Patent Grant
7949099
The present invention relates to systems and methods for providing compact X-ray sources for use in field portable or hand-held x-ray analytical instruments, and relates in particular to the design and construction of low power high voltage x-ray sources for use in field portable or hand-held x-ray analytical instruments.
Interest in the measurement of material properties using x-ray techniques has resulted in the development of compact, low power consumption x-ray sources for portable x-ray analytical instruments. Examples of such instruments are the hand-held x-ray fluorescence analyzers currently available from companies such as ThermoFisher Scientific Inc., Niton Analyzers, of Billerica, Mass., InnovX Systems of Woburn, Mass., and Oxford Instruments Company of Oxon, United Kingdom. In such conventional systems, however, the voltages of the x-ray sources have been generally limited because of the size requirements for the x-ray tube and the high voltage power supply, as well as the associated electrical insulation and radiation shielding requirements.
For example, as shown in
During use, heater power is supplied to the cathode electron emitter 34, and a high voltage (e.g., 30-50 kV) is applied between the cathode end 18 and the anode end 16. The electric field produced by the applied high voltage accelerates electrons from the electron emitter through the vacuum to the x-ray producing target 24. The intensity of the x-rays produced at the target increases with increasing high voltage, electron beam current, and atomic weight of the target material. A portion of the x-rays produced in the target exit the tube via the x-ray transmission window 26, and exit the housing 12 via the x-ray output region 14 of the housing 12. The high voltage at the cathode end is typically provided as a negative high voltage (e.g., −50 kV) and the voltage potential at the anode end is typically provided at a reference ground potential of the system. This permits the anode end 16 of the tube 10 to be coupled directly to the housing 12. The x-ray tube 10 may be packaged in a hand held device that includes a high voltage power supply and a power source to drive the electron emitter.
For fixed values of the high voltage and electron current, the intensity of the x-rays at a location outside the x-ray tube decreases rapidly with increasing distance to the x-ray producing target. The x-ray intensity may be further reduced by the presence of intervening materials that scatter or absorb x-rays. Therefore, in order to maximize x-ray intensity at a given location, it is advantageous to minimize the distance from a sample or detector to the x-ray producing target and to eliminate to the extent possible any materials that scatter or absorb x-rays from the x-ray path. For these reasons, the x-ray producing target is placed as close as possible to the x-ray transmission window, and the x-ray transmission window is generally provided at an exterior surface of the housing at the output region. For example, the x-ray producing target and x-ray transmission window may be provided at a protruding portion or nose of a hand-held device, a portion of an example of which is shown in at 12
The accurate identification and quantification of elements at depths within certain materials, as well as the identification of certain heavy elements (e.g., lead and cadmium), generally requires the use of higher voltage sources (e.g., 80 to 150 kV) for x-ray production. Increasing the voltage level of the high voltage, however, generally requires that the length and diameter of the x-ray tube be increased in order to provide sufficient high voltage insulation between the anode and cathode conductors inside the vacuum envelope of the x-ray tube. Increased x-ray tube size therefore, requires an increase in the size of the hand-held x-ray inspection device. Further, providing sufficient electrical insulation between the housing and electrodes at significantly higher voltages also requires larger distances and thicker insulation. The doubling of the voltage level of a 50 kV tube, therefore, requires a substantial increase in size of a hand-held device that includes the higher voltage x-ray tube.
There remains a need, therefore, for a high voltage hand-held x-ray inspection device that is small-scale (uses a miniature x-ray source), yet is capable of operating in the range of approximately up to, for example, 150 kV.
A general object of the present invention is to provide a compact, self-shielded x-ray source for applications in which small size, low weight, and low power consumption are important.
Another object of the invention is to provide a miniature x-ray tube for use in hand-held or field-portable x-ray analytical instruments.
Another object of the invention is to provide a miniature x-ray tube and power supply module that is capable of operating at voltages up to 120 kV to 150 kV for use in hand-held or field-portable x-ray analytical instruments.
A further object of the invention is to provide a miniature x-ray tube and power supply module for use in hand-held XRF analyzers for the detection of lead in paint, solder, or other industrial materials.
A further object of the invention is to provide a miniature x-ray tube and power supply module for use in hand-held or field-portable XRF analyzers for the in vivo detection of lead in bone.
A further object of the invention is to provide a miniature x-ray tube and power supply module for use in hand-held x-ray imaging systems for security and medical applications.
In accordance with various embodiments, the invention provides an x-ray system that includes a bipolar x-ray tube. The bipolar x-ray tube includes two insulators that are separated by an intermediate electrode in an embodiment, wherein each insulator forms a portion of an outer wall of a vacuum envelope of the bipolar x-ray tube surrounding at least a portion of a path of an electron beam within the vacuum envelope. In further embodiments, the bipolar x-ray tube includes a first electrode at a positive high voltage potential relative to a reference potential, a second electrode at a negative high voltage potential relative to the reference potential, and an x-ray transmissive window that is at the positive high voltage potential.
In accordance with further embodiments, the invention provides an x-ray system that includes a housing, an x-ray tube, and an insulating region. The housing is at a reference potential, and the x-ray tube has an anode at a positive high voltage potential relative to the reference potential, and an x-ray transmissive window at the positive high voltage potential. The insulating region between the x-ray transmissive window and the housing, is electrically insulating and transmissive to x-rays.
In accordance with further embodiments, the invention provides an x-ray system that includes a bipolar x-ray tube with an anode and a cathode, a bipolar power supply for providing a positive high voltage potential relative to a reference potential and a negative high voltage potential relative to the reference potential, and a solid, electrically insulating material that encapsulates at least the cathode of the bipolar x-ray tube and the bipolar power supply.
In accordance with further embodiments, the invention provides an x-ray system that includes a bipolar x-ray tube, a bipolar power supply, a housing, and a passive cooling system. The bipolar x-ray tube includes an anode for receiving a positive high voltage potential with respect to a reference potential, a cathode for receiving a negative high voltage potential with respect to the reference potential, and an x-ray transmissive window. The bipolar power supply provides the positive high voltage potential relative to the reference potential and the negative high voltage potential relative to the reference potential. The housing is at the reference potential, and includes the bipolar x-ray tube and an x-ray output region that is aligned with the x-ray transmissive window of the x-ray tube. The passive cooling system is between the bipolar x-ray tube and the housing, and is for sufficiently cooling the bipolar x-ray tube during use. The passive cooling system may comprise a solid or a fluid.
In accordance with further embodiments, the invention provides a method of producing x-rays in a low power x-ray system. The method includes the steps of providing a positive high voltage potential relative to a reference potential to an anode of a bipolar x-ray tube, providing a negative high voltage potential relative to the reference potential to a cathode of the bipolar x-ray tube such that a difference voltage between the positive high voltage potential and the negative high voltage potential is employed between the anode and the cathode in the bipolar x-ray tube to cause electrons to impinge upon a target within the anode at an electron beam power of less than about 10 Watts and to thereby emit the x-rays through an x-ray transmission window of the bipolar x-ray tube, and emitting x-rays through an x-ray output region of a housing that includes the bipolar x-ray tube, wherein the x-ray output region is substantially aligned with the x-ray transmissive window of the bipolar x-ray tube.
The following description may be further understood with reference to the accompanying drawings in which:
The drawings are shown for illustrative purposes only, and are not to scale.
It has been discovered that a bipolar x-ray tube may be used in hand-held x-ray systems. Electrically insulating the high voltages of an x-ray tube from the typically grounded housing of hand-held x-ray sources is commonly achieved by maintaining the cathode at a negative high voltage potential within an electrically insulated portion of a source housing, while the anode (typically at system ground reference potential) is adjacent an output region of the housing.
Although bi-polar x-ray tubes generally use a positive high voltage potential in addition to a negative high voltage potential, it has been found that the high voltage potentials of a bipolar x-ray tube may be sufficiently electrically insulated within a hand-held source yet also produce sufficient output of x-rays through an x-ray output region of the device, and provide significantly higher x-ray energies than are possible with single polarity x-ray tubes.
As shown, in
Within the vacuum, electrons are emitted along a path 76 and pass through an intermediate shroud 78 of the intermediate electrode 56. The intermediate electrode 56 also includes an intermediate conductor 80 as well as an intermediate shield 82, which may be formed of a material such as tungsten, stainless steel, copper, lead or brass.
The anode 54 is maintained at a positive high voltage potential, and includes an x-ray producing target 84 within an anode hood 86, and an x-ray transmission window 88. The anode 54 also includes a positive high voltage shield 90 formed, for example, of tungsten, stainless steel, copper, brass, or lead.
The miniature bipolar x-ray tube may be, for example, between about 2 to 4 inches in length (from the pinch-off tube 74 to the far end of the anode 54), and the tube itself may be about 0.2 to about 0.5 inches in diameter, and is preferably about 0.3 inches in diameter as shown at A in
The intermediate electrode may be maintained at a voltage substantially half-way between the cathode and anode potentials, e.g., ground reference potential. As discussed in more detail below, the invention further provides a bipolar high voltage power supply connected to the x-ray tube, and that the x-ray tube, power supply and connection means are encapsulated in an electrically insulating material and enclosed in an electrically conducting sheath maintained at substantially ground reference potential. In certain embodiments, selected regions of the electrically insulating material may also contain x-ray shielding material. In accordance with other embodiments, the intermediate electrode may be omitted from a bipolar tube, using the positive and negative high voltage potentials at the anode and cathode respectively.
The embodiment of
The electron beam is generated by the electron emitter at cathode potential and accelerated to the x-ray emitting target at anode potential. In traversing the region between the cathode and anode conductors, the electron beam passes through the intermediate electrode, which is maintained at local reference ground potential. The total beam energy when it reaches the anode is the electron charge e multiplied by the total voltage change from the cathode to the anode. In the embodiment shown in
The intermediate electrode 56 provides a benefit that the positive and negative regions of the tube are decoupled along the external and internal surfaces of the insulator, thereby reducing the probability of a full voltage arc along the insulated length of the tube. The positive and negative triple points where the two insulators join the intermediate conductor 80 are shielded by the intermediate shield 82 on the outside of the tube and by the intermediate shroud 78 on the vacuum side. Similarly, the triple points where the insulator sections 58 and 60 join the cathode and anode conductors are shielded by negative and positive high voltage shields 70 and 90 respectively on the outside of the tube and by the cathode shroud 68 and anode hood 86 on the vacuum side.
The intermediate shield 82 and the negative and positive high voltage shields 70 and 90 respectively may also provide additional x-ray shielding in the radial direction. The negative high voltage shield 70 may also provide x-ray shielding in the backwards axial direction, and the positive high voltage shield 90 may provide collimation of the x-ray beam in the forward axial direction. For this reason, the intermediate, negative, and positive shields may be made from a high atomic weight material such as tungsten, copper, brass, lead or other heavy metals.
The intermediate shroud 78 is configured as a conducting tube with apertures at either end. The length and diameter of the tube and apertures are chosen so as to provide a clear path for the accelerated electron beam while also helping to prevent stray ions or electrons produced in one half of the x-ray tube from reaching the other half. If the length of the intermediate conductor is significantly longer than its diameter, the region inside the conductor will be a region of low electric field and stray particles with large transverse velocity relative to their velocity along the axis of the tube will be collected on the walls of the tube with high probability. In this way, for example, secondary ions formed in the region of the x-ray tube surrounded by insulator 60 will be impeded from reaching the region of the x-ray tube surrounded by insulator 58, and secondary electrons produced in the latter region will be impeded from traveling to the former. This internal configuration helps to prevent the formation of discharges within the vacuum envelope.
Electrons produced at the cathode emitter travel trough the intermediate shroud 78 to the x-ray producing target 84. In this embodiment, the target is a thin coating of a selected material applied to the surface of the x-ray window. A portion of the x-rays produced in the target coating pass through the window 88 in the forward direction. Coating materials may include silver, gold, tungsten, rhenium or other metals and x-ray window materials may include beryllium, beryllium oxide, aluminum and other light materials. The anode hood 86 serves to prevent x-rays and stray electrons or ions from reaching the insulator surface and initiating high voltage breakdown.
With reference to
A feedback circuit may also be provided that maintains the positive and negative high voltage potentials at the desired levels, and the feedback circuit may include a voltage divider circuit including resistors 124, 126 for the positive high voltage output, and resistors 128, 131 for the negative high voltage output, each of which is coupled to a feedback controller as shown at 132. A feedback circuit may also be included (not shown) for stabilizing the electron beam current collected at the anode as is well known in the art.
The bipolar high voltage DC power supply therefore comprises two independently-controlled high frequency voltage multiplier circuits, each configured to reach a voltage corresponding to approximately half of the final electron beam energy in the x-ray tube. Examples of such multiplier circuits are cascade multipliers or Cockroft Walton voltage multipliers. A filament isolation transformer provides power to electron emitter, which may be a high temperature filament, or an oxide-coated or dispenser cathode. X-ray tube current is measured using a current sense resistor and high voltage is measured using a voltage divider resistor. In certain embodiments, an insulating encapsulant may surround the high voltage power supply, and the encapsulant may not contain x-ray shielding material, except in the regions adjacent to the bipolar x-ray tube. In other embodiments the high voltage insulation may be provided by an insulating liquid such as Fluorinert or oil.
The outputs of the voltage multipliers 104 and 108 are provided to the anode and cathode electrodes 114, 118 via series resistors 112 and 116 respectively as discussed above. The feedback circuit discussed above may be included with the voltage multipliers 104 and 108, and power is applied into the grounded housing 126 and the components therein via a power cable 128. Power may be provided by a battery, alternating currently supply, portable generator, solar cell or other source of electricity together with a local oscillator (not shown in
As further shown in
The region between the x-ray output aperture 130 and the x-ray transmissive window 132 must provide electrical insulation between the anode 114 at the positive high voltage potential an the housing 126 at the reference ground potential while being highly transparent to the x-rays emitted through the x-ray transmissive window 132. In certain embodiments, the region between the x-ray transmissive window 132 and the x-ray output aperture of the housing 130 may be filled with the same material that fills the remainder of the interior region 129 (as shown in
For example,
As shown in
In accordance with a further embodiment as shown in
The system also includes two insulators 166 and 168 on either side of an intermediate electrode 170 that is coupled to a system reference ground. The embodiment of
In the embodiment of
The x-ray transmission interface 172 may be filled with encapsulating material that is left free from x-ray shielding material, thus allowing the x-rays to pass to the outside of the module with minimal attenuation and scattering. The thickness of this region is kept as small as possible to permit efficient transmission of x-rays. This thickness is typically less than 0.5 inches thick and preferably between 0.1 and 0.3 inches thick. This shielding-free channel provides collimation of the x-ray beam, and the shape of this region may be chosen to provide the desired x-ray beam spatial profile as discussed above with reference to FIGS. 6 and 7A-7C. In accordance with other embodiments, if attenuation and scattering of the x-ray beam in the encapsulant material is an issue, the x-ray transmission interface 172 between the x-ray tube window and the outer surface of the encapsulant may be filled with sulfur hexafluoride gas, either pressurized or at atmospheric pressure. Sulfur hexafluoride gas is preferred for certain applications because it is an excellent electrical insulator and because its high molecular weight makes it easy to contain in a sealed cavity.
In accordance with a further embodiment as shown in
Within the vacuum, electrons are emitted along a path 226 and pass through an intermediate shroud 228 of the intermediate electrode 206. The intermediate electrode 206 also includes an intermediate conductor 230 as well as an intermediate shield 232, which may be formed of a high atomic weight material such as tungsten, stainless steel, copper, brass, lead or other heavy metal.
The anode 202 is maintained at a positive high voltage potential, and includes an x-ray producing target 234 within an anode hood 236 and an x-ray transmission window 238. The anode 202 also includes a positive high voltage shield 240 formed, for example, of a tungsten, stainless steel, copper, brass, or lead.
The miniature bipolar x-ray tube may be, for example, between about 2 to 4 inches in length (from the pinch-off tube 224 to the far end of the anode 202), and the tube itself may be about 0.2 to about 0.5 inches in diameter, and is preferably about 0.3 inches in diameter as shown at B in
As further shown in
Similar to the embodiment of
Electrons produced at the cathode emitter 212 travel through the intermediate shroud 230 to the x-ray producing target 234. The x-ray producing target may be a solid piece of target material or a thin layer of target material applied to a substrate and disposed at an angle to the direction of the electron beam path. In this embodiment, a portion of the x-rays produced in the target 234 impinge on the x-ray transmissive window 238. The portion of the x-rays that reach the window 238 are passed out of the bipolar x-ray tube through an x-ray output transmission interface 262 disposed between the x-ray transmissive window 238 and the housing 250 (shown in
In accordance with a further embodiment shown in
An optional intermediate electrode 326 may be included between ceramic insulators 327 and 328, and may be coupled to a system reference ground. The system of
As shown in
In the example of
The positive high voltage potential and the negative high voltage potential may be provided as discussed above in connection with each of the previous embodiments, employing step up transformers and voltage multipliers. The power source may be provided by battery or an alternating currently supply, together with a local oscillator as is well known in the art. The housing 368 and 418 of the embodiments of
Those skilled in the art will appreciate that numerous modifications and variations may be made to the above disclosed embodiments without departing from the spirit and scope of the invention.
This application claims priority to U.S. Provisional Patent Application Ser. No. 60/948,111 filed Jul. 5, 2007.
| Number | Name | Date | Kind |
|---|---|---|---|
| 2871402 | Lempert | Jan 1959 | A |
| 5781609 | Gerling | Jul 1998 | A |
| 5978447 | Carlson et al. | Nov 1999 | A |
| 6111934 | Foerst et al. | Aug 2000 | A |
| 6128367 | Foerst et al. | Oct 2000 | A |
| 6141401 | Gerling | Oct 2000 | A |
| 6882703 | Price et al. | Apr 2005 | B2 |
| 7218707 | Holm | May 2007 | B2 |
| 20030210764 | Tekletsadik et al. | Nov 2003 | A1 |
| 20070121788 | Mildner et al. | May 2007 | A1 |
| 20070237298 | Sundaram | Oct 2007 | A1 |
| 20080049902 | Holm et al. | Feb 2008 | A1 |
| 20080079525 | Sundaram | Apr 2008 | A1 |
| 20080095317 | Lemaitre | Apr 2008 | A1 |
| Number | Date | Country |
|---|---|---|
| 0488991 | Jun 1992 | EP |
| 0924742 | Jun 1999 | EP |
| Number | Date | Country | |
|---|---|---|---|
| 20090010393 A1 | Jan 2009 | US |
| Number | Date | Country | |
|---|---|---|---|
| 60948111 | Jul 2007 | US |
