X-ray or gamma ray systems or devices – Source – Electron tube
Reexamination Certificate
2001-01-16
2002-04-02
Porta, David P. (Department: 2882)
X-ray or gamma ray systems or devices
Source
Electron tube
C378S117000, C378S141000
Reexamination Certificate
active
06366642
ABSTRACT:
BACKGROUND OF THE INVENTION
1. The Field of the Invention
The present invention relates generally to x-ray devices. More particularly, embodiments of the present invention relate to an x-ray tube cooling system which includes features that serve to permit monitoring various coolant flow parameters, and thereby facilitate safe and reliable operation of the x-ray device.
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. For example, such equipment is commonly used in areas such as diagnostic and therapeutic radiology; semiconductor manufacture and fabrication; and materials analysis and testing. While used in a number of different applications, the basic operation of x-ray tubes is similar. In general, x-rays, or x-ray radiation, are produced when free electrons are generated, accelerated, and then impinged upon a material of a particular composition.
Typically, this process is carried out within a vacuum enclosure. Disposed within the evacuated enclosure is an electron source, or cathode, and a target anode, which is spaced apart from the cathode. In operation, electrical power is applied to a filament portion of the cathode, which causes electrons to be emitted. A high voltage potential is then imposed between the anode and the cathode, thereby causing the emitted electrons to rapidly accelerate towards a target surface positioned on the anode. The anode may be a stationary type anode, as is often employed in the context of analytical x-ray tubes, or a rotating type as is commonly employed in the context of diagnostic x-ray devices used in medical applications. During operation of an x-ray tube, the electrons in the beam strike the target surface (or focal track) at a high velocity. The target surface on the target anode is composed of a material having a high atomic or “Z” number, and a portion of the kinetic energy of the striking electron stream is thus converted to electromagnetic waves of very high frequency, i.e., x-rays. The resulting x-rays emanate from the target surface, and are then collimated through a window formed in the x-ray tube for penetration into an object, such as a patient's body, or material sample. As is well known, the x-rays can be used for therapeutic treatment, for x-ray medical diagnostic examination, or material analysis procedures.
In addition to stimulating the production of x-rays, the kinetic energy of the striking electron stream also causes a significant amount of heat to be produced in the target anode. As a result, the target anode typically experiences extremely high operating temperatures. At least some of the heat generated in the target anode is absorbed by other structures and components of the x-ray device as well.
A percentage of the electrons that strike the target surface do not generate x-rays, and instead simply rebound from the surface and then impact another “non-target” surfaces within the x-ray tube evacuated enclosure. These are often referred to as “secondary” electrons. These secondary electrons retain a significant amount of kinetic energy after rebounding, and when they impact these other non-target surfaces, a significant amount of heat is generated. This heat can ultimately damage the x-ray tube, and shorten its operational life. In particular, the heat produced by secondary electrons, in conjunction with the high temperatures present at the target anode, often reaches levels high enough to damage portions of the x-ray tube structure. For example, the joints and connection points between x-ray tube structures can be weakened when repeatedly subjected to such thermal stresses. In some instances, the resulting high temperatures can even melt portions of the x-ray tube, such as lead shielding disposed on the evacuated enclosure. Such conditions can shorten the operating life of the tube, affect its operating efficiency, and/or render it inoperable.
In view of the significant dangers posed by excessive heat levels in x-ray tubes and devices, various types of cooling systems have been devised to aid in the removal of heat from x-ray devices. For example, many conventional x-ray tube systems utilize some type of liquid cooling arrangement wherein a flow of coolant is generated and directed into contact with various surfaces and components of the x-ray tube so as to remove some of the heat generated there. The heated coolant is typically returned to an external cooling unit which removes heat from the coolant and then returns the coolant to the x-ray device. As discussed below, the configuration of the cooling system may vary somewhat depending on the type of x-ray device with which it is employed.
In the case of stationary anode type x-ray tubes, for example, the liquid coolant is typically injected, by way of a coolant injection nozzle, into a passage defined by the anode. The coolant absorbs heat from the anode and then exits the passage before returning to the external cooling unit.
The configuration of the cooling system is somewhat different in the context of typical rotating anode type x-ray tubes. In particular, many rotating anode x-ray tubes contain structures through which, or over which, a flow of coolant is directed. The coolant absorbs heat as it contacts these structures, and then ultimately returns to the external cooling unit.
It is well known that the ability of a coolant to remove heat is at least partially a function of the flow rate of that coolant. In particular, where two coolant streams are substantially equivalent in all other regards, a coolant stream characterized by a relatively higher flow rate will generally remove heat at a relatively higher rate than a coolant stream having a relatively lower flow rate.
Generally, the coolant flow rate in an x-ray tube cooling system is a function of the amount of heat produced by the x-ray device. Because the failure to maintain an adequate coolant flow rate may result in damage to the x-ray device, x-ray cooling systems using a liquid coolant are typically designed to ensure that a certain minimum of coolant flow rate is maintained. Various types of instrumentation and control systems have been devised and employed in conjunction with liquid cooling systems in attempt to ensure maintenance and/or verification of a minimum acceptable coolant flow rate. As discussed in detail below however, known devices and systems suffer from a variety of shortcomings.
In one known type of cooling system, a direct flow measuring device such as a turbine meter, plunger, or rotameter is included “in-line” in the coolant circuit. That is, the coolant must pass through the direct flow measuring device in order for the device to be effective in measuring the coolant flow rate. Typically, such direct flow measuring devices include an electrical switch or the like arranged so that upon achievement of a desired coolant flow rate through the device, contacts on the electrical switch close and complete a circuit. Generally, the circuit includes some type of visual indicator or the like to show that at least the minimally acceptable coolant flow rate has been established.
While direct flow measuring devices are generally effective in indicating coolant flow rates, they nevertheless suffer from some significant shortcomings. One such shortcoming relates to the fluid system energy losses imposed by such devices.
As is well known, the energy of a fluid system is often referred to as the “system head” and includes the energy represented by the velocity and pressure of the fluid in the system. In general, it is desirable to minimize losses in the energy of the system, or “head loss,” which would tend to compromise performance of the fluid system. As discussed below however, some head loss is unavoidable.
In particular, the system head is affected by a variety of factors. For example, friction between the fluid and the piping through which it passes tends to reduce the velocity of the fluid, and thus, the overall energy of the system. Further, by virtue of their geometry and othe
Barber Therese
Porta David P.
Varian Medical Systems Inc.
Workman & Nydegger & Seeley
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