Patent Publication Number: US-6661876-B2

Title: Mobile miniature X-ray source

Description:
Priority is claimed of U.S. Provisional Patent Application No. 60/308,637, filed Jul. 30, 2001. 
    
    
     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 π 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 &#39;562 patent, it is important to note that the foil is not a transmission type anode, but an electron window. With respect to the &#39;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&#39;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 lifetime of several tube parts. Thermal shock is especially destructive. Therefore, when operating at sufficient power, most X-ray tubes need to be cooled with a flowing liquid or forced air. The cooling effectiveness is limited primarily by the slow conduction of thermal energy through thick portions of the tube (e.g. the anode, in particular). Miniaturization reduces this problem to some extent, but cooling is still required for the inventions of U.S. Pat. No. 6,075,839 (cooling by oil, SF 6 , or forced air) and U.S. Pat. No. 6,044,130 which has exterior protrusions to aid in cooling by forced air. To obtain sufficient X-ray flux, all of the currently available X-ray tubes must be so large that they must be cooled. That is, a sufficiently powerful tube that is cooled only by ambient air is currently unavailable. 
     Another important feature is stability of the X-ray flux over the period of time required to calibrate the tube and measure the samples. This stability should be of the order of ±0.1%. Typical small high-voltage DC power supplies do not meet this criterion, and the resistivity of the tube can change over short periods of time. Thus, high-voltage stability presents a problem for mobile X-ray tubes. 
     Although X-ray tubes were first constructed over 100 years ago, no mobile X-ray tube is available for mobile applications such as those addressed by  109 Cd radioactive sources. This is surprising because so many types of X-ray instruments are in use in science and industry. There is clearly a long-felt need for mobile, electronic, X-ray tubes and instrumentation. 
     SUMMARY OF THE INVENTION 
     It has been recognized that it would be advantageous to develop a mobile, miniature x-ray source. In addition, it has been recognized that it would be advantageous to develop an x-ray source for field applications. In addition, it has been recognized that it would be advantageous to develop a low-power consumption x-ray source. In addition, it has been recognized that it would be advantageous to develop an x-ray source that is not radioactive. It has also been recognized that it would be advantageous to develop an x-ray source with improved life or durability. 
     The invention provides a mobile, miniature x-ray source with a low-power consumption cathode element for mobility, and/or an anode optic creating a field free region to prolong the life of the cathode element. The x-ray source includes an evacuated tube. An anode is disposed in the tube and includes a material to produce x-rays in response to impact of electrons. A cathode is disposed in the tube opposing the anode. An electric field is applied to the anode and cathode. The cathode includes a cathode element to produce electrons that are accelerated towards the anode in response to the electric field between the anode and the cathode. A power source is electrically coupled to the anode, the cathode, and the cathode element. The power source provides power for the cathode element, and provides the electric field between the anode and the cathode. 
     In accordance with a more detailed aspect of the present invention, the tube is configured to be both miniature and mobile. The tube can have a length less than approximately 3 inches, and a diameter or width less than approximately 1 inch. The cathode element can include a low-power consumption cathode element with a low power consumption less than approximately 1 watt. The power source can include a battery power source. 
     In accordance with another more detailed aspect of the present invention, the battery power source provides an electric field between the anode and the cathode of at least approximately 15 kilo-volts. 
     In accordance with another more detailed aspect of the present invention, a window can be disposed in the evacuated tube at the anode. The window can be aligned with a longitudinal axis of the evacuated tube to release x-rays substantially along the longitudinal axis. Alternatively, the window can be disposed in a side of the evacuated tube to release x-rays transverse to the longitudinal axis. 
     In accordance with another more detailed aspect of the present invention, a field-free region can be positioned at the anode to resist positive ion acceleration back towards the cathode element. The electrons can impact the anode and heat the anode, releasing residual gas molecules. In addition, the electrons can ionize the residual gas molecules to positive ions. Such ions would normally be accelerated back to the cathode and sputter-erode the cathode element. 
     In accordance with another more detailed aspect of the present invention, an anode tube can be disposed at the anode between the anode and the cathode, and electrically coupled to the anode so that the anode and the anode tube have the same electrical potential. The anode tube can create the field-free region. 
     In accordance with another more detailed aspect of the present invention, a cathode optic can be disposed proximate the cathode element. The cathode optic can including a plate with an aperture therein configured to allow electrons to pass through the aperture towards the anode. 
     Additional features and advantages of the invention will be apparent from the detailed description which follows, taken in conjunction with the accompanying drawings, which together illustrate, by way of example, features of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional side view of a mobile, miniature x-ray source in accordance with an embodiment of the present invention; 
     FIG. 1 b  is a cross-sectional schematic view of the x-ray source of FIG. 1; 
     FIG. 2 is a partial cross-sectional side view of the x-ray source of FIG. 1; 
     FIG. 3 is a partial cross-sectional side view of the x-ray source of FIG. 1; 
     FIG. 4 is a cross-sectional side view of another mobile, miniature x-ray source in accordance with an embodiment of the present invention; 
     FIG. 5 a  is a perspective view of a field emitter of a mobile, miniature x-ray source in accordance with an embodiment of the present invention; 
     FIG. 5 b  is an exploded view of the field emitter of FIG. 5 a ; and 
     FIG. 6 is a schematic view of a mobile, miniature x-ray source with an integral power source in accordance with an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     Reference will now be made to the exemplary embodiments illustrated in the drawings, and specific language will be used herein to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended. Alterations and further modifications of the inventive features illustrated herein, and additional applications of the principles of the inventions as illustrated herein, which would occur to one skilled in the relevant art and having possession of this disclosure, are to be considered within the scope of the invention. 
     As illustrated in FIGS. 1-3, a mobile, miniature x-ray source, indicated generally at  10 , in accordance with the present invention is shown. The x-ray source  10  advantageously includes a low power consumption cathode element suitable for use with a battery power source to allow the x-ray source to be mobile for field applications. In addition, the x-ray source  10  advantageously includes an anode optic to create a field-free region at the anode for resisting positive ion acceleration back towards the cathode element, to resist sputter-erosion of the cathode element and to increase the life of the cathode element. “Field applications”, such as X-ray fluorescence (XRF) of soil, water, metals, ores, well bores, etc., as well as diffraction and plating thickness measurements, are fields that can benefit from such an x-ray source  10 . 
     The x-ray source  10  includes an evacuated tube or bulb  14 . The x-ray source  10  can be a transmission-type x-ray source, and the tube  14  can be a transmission type x-ray tube, as shown. The tube  14  can include an elongated cylinder  16 , and in one aspect is formed of a ceramic material, such as aluminum oxide. Ceramic is believed to be superior to the traditionally used glass because of its dimensional stability and its ability to withstand higher voltages. To remove embedded gas, the ceramic is pre-treated by vacuum heating. Extensions  18  and  22  can be attached at opposite ends of the tube  14 . The extensions  18  and  22  can be formed of a metal material and brazed to the ceramic tube  14 . 
     A getter  26  or getter material is disposed in the tube  14 , and can be attached to the extension  22  to remove residual gasses in the tube after vacuum sealing. The getter  26  can be positioned in a field free position or region, as described in greater detail below. If high cleanliness standards are maintained and evacuation is performed properly, a getter may be unnecessary for tubes with thermionic emitters. The getter can be formed of ST 122/NCF, a Ti/Zr/V/Fe alloy. It can be activated by heating for a period of up to 24 hours. 
     As stated above, the x-ray source  10  advantageously is mobile and suited for field applications. The x-ray tube or bulb  14  advantageously has a length less than approximately 3 inches, and a diameter or width less than approximately 1 inch, to facilitate mobility and use in field applications. 
     An anode, indicated generally at  30 , and a cathode, indicated generally at  34 , are disposed in and/or form part of the tube  14 . The anode  30  and cathode  34  are disposed at opposite sides of the tube  14  opposing one another. An electric field is applied between the anode  30  and cathode  34 . The anode  30  can be grounded, as described below, while the cathode  30  can have a voltage applied thereto. The cathode can be held at a negative high voltage relative to the anode. Alternatively, the anode can be held at a positive high voltage, while the cathode is grounded. 
     As stated above, the cathode advantageously is a low power consumption cathode and includes a low-mass, low-power consumption cathode element or filament  38 . The cathode element  38  can be a thermionic emitter, such as a miniature coiled tungsten filament. The cathode element  38  produces electrons (indicated at  40  in FIG. 2) that are accelerated towards the anode  30  in response to the electric field between the anode  30  and the cathode  34 . The cathode element advantageously has a low power consumption that is intended herein to have a power consumption less than approximately 1 watt. The lower power consumption of the cathode element  38  allows the x-ray source  10  to be battery powered, and thus mobile. In addition, the cathode element advantageously has a low-mass less than approximately 100 micrograms. 
     A header or end cap  42  can be attached to the extension  18  to support the cathode element  38 . Pins or posts  46  can extend through the header or end cap  42 , and can support the cathode element  38  therebetween. High voltage wires  50  can be electrically coupled to the pins  46 , and thus the cathode element. 
     A potential of approximately 1 volt across the filament drives a current of about 200 mA, which raises the temperature to about 2300 C. This temperature is cool compared to most thermionic sources, but it provides sufficient electron emission for the intended applications of the x-ray tube. For example, only 20 μA are required to generate sufficient fluorescence from an alloy to saturate a semiconductor detector. Even higher emission efficiency is obtained if the tungsten cathode is coated with mixed oxides of alkaline earths (e.g. Cs, Ca, or Ba). They do, however, allow operation at temperatures as low as 1000 K. Such coated cathodes can still have a low mass as described above. 
     There are numerous advantages to this cool, coiled tungsten emitter compared to the conventional hot hairpin type. The cooler wire does not add as much heat, and this eliminates the need for an inconvenient cooling mechanism. The lower temperature reduces tungsten evaporation, so tungsten is not deposited on the anode, and the wire does not become thin and break. The cool tungsten coil, however, does not fall below the Langmuir limit, so space charge can accumulate between it and the Wehnelt optic or cathode optic, described below. 
     An end piece  52  can be disposed on the extension  22  at the anode  30 . The end piece  52  can form a window support structure. The extension  22  can be formed from kover while the end piece  52  can be formed of monel. A bore can be formed through the extension  22  and the end piece  52  through which the electrons  40  pass. 
     A window or target  54  is disposed at the anode  30  of the end piece  52  to produce x-rays (indicated at  58  in FIG. 2) in response to impact of electrons  40 . The window or target  54  can include an x-ray generating material, such as silver. The window or target  54  can be a sheet or layer of material disposed on the end of the anode  30 , such as a 2-μm-thick silver. When electrons  40  form the cathode  34  impact the window or target  54  characteristic silver x-ray emission  58  is largely of the same wavelengths as the popular  109 Cd radioactive x-ray sources. 
     A filter  62  can be used to remove low-energy Bremsstrahlung radiation. The filter  62  can be disposed at the anode  30  on the target material  54 . The filter  62  can include a filter material, such as beryllium. In addition, the filter can be a thin layer or sheet, such as 130 μm of beryllium. The filter  62  or material thereof can coat the window or target  54 . With such a configuration, silver L lines may be emitted, but they are absorbed after traveling a very short distance in air. It will be appreciated that additional filtering can be added after or instead of the beryllium. For example, one could use a balanced filter of the type described by U. W. Arndt and B. T. M. Willis in  Single Crystal Diffractometry,  Cambridge University Press, New York, 1966, p. 301. 
     The various components described above, such as the cylinder  16 , the extensions  18  and  22 , the end cap  42 , the end piece  52 , and the window or target  54  form the evacuated tube  14 . A shield  66  can be disposed around the tube  14  to provide electrical shielding and shielding from stray x-rays. The shield  66  can be electrically coupled to the anode  30  to provide a ground for the anode. In addition, the shield  66  can be metallic to be conductive and shield x-rays. The shield  66  can be a tubular or frusto-conical shell to allow insulation between the x-ray tube  14  and the shield while contacting the anode  20 . A space  70  between the shield  66  and the tube  14  can be potted with a potting compound, such as silicone rubber. In one aspect, the potting material has high thermal conductivity and can include high thermal conductivity materials, such as boron nitride. 
     The x-ray source  10  also advantageously includes a battery operated, high voltage power supply or battery power source, represented by  74 , electrically coupled to the anode  30 , the cathode  34 , and the cathode element  38 . The battery power source  74  provides power for the cathode element  38 , and the electric field between the anode  30  and the cathode  34 . The battery power source  74  and the low-power consumption cathode element  38  advantageously allow the x-ray source to be mobile for field applications. 
     In analytical applications, it is important to maintain a constant intensity of the x-ray emission. Therefore, a feature of the power supply is the stability that is maintained by feedback that is proportional to the emission current. Any drift in the resistivity of the tube is quickly neutralized by this means so that the tube current remains constant. The power supply can be similar to that described in U.S. Pat. No. 5,400,385, but in the present invention, the power supply is small and battery powered. 
     In addition, the x-ray source  10  advantageously includes an anode optic, indicated generally at  80 . The anode optic  80  is located in the x-ray tube  14  at the anode  30 , and creates a field free region to resist positive ion acceleration back towards the cathode element  38 . Although, the x-ray tube  14  is evacuated, and can include a getter  26 , the impact of electrons  40  on the window or target  54  can heat the anode  30 , causing the release of residual gas molecules. The electrons  40  from the cathode element  38 , in addition to impacting the window or target  54  to produce x-rays  58 , can also ionize the residual gas from the heated anode  30  to positive ions. Normally, such positive ions would be accelerated back to the cathode  34 , and can sputter-erode the cathode element  38 . Because the cathode element  38  is a low power consumption element, it can have a low mass. Thus, such sputter-erosion from the positive ions can significantly damage the cathode element, and detrimentally affect the life of the cathode element. The field free region created at the anode by the anode optic  80 , however, resists the acceleration of positive ions back towards the cathode element  38 , thus resisting sputter erosion of the cathode element, and improving the life of the cathode element and x-ray tube. 
     The anode optic  80  can include an elongated anode tube  84  disposed at the anode  30  and window or target  54 . One end of the elongated anode tube  84  can be in contact, or immediately adjacent to, the window or target  54 . The anode optic  80  and tube  84  are at the same electrical potential as the window or target  54  or the anode  30 . Thus, the anode tube  84  and anode  30  can be grounded. The field free region can be formed in a hollow of the tube. The tube  84  can be formed of silver, and can have an inner diameter of 1.6-mm. The anode optic  80  operates on the diverging beam of electrons  40  to focus them at the window or target  54 . The anode optic  80  can be focused by having the proper distance between its open end and the cathode. Focusing may be necessary to create a small spot where x-rays are emitted, and also to prevent stray electrons from striking the inside of the tube. If any stray electrons strike the inside of the tube, the resulting emission of x-rays is of the same wavelengths as those of the target, which is composed of the same material. The tube  84  should completely cover the extension  22  and the end piece  52 . As stated above, the tube  84  should extend or reach all the way to the window or target  54 , otherwise a halo of unwanted wavelengths can appear around the x-ray beam. 
     In one aspect, the anode tube  84  and the anode  30  can include the same material, or can be formed of the same material, to prevent contamination of the output spectrum. For example, the anode  30  and the anode tube  84  can be formed of silver, palladium, tungsten, rhodium, titanium, chromium, etc. 
     It will be appreciated that the anode optic  80  and the low-power consumption cathode element  38  work together to provide a mobile x-ray source. The lower-power consumption cathode element  38  allows for a battery power source, while the anode optic  80  resists untimely erosion of the low-power consumption cathode. 
     The x-ray source  10  also can include a cathode optic  90  disposed near the cathode  34 . The cathode optic  90  can include a disc disposed between the cathode  34  and anode  30 . An aperture  94  can be disposed in the disc and aligned along a path of travel between the cathode element  38  and the window or target  54 . An indentation can be formed in the disk and can surround the aperture. The disc can be formed of metal. The cathode optic  90  can be a type of Wehnelt optic, but its shape is the inverse of the reentrant Wehnelt (or IRW). The voltage of the cathode optic  90  can be independently controlled, but is kept at the cathode potential in the current configuration. The cathode optic  90  limits the divergence of the emitted electron stream sufficiently that the anode optic  80  or tube  84  can focus the electrons without the major aberrations present with the fully divergent beam. Although the coiled thermionic emitter is large compared to the hairpin type, the aperture of the cathode optic exposes an area of space charge that can be focused on the anode. In fact, this aperture and the aperture of the anode optic are at different electrical potentials, and they form an electrostatic lens. The electron beam focus at the anode is surprisingly tight. In addition, it is not necessary to center the filament in this configuration because the cathode optic positions the source of electrons with respect to the anode. 
     Without the anode and cathode optics  80  and  90 , the electron beam is weak and diffuse at the target. Only about 30% of the current emitted by the filament actually strikes the window. By contrast, if both the anode and cathode optics are present, more than 60% of the emission current strikes the anode target. What is more, the filament is imaged on the target with close to a 1:1 magnification. The result is emission of x-rays from a spot that has only a 0.3 mm diameter. This is far smaller than the size of typical x-ray sources. In addition, the x-rays are generated within the thin window so the distance between the point where the x-rays arise and the sample can be as short as a few millimeters. In another aspect, a Pierce-type electron gun can replace the cathode optic. The x-ray tube advantageously produces a sub-millimeter spot on the anode from which x-rays are emitted. In addition to being important for micro-XRF applications, a small X-ray source can be necessary for high-resolution imaging and for accurate crystallography. 
     An x-ray collimator  102  can be disposed on the end of the x-ray tube  14  at the anode  30  to direct x-rays in a desired direction. The collimator  102  can be disposed on the target  54  or filter  62 . The collimator  102  includes a bore therethrough aligned with the path from the cathode element  38  to the window or target  54 . The collimator  102  intercepts x-rays that exit at angles that are large relative to the window normal. The collimator  102  can be formed of silver to prevent the generation of unwanted x-ray wavelengths. The x-ray collimator  102  can be held at ground potential to avoid the possibility of electric shock to the operator of the device. 
     With the configuration described above, it has been found that the spectrum of the anode includes silver K α  and K β  lines, and surprisingly intense L lines, as well as Bremsstrahlung radiation extending to the acceleration potential and also to the lower limit. This low-energy limit is determined by absorption in the target anode and in the x-ray filter. The spectrum is responsive to the magnitude of the bias voltage. The intensity ratios of the K and L lines can be changed by the thickness of the target anode, which absorbs L lines better than K lines. The spectrum can also be adjusted by changing the material and thickness of the x-ray filter, or by the addition of the previously mentioned balanced filter. 
     As described above, the cathode element  38  can be a thermionic emitter. Other types of electron emitters also can be used. For example, referring to FIG. 5, a field emitter  120  can be used. The field emitter  120  can include a centering frame  124 . The centering frame  124  can be formed of ceramic. The centering frame  124  can be mounted onto the end cap or header  42 . A disc  128  can overlay the field emitter  120 . The disc  128  can include an aperture to allow transmission of electrons. The disc can be formed of stainless steel. The field emitter tips can be ruined if operated with a back-pressure of more than about 10 −8  mm Hg, so a getter is advisable in the x-ray tube. The cathode portion of the power supply can place about 80 volts on the field emitter gates. In contrast to the thermionic filament, the field emitter draws virtually no current other than that emitted toward the anode, and they do not heat other parts of the tube. What is more, while the high voltage remains on, the gate voltage can be switched or modulated to control electron emission and subsequent x-ray emission at very high frequencies. 
     As another example of an electron emitter, a ferroelectric solid can be provided to emit electrons. Again, the cathode portion of the power supply can be adapted. In this case, the power supply can provide pulses of higher voltage to the ferroelectric. 
     Other electron emitters can be used, including metal tip arrays, gate-modulated emitters either in arrays or field emitting surfaces, carbon nanotubes with or without modulating gates, heated lanthanum hexaboride (LaB6), etc. 
     As described above, the x-ray source  10  is configured to emit x-rays  58  along its longitudinal axis. The cathode element  38  and window or target  54  are aligned along a longitudinal axis of the x-ray tube  14 . The anode optic  80  and cathode optic  90  are similarly aligned along the longitudinal axis. 
     Referring to FIG. 4, another x-ray source  130  is shown that is similar in many respects to the x-ray source described above, but that is configured to emit x-rays  58  transverse to the longitudinal axis, or to emit laterally. The x-ray source  130  can include an anode  30   b  with a target or target material  134  that includes a more massive block of metal, such as copper for x-ray diffraction applications. Other electrically conductive targets could be used. The target  134  can have an angled surface  138  oriented at an acute or obtuse angle (as opposed to a right angle) with respect to the electron  40  path or longitudinal axis of the x-ray tube  14 . The angled surface  138  directs x-rays  58  laterally or transverse to the longitudinal axis. A window  142  can be form in or disposed at the anode  30   b , and positioned on a lateral side of the anode adjacent the angled surface  138  so that x-rays from the target pass through the window. The window can be formed of a material including beryllium. The window can be made of other materials, and it can be covered with a filter material, such as nickel that can provide a β filter for copper emissions. 
     The x-ray source  130  also can have an anode optic  80   b  to create a field free region as described above. The anode optic  80   b  can include an axial hole formed through the extension  22   b  and/or end piece. 
     The x-ray head can be connected to the high voltage power supply by flexible electrical cables to make it easy to maneuver the head, and to allow the head to fit into long, narrow spaces. An alternative is to build the head as an integral part of the power supply, making a single unit with no exposed cables. The head/power supply combination may be small enough for spaces of moderate size. Referring to FIG. 6, an integral x-ray unit  150  can include an x-ray source  10  or  130  embedded in a power supply  154  to provide a single unit, rather than a head connected by wires to a power supply. The power supply  154  can be electrically coupled to a battery. 
     To prolong the life of the tube, the following sequence is followed when turning the tube on: 
     1) a high voltage bias is applied; and 
     2) a filament current is ramped up over at least several milliseconds so the cathode structure does not receive a thermal shock. 
     A synergy is developed by the components of the present invention to provide improved performance. For example, typical transmission type tubes with thermionic emitters can have unacceptable window heating and serious voltage instabilities. The x-ray tube of the present invention, however, is stable and cool because of the interaction of the following features: 
     1) The conical end of the tube and the close proximity of the source of x-rays to the sample allow the tube to function at very low x-ray fluxes. 
     2) The anode window is thin, but with high thermal conductivity, and the window is sufficiently cooled by ambient air, thus avoiding the necessity of liquid cooling, forced air, fins on the tube exterior, etc. 
     3) Because the X-ray flux can be small, the cathode can be small, rugged, and it can be operated at an unusually low temperature so that its lifetime is significantly extended. 
     4) The typical fluctuations in the resistivity of transmission type tubes is compensated by using feedback from the emission current to adjust the filament current, thus providing a level of stability that is more than adequate. 
     The power supply should match the electron emitter in addition to proving a high DC bias voltage. For example, a thermionic emitter requires an electrical current to heat the filament. The field emitter needs 60 to 80 volts for electron extraction, and a ferroelectric source requires high voltage pulses. 
     Although ferroelectric electron emitters show promise, they are somewhat expensive and can be less reliable for mobile applications. A thermionic emitter has certain advantages such as immunity to background gas of pressure up to about 10 −5  mm Hg. 
     Thermionic emitters, however, require a power supply for the filament current, generate heat by the filament, and life of the filament is limited if operated at high temperature to increase electron emission. The lifetime in hours can be estimated well by t=32/ρ 0  where ρ 0  is the temperature-limited current density. In addition, filaments are extended sources, whereas the ideal electron source is a point source (or at least a very small source). Filament wire is typically bent into a “V” (the hairpin filament). Although this decreases the size of the emitting spot, it creates other problems. The stresses in the bend create hot spots near it, and filament metal evaporates more rapidly in these areas. The resistance of the filament rises, and the heater current will decrease correspondingly if the power supply maintains constant voltage. If the power supply is of the constant current variety, the thin spots in the filament become increasingly hotter and rapidly thinner, resulting in imminent failure. However, if the generator impedance (including the leads) is one third that of the wire, the filament will maintain the same temperature throughout its life. 
     As an alternative, one could use field emitters in place of a thermionic emitter. Although field emitters generate virtually no heat, they require the maintenance of substantially harder vacuum. Even then, they tend to become dull and inefficient due to erosion of the emitting surface by ions generated by the bombardment of residual gasses by the electron beam. All of the electron emitter types have limited lives, and the emitters are usually the cause of tube failure. Improvements are needed for the production of practical mobile X-ray tubes. 
     Possibly for thermionic emitters, and certainly for field emitters, a getter is needed to clean up the tube vacuum. Getters are typically metallic and they will distort the electric fields in the tube unless they are placed in a field-free region. 
     It is to be understood that the above-referenced arrangements are illustrative of the application for the principles of the present invention. Numerous modifications and alternative arrangements can be devised without departing from the spirit and scope of the present invention while the present invention has been shown in the drawings and described above in connection with the exemplary embodiments(s) of the invention. It will be apparent to those of ordinary skill in the art that numerous modifications can be made without departing from the principles and concepts of the invention as set forth in the claims.