is, for example, in the order of several tens of
microns.
Although electron beam e hits a relatively large inner surface area
of the conical opening 9 and the portion of the foil 7 exposed at
the opening by the apex, there is no fear of local elevation of the
temperature at the portions bombarded by the electron beam since
the opening 9 is formed in the disc 6 which has a sufficient
thickness and is formed of a metal having a good thermal
conductivity. The foil 7 is bombarded and heated by the electron
beam only at its exposed portion which, as noted, has a diameter of
several tens of microns. Since the exposed portion is in contact
directly with the massive discs 5 and 6 formed of a good heat
conductive material and surrounding the exposed portion of the foil
and having cooling water passages 10 and 11 contiguous with the
exposed portion, the vicinity of the exposed portion of the foil 7
is maintained at a temperature substantially equal to that of the
cooling water. Assuming that the thickness of the foil 7 is s and
that the calorific value Q due to the electronic impulse is
generated uniformly in the foil and the density of the electron
beam is uniform for simplifying the analysis, calories Q.sub.1
generated within a circle with a radius r from the center of the
exposed portion of the foil 7 may be expressed as follows:
Where the temperature gradient at radius r is dt/dr and the thermal
conductivity of foil 7 is h, the calories Q.sub.2 in a circular
cross section of the foil 7 with radius r is:
If the scattering of heat due to radiation and convection is
ignored, since calorific value Q.sub.1 and the propagating calories
Q.sub.2 are equal under normal conditions,
thus,
Therefore, the temperature difference T between the center of the
exposed foil portion and its periphery is: ##SPC1##
Hence, if the temperature of cooling water is T.sub.1 , the
temperature at the center of the exposed portion of the foil 7 may
be obtained in the following manner:
Thus the temperature at the center of the exposed foil portion
where it is heated most extremely has no relationship to the radius
R of the exposed portion.
If the acceleration potential of the electron beam is constant, the
total calorific value at the exposed portion of the foil will
depend upon the value of the electron current hitting the exposed
portion. Therefore, if the highest permissible temperature (i.e.,
without melting or damaging the foil 7) is constant, the exposed
foil portion regardless of its radius R may be bombarded with a
constant electron beam whereby X-rays are generated. In other
words, with the smaller X-ray focal point provided by forming
smaller apices of the conical openings 8 and 9, the electron beam
density is made greater, whereby the magnitude of the electron
current hitting the exposed portion of the foil 7 may be always
maintained at a constant value. In this manner, X-rays of a
substantially constant intensity may be provided regardless of the
size of the X-ray focal point. Thus according to the present
invention, an X-ray tube having an extremely small focal point and
capable of generating strong X-rays may be provided.
According to the present invention, since the X-ray generating
portion of the X-ray tube is directly cooled by water, the cooling
at this portion may be carried out very effectively and
efficiently. Furthermore, X-rays of a constant intensity may always
be provided regardless of the size of the focal point and since the
cooling efficiency is high, the electron-beam-accelerating
potential may be increased. In such a case, the electron beam
permeates deeper into the foil to generate X-rays at deeper
portions of the foil, thus to emit stronger X-rays from the back
surface of the foil.
It is a further advantage of the present invention that with the
smaller focal point, a lower atmospheric pressure is applied to the
foil. Therefore, a thinner foil may be used whereby even stronger
X-rays may be obtained.
In the second embodiment of the present invention shown in FIGS. 4
and 5, the cooling water introducing member 6 as shown in FIGS. 2
and 3 is omitted while an X-ray window 12 is formed through the
wall of the tube 1 and the window 12 is closed and hermetically
sealed with a beryllium cover plate 13. In this embodiment, laminar
electron beams e bombard the apex of the opening 8. The manner of
emitting X-rays X.sub.1 from the back surface of the foil 7 through
the opening 8 is similar to that of the embodiment shown and
described with reference to FIGS. 2 and 3. In this embodiment,
however, when the window 12 is so positioned as to face the side of
a wide electron flux e, X-rays X.sub.2 emitted from the surface of
the foil 7 through the window 12 and the beryllium cover plate 13
form a linear focal point as shown, when the window is positioned
to face the side of a narrow electron flux, X-rays X.sub.2 form a
spot focal point effectively. The X-ray tube of this form is of a
multipurpose type; i.e. X-ray microscopic photographs may be taken
by utilizing X-rays emitted from the opening 8, while, for example,
the observation of X-ray diffraction may be carried out using
X-rays emitted through the window 12.
The third embodiment of the present invention is next described
with reference to FIGS. 6 through 10. In this embodiment, the
elongated metal tube 1, which is similar to that shown in FIG. 1,
is fitted at its one end with a thick disc-shaped target holder 105
formed of a good heat-conductive material such as steel to form a
part of the wall of an airtight casing. This portion is shown on
enlarged scale in FIGS. 7 and 8 and, as clearly illustrated, the
target holder 105 is formed with an axial hole 106 along its axis
and a radially extended hole 107 at the outer end. A metal target
foil 108 is secured to the outer end face of the holder 105 and the
open ends of the radial hole 107 are respectively closed in
airtight condition by cover plates 109 made of a highly X-ray
permeable material such as beryllium. The cooling water pipe 110 is
provided with a hole into which the end of the tube 1 is fitted,
thus to form a passage 111 for cooling water to cool the outer
surface of the foil 108.
Cooling water is passed through the passage 111 as shown by arrow a
in FIG. 7, the electron current discharge from the filament 3 being
accelerated through the elongated tube 1 to a voltage of several
tens of thousands to form X-ray bean e as shown by dotted lines in
FIG. 8. The X-ray beam e is fluxed by coil 4 and the flux is passed
through the axial hole 106 to form focal point P on the surface of
the target foil 108 whereby to emit X-rays from the foil 108. A
portion of the X-rays is externally passed through the radial hole
107, which may be termed an X-ray outlet hole, to utilize it for
any desired purpose such as for the observation of X-ray
diffraction.
FIG. 9 shows a section of the focal point P, on enlarged scale, in
which r designates the radius of the focal point P of the electron
beam e, and B is a hemisphere with radius R; the heat radiation E
may be obtained as follows:
wherein, T.sub.1 represents the temperature at the focal point P,
TT.sub.2 represents the temperature of the hemisphere B and k
represents the thermal conductivity of the target foil 108. If the
electric current value of the electron current is I and the
velocity thereof is V, the power applied to the focal point P will
be VI, therefore, the value VI should be maintained below the value
E obtainable by expression (7) given hereinabove. Assuming that all
values T.sub.1, T.sub.2, k and r are constant, it can be seen that
there is a relationship between the radius R and the heat radiation
E as shown in FIG. 5. Accordingly, it is clear that the radius R of
the hemisphere B becomes smaller at a constant temperature T.sub.2
, the permissible power VI of electron beam may be increased. In
the above-described X-ray tube, a thin foil 108 is used as the
target provided with a cooling water passage 111 on its back
surface to be cooled directly, and a member 105 for holding the
target foil 108 is formed with a good heat conductive material and
provided with an axial hole 106 over which the foil 108 is directly
secured. When the temperature of the cooling water is T.sub.2, the
back side of the foil 108 and the hole 105 is naturally maintained
at the temperature T.sub.2 . Therefore, the heat radiation E may be
obtained by substituting for the radius R in expression (7) the
thickness d of the foil 108. As shown in FIG. 10, the smaller
becomes the value R substituted by thickness d, the more heat
radiation E increases. Thus, the power VI to be applied to the
target becomes higher due to the stronger bombardment of electron
beam, consequently emitting stronger X-rays. Since the target foil
108 is subjected to atmospheric pressure at only the portion
exposed through the axial hole 106 and the radial hole 107, a very
thin foil may be used as the target. Since the foil is simply
secured to and held by the holder 105, the manufacture of the X-ray
tube may be greatly facilitated.
FIG. 11 shows the fourth embodiment of the present invention. In
this embodiment, the end of the tube 1 opposite its electrode side
is fitted with a cover plate 215 formed of a metal having a good
heat conductivity. The cover plate 215 is formed with a central
opening 216 the open end of which is covered with a thin-target
foil 217 of a suitable metal such as steel, molybdenum, iron or
tungsten such that the target foil 217 forms of a portion of the
wall of the hermetically sealed casing. The elongated tube 1 is
surrounded by an electron beam fluxing coil and electron beam e
emitted from a filament by grounding the tube 1 and applying a high
negative voltage to the filament and a Waynelt electrode is
accelerated through the tube 1 to a voltage of several tens of
thousands. The beam is fluxed by the coil so that a focal point is
formed on the surface of the foil 217 through the axial hole 216.
The size of the focal point thus formed on the target foil is very
small such as of several microns to several tens of microns. The
thickness of the foil 217 is also within the same range. Since the
penetration of electron beam e is shallow relative to the thickness
of the foil, X-rays are generated at the inner surface of the foil,
and a portion of X-rays passes through the foil and is emitted to
the outside as shown by arrows x in FIG. 11. In the embodiment
illustrated in FIG. 11, the outer surface of the target foil 217 is
coated with a heat-resistive antioxidizing layer 218. In case a
steel foil is used as the target foil 217, one surface of the steel
foil is preplated with platinum so that the plated platinum layer
will serve as the antioxidizing layer. When an aluminum foil is
used as the target foil, one surface of the foil may be covered
with an alumite layer for the same purpose.
In known X-ray tubes, when it is attempted to obtain strong X-rays
by increasing the intensity of the bombardment of the electron beam
against the target, the temperature of the target surface rises. In
an X-ray tube having the target contained in a hermetically sealed
casing, the bombardment intensity is limited by the temperature
which should be lower than that which melts the tube. It has been
found that in such a known type of thin-target X-ray tube, the
target foil 217 is gradually oxidized and deteriorated due to the
temperature rise. It usually cracks and causes leaks. Therefore,
such X-ray tubes of the prior art have been generally found to be
short lived due to oxidation of their target foils. According to
the present invention, however, the target foil 217 is covered with
a heat-resistive antioxidizing layer, whereby to avoid the
formation of cracks in the foil thus to increase the life of the
X-ray tube. Since the target foil of the present invention is
prevented from oxidizing, the level of permissible temperature may
be elevated to allow increase of the density of the electron beam
to thereby increase the intensity of X-rays emitted. The
antioxidizing layer can be made very thin. Therefore, its
absorption of X-rays may be minimized. However, in some
applications, the tube may be arranged to emit secondary X-rays of
suitable wavelengths from the antioxidizing layer for any
particular purpose.
The discs 5 and 6 and the element 215 noted hereinabove can be made
of metals having good thermal conducting qualities, such as, for
example, copper.
* * * * *