The present invention relates to x-ray tubes and is particularly related to an apparatus for use in improving the performance and/or extending the service life of an x-ray tube by selectively improving performance of gas pressure reducing materials in x-ray tubes installed in operating x-ray imaging systems. The present invention finds particular application in conjunction with activating a getter by depositing and/or actuating getter material surface area in x-ray tubes installed in operating imaging systems when improved getter pumping rates are desired. It will be appreciated, however, that the invention is applicable in other applications where it is necessary to improve getter pumping rate in a vacuum tube or envelope.
The useful life and performance of an x-ray tube are affected by the maintenance of an appropriate vacuum within the tube throughout its operative life. For this reason, creating and maintaining an adequate vacuum over the planned life of the x-ray tube is important. Contaminants that are present in an operating x-ray tube evolve into gases over the life of the tube during normal tube operation. These evolved gases reduce the vacuum level in the tube during its service life.
Production of a tube or envelope that can maintain an adequate vacuum during operation includes removing residual contaminants left on the tube components. During manufacture the tube components are cleaned and baked in vacuum furnaces. This procedure reduces the amount of surface contaminants available to evolve into gases when the tubes are in service.
The cleaned x-ray tube components are then assembled and placed within an envelope. The envelope is evacuated using a vacuum pump and sealed from the outside environment. During the pumping process the x-ray tube is heated to further reduce contaminants.
A getter material typically is located in the vacuum envelope. After the vacuum pump has established a vacuum, the getter material is activated. Depending on the type of material, getters are classified as either (i) evaporable or (ii) non-evaporable e.g, a bulk getter. Activating the getter material may include (i) flashing the getter for an evaporable material or (ii) actuating the getter by raising its temperature for a bulk or non-evaporable getter.
One method of activating (flashing) an evaporable getter is accomplished by locating a source for an electromagnetic field, usually an RF field, outside the evacuated envelope proximate to the getter coil located inside the envelope. The electromagnetic field is generated and couples with the getter coil, thereby inducing current flow in the coil and heating the getter material in contact with the coil. The heated getter material evaporates and atoms leaving the getter surface are deposited on interior surfaces of the envelope and other internal components. The freshly deposited getter material plated on the interior surfaces absorb additional amounts of residual gas molecules. However, once an x-ray tube is installed in a housing this method of flashing the getter is not easily accomplished. Another method of activating the getter by flashing includes applying electric current directly to the getter material via dedicated terminals. The getter is heated by resistance heating thereby raising the temperature of the getter material to that necessary to evaporate the getter.
In these types of getters, the getter film produced by flashing reacts with all the residual active gasses and, by chemisorption, removes them from the gas phase to further reduce the gas pressure within the evacuated envelope. However, the sorption capacity of the deposited getter material is limited. As sorption capacity is approached, the ability of the getter to sorb additional gas molecules is diminished. The reacted layer includes oxides and additional compounds that include the other evolved and absorbed gasses.
Non-evaporable, or bulk, getters are temperature activated and do not need to be evaporated to be activated. These types of getters are activated by supplying specified electric current levels to the getter causing resistance heating to elevate the getter material to a desired temperature. Once the getter reaches the desired temperature the specific gas molecules are absorbed into the getter. The vacuum in the x-ray tube may be getter pumped to the desired gas pressure level using either a flashed getter or a bulk getter.
The completed x-ray tube is then mounted into the housing or enclosure that is filled with dielectric oil. Once the oil filled housing containing the x-ray tube is installed in an operative x-ray system, the components continue to evolve gas into the evacuated envelope. The evolved gas is also produced from contaminants that migrate via diffusion through the tube components to their surfaces. These gases react with the getter material that was flashed during the original manufacturing process. As the tube continues to operate and contaminants are absorbed by the getter, sorption capacity of the getter material is reached. The getter material becomes less efficient in removing gas molecules and the gas pressure in the evacuated envelope increases.
When gas pressures within the evacuated region of the x-ray tube increase, the mean free path between gas molecules is reduced such that a chain reaction is more likely to occur when the gas molecules in the vacuum envelope are ionized by the high electric fields generated during normal tube operation. This chain reaction is called avalanche and is a form of arcing. An arc is an undesired surge of electrical current between two elements which are at a different electrical potential and typically occurs through the gas molecules present in the x-ray tube. In x-ray tubes, this tendency to arc often increases as the tube ages due to such factors as degradation of the vacuum within the tube due to the existence of additional undesired gas molecules. When the x-ray tube arcs, a current on the order of hundreds of amperes can flow between the cathode and the anode. Once an x-ray tube starts to arc an avalanche type effect may occur, sputtering metal and metal atoms as well as ionizing the contaminants in the vacuum. In addition, arcing in an x-ray tube used in a Computed Tomography (CT) imaging system contaminates the signal collected at the detectors and affects proper image reconstruction. This may result in an un-usable set of data requiring another CT scan of the patient.
Arcing typically occurs in the area of the x-ray tube having the highest electric field strength. As such, arcing in an x-ray tube may commonly occur in the same region as where the cathode is supplying the anode with electrons for the production of x-ray emissions. The sputtering of metal from the cathode produced during arcing often lands on the internal surface of the glass envelope in proximity to the cathode. The existence of the metal deposits on the glass envelope can deleteriously affect x-ray tube performance. Sputtering of metal within the envelope may as internal pressure increases, even in the absence of an arc.
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