X-ray or gamma ray systems or devices – Source – Target
Reexamination Certificate
1999-09-30
2002-05-21
Kim, Robert H. (Department: 2882)
X-ray or gamma ray systems or devices
Source
Target
C378S119000, C378S121000
Reexamination Certificate
active
06393099
ABSTRACT:
BACKGROUND OF THE INVENTION
1. The Field of the Invention
The present invention relates to stationary anode assemblies used in certain types of x-ray tubes. In particular, the present invention relates to a stationary target anode that improves the quality and intensity of the x-ray signal generated by the x-ray tube.
2. The Relevant Technology
X-ray producing devices are extremely valuable tools that are used in a wide variety of applications, both industrial and medical. Such equipment is commonly used in areas such as diagnostic and therapeutic radiology; semiconductor manufacture and fabrication; and materials testing.
The basic operation for producing x-rays in the equipment used in these different industries and applications is very similar. X-rays, or x-radiation, are produced when electrons are produced and released, accelerated, and then stopped abruptly. Typically, this entire process takes place in a vacuum formed within an x-ray generating tube. An x-ray tube ordinarily includes three primary elements: a cathode, which is the source of electrons; an anode, which is spaced apart from the cathode and oriented so as to receive electrons emitted by the cathode; and some mechanism for applying a high voltage for driving the electrons from the cathode to the anode.
The three elements are usually positioned within an evacuated tube, and connected within an electrical circuit. The electrical circuit is connected so that the voltage generation element can apply a very high voltage (ranging from about five thousand to in excess of hundreds of thousands of volts) between the anode (positive) and the cathode (negative). The high voltage differential causes a stream, or beam, of electrons to be emitted at a very high velocity from the cathode towards an anode target portion of the anode assembly. The anode target typically is comprised of a metal so that when the electrons strike the target, the kinetic energy of the striking electron beam is converted to electromagnetic waves of very high frequency, i.e., x-rays. The characteristics of the x-rays that are produced, for instance wavelength, depend on the type of metal used for the anode target material. Different metals will produce x-rays having different characteristics. The resulting x-rays emanate from the anode target, and are then collimated onto an object, such as an area of a patient's body or an industrial device. As is well known, the x-rays that pass through the object, or that fluoresce from the object, can be detected and analyzed so as to be used in any one of a number of applications, such as x-ray medical diagnostic examination or material analysis procedures.
In some x-ray devices, the anode target is positioned on a rotary disk that rotates during operation. Rotation of the anode target reduces the amount of heat present at a particular point on the target at any given time. Other x-ray tubes however, for example certain types used in devices for analytical work such as x-ray fluorescence and x-ray diffraction, use a stationary target anode assembly.
FIG. 1
illustrates one example of a portion of an x-ray tube device
8
that utilizes a stationary anode assembly
10
. The stationary anode assembly
10
includes an anode substrate
12
portion, and an anode target
14
that is affixed to the target end
16
of the substrate
12
by a brazing interface
18
or the like. X-ray tube device
8
also includes a cathode assembly, shown as comprising a shield
24
and a filament
25
. In operation, an electrical current is passed through the filament
25
, which heats up and then discharges a cloud of electrons. As noted, a large voltage potential is placed between the cathode and the anode, which causes the electrons to accelerate to extremely high speeds towards the anode. When the accelerating electrons impinge upon the surface
20
of anode target
14
, x-rays are produced, schematically represented at lines
26
. Preferably, the x-rays
26
are directed through a window
28
formed on the x-ray tube device
8
and towards an x-ray subject.
The generation of quality x-rays is dependent on several factors, including the type of materials used on the anode target
14
, and the physical orientation of the anode target with respect to the cathode. For example, the anode target layer
14
is made from a metallic material having a specific atomic number (Z), which is capable of efficiently generating x-rays when impinged with the high velocity electron stream. In contrast, the underlying anode substrate
12
portion is typically constructed of a different type of metal than the target. For example, copper is often used as a substrate. The selection of this substrate material is based upon several factors. First, its ability to efficiently conduct and dissipate the heat created at the anode target
14
as a result of the impinging electrons is important. Second, the substrate material used is often different from the target material due to the fact that target materials are typically very expensive, and are difficult to machine and manufacture. Thus, use of a different material in the substrate is usually more practical. However, use of a different material for the substrate can give rise to other problems. For instance, the substrate material will emit characteristic x-rays that are different from those emitted from the target. As such, if the anode substrate is impinged with electrons, it is typically a contaminating source of x-rays that can adversely interfere with the x-rays emitted from the target. The x-rays that are emitted from a substrate can be destructive in other ways as well. For instance, in an x-ray fluorescence device, x-rays must be produced from an anode target material that is different from the type of material being analyzed, or the resulting analysis would be inconclusive. Thus, if the substrate material is the same as the material being analyzed, any x-rays generated at the substrate would be destructive.
Generating x-rays that have a specific and consistent wavelength and intensity also requires that the cathode be oriented with respect to the anode target
14
in an appropriate manner. For instance, the filament
25
must be positioned relative to the anode assembly
10
in a manner so that the electrons within the electron stream strike the anode target and thereby generate x-rays. At the same time, the distance (denoted as “s” in
FIG. 1
) between the stationary anode assembly
10
and the cathode shield
24
, must be large enough to prevent an electrical short from occurring between the anode and the cathode.
Attempts to maintain an optional distance “s” may, however, give rise to other circumstances that can adversely affect the quality of the x-rays generated. For instance, some electrons from the electron stream
22
may have a primary impact upon face
20
of anode target
14
without producing any x-rays. These electrons can then rebound from face
20
of the anode target layer
14
and result in a secondary electron stream (designated at
30
in
FIG. 1
) that can impinge upon the anode substrate
12
portion of the anode. As noted, the substrate material that is used to construct the anode substrate
12
is a contaminating source of x-rays. As such, this secondary impact stream may result in the production of an errant x-ray beam (denoted at
32
in FIG.
1
), the characteristics of which are often significantly different from the primary x-ray beam
26
. As noted, the interaction between the errant beam and the primary beam can adversely affect the quality, the intensity and the focusing of the x-rays that are ultimately produced and released by the x-ray tube device
8
, which can ultimately affect the quality of any resulting analyses obtained via the x-rays.
Preventing the formation of such errant x-ray signals has proven difficult. One approach has been to reduce the size of the x-ray device window to prevent the errant x-ray signal from exiting the device and interfering with the primary x-ray signal. However, this approach may also limit the amount of primary x-rays that
Kim Robert H.
Song Hoon K.
Varian Medical Systems Inc.
Workman & Nydegger & Seeley
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