X-ray or gamma ray systems or devices – Source – Electron tube
Reexamination Certificate
2002-07-29
2003-12-09
Dunn, Drew A. (Department: 2882)
X-ray or gamma ray systems or devices
Source
Electron tube
C378S119000, C378S102000, C378S123000, C313S553000
Reexamination Certificate
active
06661876
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to the production of an x-ray beam by electron impact on a metal target. More particularly, the present invention relates to a transmission type x-ray source that is mobile, miniature, with a configuration allowing placement of a sample close to the point where X rays are generated, with a configuration allowing close placement of a detector in XRF application, and with an electron optical element configuration that allows the generation of a small diameter spot as the source of X rays.
2. Related Art
In an X-ray tube, electrons emitted from a cathode source are attracted to an anode by the high bias voltage applied between these two electrodes. The intervening space must be evacuated to avoid electron slowing and scattering, but primarily to prevent ionization of containment gas and acceleration of the resulting ions to the cathode where they erode the filament and limit tube life. Characteristic and Bremsstrahlung X rays are generated by electron impact on the anode target material. Every material is relatively transparent to its own characteristic radiation, so if the target is thin, there may be strong emission from the surface of the target that is opposite the impacted surface. This arrangement is termed a transmission type X-ray tube. By comparison, a side-window tube has a thick anode in the vacuum space; and its X-ray emission passes from the tube via an X-ray transparent window placed in the side of the vacuum chamber. Each type has its advantages and disadvantages, depending upon the intended application.
Typical X-ray tubes are bulky and fragile, and must be energized by heavy, high-voltage power supplies that restrict mobility. Thus, samples must be collected and brought to the X-ray unit for analysis. This is very inconvenient for popular X-ray applications. Certain “field applications” include X-ray fluorescence (XRF) of soil, water, metals, ores, well bores, etc., as well as diffraction and plating thickness measurements.
A popular approach to portability of low power X-ray sources is the use of
109
Cd which emits silver K X-ray lines during radioactive decomposition. There are many such instrumental sources currently in use, and software has been developed to make XRF with the silver line sensitive and reliable. Unfortunately the intensity of emission from
109
Cd decays exponentially with a half-life of about 1.2 years. This necessitates frequent recalibration and eventual disposal of the
109
Cd. The size of such radioactive sources are 1 or 2 Curies, so a license is required for transportation and possession of this isotope in the quantities useful for XRF.
Miniature size X-ray tubes have been demonstrated for medical purposes. For example, see U.S. Pat. Nos. 5,729,583 and 6,134,300. The geometry, however, is wrong for analysis. Such tubes are designed to send X rays into at least &pgr; steradians for therapeutic reasons, rather than concentrating radiation in a spot that is easily accessed by a detector. Thus, such therapeutic X-ray tubes are inadequate for XRF work in the field because of the divergent beams. Another type of medical tube is a combination device where the X rays are for diagnostic purposes, i.e. tube placed in the body internally. Emitted X rays pass through tissue to film that is external to the body. This reveals the position of tumors or anatomic maladies. For example, see U.S. Pat. Nos. 5,010,562 and 5,117,829. With respect to the '562 patent, it is important to note that the foil is not a transmission type anode, but an electron window. With respect to the '829 patent, an interesting nozzle is shown, but the rest of the apparatus is large and inadequate for mobile field work.
Another type of x-ray tube includes a rod anode used for insertion into pipes and boilers for X-ray inspection. The metal anode rod is hollow from the point the electron beam enters to its opposite end, which is the target for the production of X rays. The whole rod structure is at the anode potential. A window in the side of the rod allows X rays to be emitted from the anode. To focus the electron beam on the target at the end, a magnetic coil is positioned along the rod. This electromagnet is heavy and requires considerable power from a large battery if it is to be portable. What is more, a long anode is of little value in typical analytical applications. Such rod-anode tubes are not of the transmission type.
To obtain a source of X rays that is of small diameter at the anode target of an X-ray tube, electrodes or apertures or both have been used in the tube. These are designed to focus the electron beam to a small spot on the target. One of these electrodes is termed a Wehnelt aperture. It is near the cathode, as is done in electron guns for microscopes. This seriously limits the electron flux. It is more important to limit the diameter of the electron beam where it strikes the anode, since this is the proximal source of X rays intended to strike a small portion of the analyte. This typically requires other electrodes. One type is a focusing electrode extending from the cathode region to approximately half way to the anode. This typically cylindrical tube reduces the distance between points of high and low voltage and it can lead to electrical breakdown in the tube.
An important feature of an X-ray tube used to excite X-ray fluorescence for elemental analysis is that the point where the X rays are generated be as close as possible to the sample being irradiated. This is necessary because the intensity of the X rays drops off in proportion to the reciprocal of the square of the distance from the target spot. It is a further advantage if the X-ray flux is focused to a small spot on the sample for reasons of spatial resolution, which allows analysis of discrete, small portions of a complex sample. In XRF, this X-ray beam is used to excite elements in the sample. They, in turn, fluoresce characteristic radiation in a Lambertian pattern, so XRF sensitivity is maximized if there is an angle of about 45° between the beam illuminating the analyte and the fluoresced X rays going to the detector. For generic X-ray tubes, the spot impacted by electrons is broad and blunt, so the detector must be placed to one side with an angle that is 90° or more instead of the desired 45°.
An object of the Treseder patent (U.S. Pat. No. 6,075,839) is to make the target accessible to the sample, but the exit window end of this invention is necessarily broadened (greater than 20 mm). In addition, the anode is seriously recessed from the window because the tube's electron gun is placed at the side of the anode instead of generally behind it. What is more, it is impossible to modify the Treseder design because the target must be well separated from the X-ray window to make room for the curvature of the electron beam. The result is a large distance between the target and the sample, as shown in FIG. 3 of that patent.
Another requirement for sensitive XRF is irradiation with the correct band of wavelengths for exciting the sample. Higher bias voltage not only increases X-ray flux, but it changes the spectrum of the output. The bias should be subject to selection by the operator, and this setting should be independent of the tube current setting. In general, the higher the X-ray flux (and corresponding tube current), the more sensitive and accurate will be the measurements, whether they are XRF, plating thickness, or diffraction. However, once the detector is saturated, additional power is of no use. The current of the electron beam should be adjusted to produce adequate but not excessive x-ray intensity.
For generic X-ray tubes, substantial cooling is required because upon electron impact, and less than 1% of the electron beam power is converted to X-ray power. The rest of the energy becomes heat in the target. Heat also arises from thermionic electron sources. The heat cannot be allowed to accumulate and raise the temperature of the tube because high temperature decreases the lifet
Lines Michael
Lund Mark W.
Moody Paul
Pew Hans K.
Reyes Arturo
Dunn Drew A.
Moxtek, Inc.
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