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The present invention relates to reducing the effects of
excessive heat buildup on the x-ray transmission window and the tube
envelope surfaces of a high power metal envelope x-ray generating
tube.
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In x-ray generating tubes, a stream of electrons is emitted
from a cathode and accelerated in a high voltage potential difference
to strike the target area of an anode surface. Electromagnetic
energy is thus produced in the form of x-rays. In many applications,
it is desirable to narrowly focus the stream of electrons onto a small
area of a rotating anode, known as the "focal spot." In addition, it
is often desirable to maximize the energy of the electron stream in
order to produce a large amount of high energy x-rays.
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In x-ray tubes, the tube envelope surfaces are heated by
several means. Heat is generated in the target from bombardment
of the primary electron beam created by the cathode. This heat is
radiated to the envelope of the x-ray tube. Heat is also generated
in the envelope from secondary electron bombardment from
electrons back scattered from the focal spot on the target. In this
context, reference to "secondary" electrons includes backscattered
electrons. Heating of the envelope is higher in the window area due
to the contribution of electrons generated by back scattered
secondary electrons. If the temperature in the area of the window
rises too high, dielectric oil on the exterior of the x-ray tube will boil
or break down. This is highly undesirable since the oil must
maintain its electrical dielectric properties in the x-ray tube housing.
In certain medical applications such as computed tomography ("CT"
or "CAT") scanning, some diagnostic techniques require high energy
exposures for relatively long periods of time. The energy input from
these exposures can cause overheating of the window area of the x-ray
tube envelope causing the dielectric oil to break down due to
secondary electron bombardment of the window area and heating
from the target. Reducing the energy input to a level that prevents
this heating can cause delays that put limits on the time available to
gather diagnostic information for the radiologist.
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The buildup of thermal energy in the anode is the first
limiting factor in power output, longevity, and efficiency of the x-ray
generating tubes. The need for continuous use, high power x-ray
tubes has become even stronger with the advent of new types of
medical equipment such as CAT scanners and other high power
x-ray applications such as digital radiography, and angiography.
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Only a small fraction of the electron energy is convened to
x-rays. While most of the electron energy is convened directly to
heat energy, some electrons have enough energy to leave the target
surface and fly off in random directions. These electrons, still subject
to the high voltage field, tend to be reabsorbed back into the target
or any other surface which intercepts their course. These "stray"
electrons are called "secondary electrons" as opposed to the electrons
in the primary beam from the cathode which generate the desired
x-rays. Secondary electrons cause undesirable heating when they
bombard the envelope surface of the x-ray tube near the focal spot
area, which may damage the sealing components, wall material, or
cause overheating of the dielectric oil external to the x-ray tube.
Radiation produced from the secondary electrons is called "off focal
radiation" and is undesirable because it creates a background
radiation pattern which does not contribute to the x-ray image.
Energy of the secondary electrons, rather than producing x-rays, is
released as heat, thereby elevating the anode and envelope tube
surface temperatures.
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The use of a rotating anode disperses the energy of the
electron stream over a large area, while maintaining a narrow focal
spot. Rotating anode x-ray generating tubes are now common, and
details of their construction and operation are readily available.
However, even with a rotating anode design, the buildup of thermal
energy in the anode structure remains a problem. Since the anode
structure operates in a vacuum, heat cannot be carried away from
the anode surface by convection. Some heat can be conducted to
the exterior of the tube through the bearing structure of the rotating
anode. However, heat buildup in the bearing structure is a major
cause of tube failure. Generally, it is desirable to thermally isolate
the bearing, thereby minimizing the heat loss by conduction.
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High power x-ray generating tubes typically include a glass
center portion immersed in dielectric oil. However, it is possible to
achieve higher power levels with tubes having a metal center section.
The propensity to overheat in the window area of x-ray tubes is seen
mainly in high power applications in metal center x-ray tubes. By
reducing the power, heat buildup can be reduced. However, this
resolution is not satisfactory for applications requiring high power.
Alternatively, attempts have been made to reduce the effects of heat
buildup.
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One approach to increasing the thermal capacity of rotating
anode x-ray tubes has been to increase the radius volume of the
anode disk, thereby increasing the mass of material capable of storing
the thermal energy imparted by The electron beam. However, such
designs do not increase the capacity of the anode to dispose of
thermal energy. Under continuous use, such designs again result in
heat buildup. Moreover, this approach has the further disadvantage
of amplifying the mechanical motions of the anode as it rotates and
increasing the difficulty of maintaining the mechanical tolerances of
the anode structure. The overall moment of inertia of the anode is
increased, thereby necessitating greater input of rotational energy.
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Secondary electron beating and off focus radiation has been
addressed in stationary anode x-ray tubes. Elaborate means have
been employed to reduce off focus radiation and anode heating.
"Hooded" anodes with beryllium windows have been used to collect
secondary electrons. These anodes are liquid cooled by various
methods, usually from behind the stationary target and inside the
hood surrounding the target. Cooling of these systems has been
limited to cooling of the anode structure alone.
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U.S. Patent No. 4,819,260 and European Patent
No. B1 0 059 238 address off focal radiation. U.S. Patent No.
4,819,260 is directed to a magnetically controlled electron beam
which prevents migration of the focal spot of an electron stream on
a rotating anode. European Patent No. B1 0 059 238 discloses a
metal screen which is inserted between the cathode and the anode
and a voltage is applied to the screen. Neither of these systems
addresses heat produced by secondary electron bombardment to cool
the vacuum envelope.
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U.S. Statutory Invention Registration No. H312 addresses heat
dissipation of the anode structure. Fins are provided adjacent to the
anode structure for enhancing heat transfer from the anode structure
to the region outside the vacuum envelope. As in the previously
discussed references, this publication does not address heat
dissipation of the vacuum envelope due to secondary electron
bombardment.
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U.S. Patent No. 4,841,557 discloses a rotating anode in which
the housing is cooled by circulating, within the housing body, the
outer immersion fluid in which the housing is retained. Interior
conduits and connections require that a complicated structure be
included for circulation of the fluid.
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None of the prior art addresses dissipating heat generated by
secondary electron bombardment to cool the tube envelope.
Traditionally, the anode is maintained at a high positive voltage while
the cathode is maintained at an equally high negative voltage to
achieve a potential of approximately 150,000 volts. Although higher
power levels are precluded by this traditional split because of
secondary electron bombardment of the tube envelope, it alleviates
intensified heating of the x-ray tube envelope and window area.
With increased demand for higher power applications, cooling of the
vacuum tube envelope must be expressly addressed.
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Accordingly, it is a primary object of the invention to provide
means for dissipating heat generated by secondary electron
bombardment from the rotating anode structure of an x-ray
generating tube having a metal center section.
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Another object of this invention is to provide an anode
structure maintained at ground potential.
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It is a further object of the invention to provide means to
reduce overheating of the x-ray tube envelope by controlling the
secondary electron heating and heat transfer from the target area.
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The present invention relates to a high power metal center
x-ray generating tube for reducing the effects of excessive heating of
the x-ray transmission window by secondary electron bombardment.
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The present invention includes an x-ray generating tube
consisting of a vacuum envelope having a metal center section, a
stationary cathode and a rotating anode. A high potential is created
between the rotating anode and the cathode to cause an electron
beam to strike the anode with sufficient energy to generate an x-ray
beam trough a window formed in the tube envelope. The present
invention seeks to dissipate heat which can be generated in two ways.
First, heat is generated in the target from bombardment of the
primary electron x-ray beam created by the cathode. This heat is
radiated to the envelope of the x-ray tube. Second, heat is generated
in the envelope from secondary electron bombardment from
electrons back scattered from the target.
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The rotating anode is preferably held at ground potential
along with the metal center section, while the cathode is at high
voltage so as to maintain a potential between the cathode and anode.
It is thus possible to provide a more intimate design between the
cooling means and anode, using a coolant, such as water, which need
not provide a dielectric standoff between the anode and cooling
means. With the anode at ground, the hardware mechanism for
cooling is less complicated. Alternatively, the anode can be at high
positive voltage and the cathode at high negative voltage, resulting
in a similar potential difference.
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In the preferred embodiment, a shield intercepts back
scattered secondary electrons, preventing them from striking the
window, thereby avoiding secondary electron bombardment of the
window. The shield is disposed adjacent to the window and includes
a contoured surface to match the high voltage field lines between the
cathode and anode. A coolant is circulated around the outer portion
of the window to dissipate heat. As discussed above, because the
metal center section is at ground potential, it is possible to circulate
water as the selected coolant.
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In an alternate embodiment, heat generation by the secondary
electrons back scattered from the anode is mitigated by way of an
insulating means. The insulating means provides a build up of
negative charge on the inner surface of the window to decelerate and
repel the back scattered electrons, reducing the amount of electron
energy bombarding the window and, thereby, reducing excessive
heating of the window. Electrons are collected on the inner surface
of the window for build up of a negative charge to repel electrons
from the window surface. A coating of conductive material may also
be formed on the window surface. In this case, the window is
electrically insulated from the envelope so as to form a floating
potential on the surface of the window.
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In another embodiment of the invention, the effects of
excessive heating are reduced by a double wall formed in the metal
center section in the vicinity of the window. A closed space is
created between an inner and outer wall for circulation of a coolant
to conduct heat through the inner wall and away from the window.
The window also includes an inner and outer window in the
respective inner and outer walls. Alternatively, a closed space can
be formed such that coolant is circulated around the entire portion
of the metal center section.
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Examples of the invention will now be described with
reference to the accompanying drawings in which:
- FIG. 1 is a cross sectional view of the preferred embodiment
of the x-ray generating tube of the present invention.
- FIG. 2 is a cross sectional view of a second embodiment of
the x-ray generating tube of the present invention.
- FIG. 3 is a cross sectional view of a third embodiment of the
x-ray generating tube of the present invention.
- FIG. 4 is a cross sectional view of a fourth embodiment of the
x-ray generating tube of the present invention.
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FIGS. 1-4 illustrate an x-ray generating tube 2 including a
vacuum envelope 4 comprising a metal center section 6. A stationary
cathode 8 and a rotating anode 10 are housed within vacuum
envelope 4. In the preferred embodiment, anode 10 is held at
ground potential. High voltage means 12 are provided for creating
a potential between cathode 8 and anode 10 to cause an electron
beam generated by cathode 8 to strike anode 10 with sufficient
energy to generate x-rays 14. A window 16 permits transmission of
x-rays 14 for high power applications. Window 16 is preferably
constructed of beryllium, but may be any material through which
x-rays can be transmitted.
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Referring now to FIG. 1, a shield 18, preferably constructed
of copper, intercepts secondary electrons 20 back scattered from
anode 10 away from window 16 to avoid secondary electron
bombardment of envelope 4 and, thereby, to prevent overheating of
the window. Secondary electrons 20, still subject to the high voltage
field, tend to be reabsorbed back into target area 11 of anode 10 or
any other surface which intercepts their course.
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An enlarged region 22 of metal center section 6 surrounds
window 16 such that the window slightly protrudes from the metal
center section, outward of vacuum envelope 4. Shield 18 is formed
integrally with or affixed to the interior surface of enlarged region 22.
A contoured surface 24 intercepts secondary electrons 20 away from
the window area. Contoured surface 24 is continuous with the inner
surface of center section 6, curving so as to extend toward the
interior of vacuum envelope 4 and match the high voltage field lines
between cathode 8 and anode 10. An interior, transverse face 26 is
generally flat and extends from a tip 30 of contoured surface 24 to
an indentation 32 for retaining window 16 in place. Interior
transverse face 26 is angled slightly from tip 30 outwardly to window
16. A generally flat, exterior, transverse face 28 extends from the
outer surface of center section 6 to the outer edge of enlarged region
22, approximately aligned with window 16.
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Juxtaposed with exterior transverse face 28 is a coil 34 for
circulating a forced coolant around window 16. Heat generated by
secondary electrons 20 bombarding shield 18 and heat radiated from
target 11 is removed by conduction through the shield and carried
away by the forced coolant through coil 34. Because anode 10 is at
ground potential along with metal center section 6, it is possible to
use water as the selected coolant immediately adjacent to the anode.
Thus, the hardware required for cooling is less complicated than
would be required for a high voltage anode.
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In operation, high voltage means 12 creates a potential
between cathode 8 and anode 10 to cause an electron beam to strike
the anode with sufficient energy to generate x-rays 14. Heat is
generated by this electron beam from secondary electrons 20. Shield
18 absorbs the secondary electrons, collecting the heat along with
heat generated by primary electron beam 14. The heat is conducted
by coolant forced through coils 34 to ensure that window 16 remains
cool.
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The embodiments shown in FIGS. 2 and 3 are similar to FIG.
1, differing from FIG. 1 only in the circulation of the forced coolant.
As discussed with reference to FIG. 1, x-ray generating tube 2 of
FIGS. 2 and 3 include metal center section 6, stationary cathode 8,
rotating anode 10, window 16 and shield 18.
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Turning first to FIG. 2, a double walled closed space 36 is
formed in metal center section 6. Closed space 36 is positioned
between an inner wall 38 and an outer wall 40 for circulation of
forced coolant The coolant enters closed space 36 through a
coolant inlet 42, circulates around vacuum envelope 4 and exits
through a coolant outlet 44. Window 16 of FIG. 2 is identical to
window 16 of FIG. 1, i.e., slightly protruding from enlarged region 22
outwardly of vacuum envelope 4. Closed space 36 surrounds shield
18 and window 16 for conduction of heat away from the window.
Heat radiated from anode 8 is conducted through inner wall 38 and
convected away with the liquid coolant circulating between inner wall
38 and outer wall 40.
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Turning now to FIG. 3, a double walled closed space 36 is
also formed in metal center section 6. Unlike the embodiment of
FIG. 2 in which the entire metal center section is double walled,
metal center section 6 of FIG. 3 is identical to the metal center
section of FIG. 1. Closed space 36 is localized in the vicinity of
window 16. It is positioned between the outer surface 38a of metal
center section 6 and an external outer wall 40 coupled to the exterior
the metal center section in the vicinity of the window along only one
side of vacuum envelope 4. An inner window 16a is identical to
window 16 of FIG. 1. Additionally, an outer window 16b is formed
in external outer wall 40 and spaced from window 16a for
transmission of x-rays 14 through both inner window 16a and outer
window 16b. In operation of FIG. 3, the coolant enters closed space
36 through a coolant inlet 42, circulates between window 16a and 16b
and exits through a coolant outlet 44. Heat generated by primary
electrons 14 and heat from secondary electrons 20 is conducted
through inner wall 38a and convected away from the circulating
cooling fluid.
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Referring to FIG. 4, x-ray generating tube 2 includes metal
center section 6, stationary cathode 8, end-grounded rotating anode
10, high voltage means 12 and window 16. FIG. 4 differs from FIG.
1 in the means for deflecting secondary electrons 20 back scattered
from anode 10 away from window 16 to avoid secondary electron
bombardment of envelope 4. FIG. 4 includes, rather than a shield
18 as shown in FIGS. 1-3, a window 16 having a dielectric material
such as ceramic or beryllium oxide material on the inner surface 16a
of the window. The material selected acts as an insulator for
collecting electrons on inner surface 16a of window 16. These
electrons have no conductive path and, therefore, build up a negative
charge 15 on inner surface 16a to cause a certain percentage of
secondary electrons 20 to decelerate or be repelled before striking
window surface 16a. Thus, the effect of providing an insulating
material on inner surface 16a serves to reduce the amount of
electron energy imparted to the window surface, thereby, reducing
excessive heating of window 16. A light coating of semiconductive
material 46 is preferably formed on inner surface 16a, electrically
insulated from vacuum envelope 4 so as to form a floating potential
on inner surface 16a. However, it is within the scope of the present
invention to construct inner surface 16a of an insulating material,
without providing a semiconductive layer.
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Variations and modifications can be made to the present
invention without departing from the scope of the present invention.
For example, the preferred embodiments describe a rotating anode
in a metal center tube; however, it is within the scope of this
invention to cool an x-ray generating tube having a stationary anode.
The embodiment of FIG. 3 may be constructed with or without a
shield. In the embodiment without a shield, the closed space, alone,
prevents excessive heating of the window. The means for preventing
excessive heating may be of any form not shown or described herein.
For example, a magnet may be employed outside of the envelope to
deflect secondary electrons from the window. A power supply may
be attached to any of the described embodiments to produce a
charge, thereby enhancing the force for repelling the secondary
electrons.
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The present application is a divisional application of parent application EP
91310751.2. The claims of the parent application are set out below. Whilst these claims are
not currently claims of the present application, their subject matter does form part of the
present application. The right to claim the subject matter of these claims in the present, or
in future divisional applications, is reserved
Claims of EP 91310751.2:
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- 1. A high power rotating anode x-ray generating tube
comprising:
- a vacuum envelope having a metal center section;
- an anode structure disposed within said center section for
rotation about an axis;
- cathode means within said center section for generating a
beam of electrons;
- high voltage means for maintaining a potential between said
anode structure and said cathode means to cause said electron beam
to strike said cathode means with sufficient energy to generate x-rays;
- a window formed in said center section for transmission of
said x-rays outside said envelope; and
- means for preventing the excessive beating of said window.
- 2. The x-ray generating tube of claim 1 wherein said means
for preventing comprises means for deflecting electrons from striking
said window.
- 3. The x-ray generating tube of claim 2 wherein said means
for deflecting comprises shield means disposed adjacent said window.
- 4. The x-ray generating tube of claim 3 wherein said shield
is configured to match high voltage field lines between said cathode
means and said anode structure.
- 5. The x-ray generating tube of claim 1 wherein said anode
structure is maintained at ground and wherein said cathode means
is maintained at high voltage.
- 6. The x-ray generating tube of claim 5 further comprising
means for circulating coolant around said preventing means.
- 7. The x-ray generating tube of claim 1 wherein said means
for preventing comprises heat transfer means for transferring heat
from said window and said envelope.
- 8. The x-ray generating tube of claim 7 wherein said heat
transfer means are constructed of copper.
- 9. The x-ray generating tube of claim 1 wherein said means
for preventing comprises means for collecting electrons on an inner
surface of said window for build up of a negative charge thereon to
thereby repel electrons from said window surface.
- 10. The x-ray generating tube of claim 9 wherein said window
is electrically insulated from said envelope so as to form a floating
potential on the surface of said window.
- 11. The x-ray generating tube of claim 9 wherein said means
for collecting comprises insulating said window from said metal center
section.
- 12. The x-ray generating tube of claim 9 further comprising
a coating of conductive material on said window surface.
- 13. The x-ray generating tube of claim 1 wherein said means
for preventing comprises an inner wall and an outer wall having a
closed space therebetween for circulating coolant between said walls
such that heat generated by transmission of said electrons is
conducted through said inner wall and convected away from said
window.
- 14. The x-ray generating tube of claim 13 wherein said inner
wall and said outer wall encircle said entire center section.
- 15. The x-ray generating tube of claim 13 wherein said inner
wall and said outer wall are localized in the vicinity of said window
and wherein said window comprises an inner window and an outer
window formed in said inner wall and said outer wall, respectively.
- 16. An x-ray generating tube comprising:
- a vacuum envelope having a metal center section;
- rotating anode means disposed within said center section
maintained at ground potential;
- cathode means maintained at high voltage and disposed within
said center section for generating a beam of electrons with sufficient
energy to generate x-rays;
- a window for transmission of said x-rays outside said envelope,
said window being constructed of beryllium; and
- shield means for deflecting secondary electrons back scattered
from said anode means away from said window to avoid secondary
electron bombardment of said envelope and, thereby, to prevent
overheating of said window, said shield means mounted to the
interior of said metal center section and having a contoured surface
to match high voltage field lines between said cathode means and
said anode means.
- 17. The x-ray generating tube of claim 16 further comprising
means for circulating fluid around said window and thereby cooling
said window.
- 18. A high power x-ray generating tube comprising:
- a vacuum envelope having a metal center section;
- anode means disposed within said center section for rotation
about an axis;
- cathode means maintained at high voltage and disposed within
said center section for generating a beam of electrons with sufficient
energy to generate x-rays;
- a window formed in said center section for transmission of
said x-rays outside said envelope, said window including insulating
means for providing a build up of negative charge on an inner
surface of said window to repel secondary electrons back scattered
from said anode to thereby reduce the amount of electron energy
bombarding said window to therefor reduce heating of said window.
- 19. The x-ray generating tube of claim 18 further comprising
a coating of conductive material on said window surface.
- 20. A high power x-ray generating tube comprising:
- a vacuum envelope having a metal center section;
- an anode structure disposed within said center section for
rotation about an axis;
- cathode means within said center section for generating a
beam of electrons with sufficient energy to generate x-rays;
- a portion of said center section including an inner wall and an
outer wall;
- a window formed in said portion of said center section for
transmission of said x-rays outside said envelope;
- a closed space for circulating liquid coolant within said closed
space such that heat generated by bombardment of said windows by
secondary electrons back scattered from said anode is conducted
through said inner wall and carried from said window by convection
to thereby cool said tube envelope.
- 21. The x-ray generating tube of claim 20 wherein said
portion of said center section comprises the entire center section.
- 22. The x-ray generating tube of claim 20 further comprising
an inner window in said inner wall and an outer window in said
outer wall generally parallel to said inner window and wherein said
portion of said center section is localized in the vicinity of said
windows such that said closed space is formed between said inner
window and said outer window.
- 23. A high power generating tube comprising:
- a vacuum envelope having a metal center section;
- rotating anode means disposed within said center section for
rotation about an axis;
- stationary cathode means disposed within said center section
and maintained at high voltage for generating a beam of electrons
with sufficient energy to generate x-rays;
- a window formed in said center section for transmission of
said x-rays outside said envelope; and
- means mounted to said center section for dissipating electrons
back scattered from said anode away from said means toward the
interior of said envelope.
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