X-ray producing device with reduced shielding

A method for reducing the amount of shielding used in radiation sources, as well as, an improved radiation source (e.g., an x-ray producing device), are provided. The inventive method involves placement of a radiation producing target at the end of a vacuum drift tube and closer to and substantially in the center of a shield for blocking radiation emitted from the radiation producing target.

FIELD OF THE INVENTION

The present invention relates generally to a method for reducing the amount of shielding used in radiation sources such as x-ray producing devices, and further relates to an improved radiation source where the weight of the shielding has been minimized.

BACKGROUND AND SUMMARY OF THE INVENTION

X-ray producing devices are extremely effective and valuable tools that are used in a wide variety of industrial and medical applications. While used in a number of different applications, the basic operation of these devices is similar.

Generally speaking, x-rays are produced when electrons are accelerated and then impinged upon a material of a particular composition. This process is typically carried out within a vacuum enclosure formed as part of the x-ray producing device. Disposed within the evacuated enclosure is an electron generator (i.e., cathode), and an anode, which is spaced apart from the cathode. In operation, electrical power is applied to a filament portion of the cathode, causing electrons to be emitted. A high voltage potential is placed between the cathode and the anode, causing the emitted electrons to accelerate towards a target surface on the anode. Typically, the electrons are “focused” into an electron beam towards a desired “focal spot” located on the target surface.

During operation of the x-ray producing device, the electrons in the beam strike the target surface at a high velocity. The target surface on the target anode is composed of a material having a high atomic number, and a small portion of the kinetic energy of the striking electron stream is thus converted to x-rays, which are electromagnetic waves of very high frequency. The resulting x-rays, which emanate from the target surface in all directions, are blocked using heavy metal shielding and collimated through a window formed in the shielding for penetration into an object.

By way of the present invention, it has been discovered that the amount of heavy metal shielding used in radiation sources such as x-ray producing devices may be reduced by placing the radiation producing target closer to and substantially in the center of shielding adopting, for example, a spherical or substantially spherical geometry. The relocation of the target in these sources or devices is made possible by the use of a vacuum drift tube.

The present invention therefore generally provides a method for reducing the amount of shielding used in radiation sources such as x-ray producing devices, which basically comprises: placing a radiation producing target at the end of a vacuum drift tube and substantially in the center of a shield for blocking radiation emitted from the target.

The present invention further generally provides a radiation source such as an x-ray producing device, which basically comprises: a vacuum drift tube; a radiation producing target; and a shield for blocking radiation emitted from the radiation producing target, wherein the radiation producing target is located at the end of the drift tube and substantially in the center of the shield.

The present invention more particularly provides an improved x-ray producing device, which comprises:(1) an electron accelerator structure defining an electron flow path and having an electron injection end and an electron exit end;(2) an electron gun having an electron source, which is located at the injection end of the electron accelerator structure, for producing and delivering a stream of electrons to the accelerator structure;(3) a vacuum drift tube having a first end and a second end, wherein the first end of the drift tube is located at the electron exit end of the accelerator structure;(4) a target located at the second end of the vacuum drift tube for producing x-rays from electrons striking a surface of the target; and(5) a shield located around the drift tube for blocking x-rays emitted from the target, wherein the shield has one or more openings for forming an x-ray beam having a pre-selected cross section from the x-rays emitted from the target,wherein, the target is located substantially at the center of the shield.

The present invention further provides a radiation (e.g., x-ray) inspection or imaging system that employs the radiation source described above. The inventive system is a lighter weight system and thus particularly advantageous for portable or mobile system applications.

Other features and advantages of the invention will be apparent to one of ordinary skill from the following detailed description and drawings.

BEST MODE FOR CARRYING OUT THE INVENTION

By way of the present invention, shielding is moved closer to a target used in a radiation source or x-ray producing device resulting in a reduction in the size and overall weight of the shielding. The shielding weight reduction achieved by way of this invention results in a reduction in the total weight of the radiation source or x-ray producing device that ranges from about 30 to about 50%. As such, the inventive device is particularly suitable for use in portable or mobile inspection or imaging systems.

Referring now toFIG. 1, a preferred embodiment of the improved x-ray producing device of the present invention is shown generally at10. Device10basically comprises:(1) a shielded electron accelerator structure12defining an electron flow path and having an electron injection end14and an electron exit end16;(2) a shielded electron gun18having an electron source, which is located at the injection end14of the electron accelerator structure12, for producing and delivering a stream of electrons to accelerator structure12;(3) a vacuum drift tube20having a first end22and a second end24, wherein the first end22of the drift tube20is located at the electron exit end16of the accelerator structure12;(4) a shield26located around the drift tube20for blocking x-rays emitted from a target28, wherein the shield26has one or more openings (not shown) for forming an x-ray beam having a pre-selected cross section from the x-rays emitted from target28; and(5) target28located at the second end24of the drift tube20and substantially at the center of the shield26, for producing x-rays from electrons striking a surface of target28.

The improved x-ray producing device10of the present invention operates as follows: The electron gun18produces a beam of energetic particles or electrons directed toward the electron accelerator structure12, which in turn accelerates these particles toward target28located at the second end24of drift tube20. The electrons in the beam strike the surface of target28, causing x-rays to be emitted from a side of target28opposite from the electron collision. The emitted x-rays are blocked by shield26and collimated through the one or more openings in shield26for penetration into an object.

The electron accelerator structure12of the improved x-ray producing device10of the present invention is known and, in one embodiment, is an elongate accelerator structure that defines a linear electron flow path. Such an accelerator structure is generally made up of two basic sections, namely, a coupler section, and an accelerator section. The coupler section is a device that serves to transmit microwave power into the accelerator section. The accelerator section is composed of a series of identical cavities in which the transmitted microwave power is used to accelerate an electron beam. The cavities are brazed together to establish good electrical contact for the flow of microwave current and to provide an ultra-high vacuum seal.

In a preferred embodiment, which is best shown inFIG. 2A, the coupler section of accelerator structure12is made up of a series of in line couplers30instead of side couplers32, which are shown inFIG. 2B. More specifically, in the preferred embodiment shown inFIG. 2A, accelerator structure12is provided with in line “pancake” couplers30that are located in the walls between the cavities in the accelerator section. Electron accelerator structures employing such an in line coupler design are described in S. O. Schriber,IEEE Transactions on Nuclear Science, Vol. NS-22, page 1343 (June 1975), which is incorporated herein by reference.

Referring now toFIGS. 3A and 3B, a comparison of the x-ray producing devices shown in these figures, which employ either a side coupled or in line coupled accelerator12, will indicate that less shielding material (e.g., lead, heavy alloy) is needed when the accelerator design contains in line couplers, due to the obvious decrease in the outside diameter of accelerator structure12.

The electron gun18of the improved x-ray producing device10of the present invention is also known and, in one embodiment, is a triode gun that produces a pulsed electron beam and comprises an electron source (e.g., cathode), a focus electrode, an accelerating electrode, and a control grid placed between the electron source and accelerating electrode, to control the flow of electrons through the gun body.

The vacuum drift tube20of the improved x-ray producing device10of the present invention serves as a connecting passage for carrying the electron beam to target28, thereby allowing target28to be placed closer to and substantially in the center of shield26.

As is well known to those skilled in the art, the kinetic energy resulting from the electrons striking a target produces a significant amount of heat in the target and surrounding region. As a result, the area of the target typically experiences extremely high operating temperatures. This heat can cause the expansion of drift tube components, thereby modifying the geometry of the drift tube and the dynamics of the charged particle or electron beam, including its frequency.

In one embodiment contemplated by the present invention, a drift tube employing a novel means for cooling target28and the surrounding region is provided. More specifically, the vacuum drift tube20employed with improved x-ray producing device10basically comprises:(a) an inner vacuum tube;(b) an outer tube concentric with and spaced from the inner vacuum tube; and(c) means for directing cool water through the space defined by the inner and outer tubes to target28.

Referring now toFIGS. 4 and 5, a preferred vacuum drift tube is shown generally at34, and basically comprises:(a) an inner vacuum tube36;(b) an outer tube38concentric with and spaced from the inner vacuum tube36;(c) a water inlet port40communicating with the space defined by the inner vacuum tube36and the outer tube38;(d) a water outlet port42communicating with the exterior of the vacuum drift tube34;(e) a target44for producing x-rays from electrons striking its surface;(f) baffles or diverters46a,b(FIG. 5) extending longitudinally within the space defined by the inner vacuum tube36and the outer tube38, for directing water toward and away from target44; and(g) water channels48a,bformed by diverters46a,b.

In operation, water enters inlet port40and passes down water channel48atoward target44. Diverter46aextends longitudinally within the space between the inner and outer tubes36,38, and forms water channel48a, while diverter46bextends longitudinally within the space between the inner and outer tubes36,38, and forms water channel48b.Diverters46a,bend prior to reaching target44, allowing water to pass within from about 3 to about 10 millimeters (mm) of target44before passing to the other side of tube34and down to the water outlet port42. The shape of water channels48a,band the abrupt connection of these channels near the target area ensures maximum turbulence for effective cooling. A further advantage is that the target44or target area can be made completely axially symmetric and thus will throw no shadows when used as an anode for panoramic applications.

In a preferred embodiment, the inner vacuum tube36of the vacuum drift tube34is comprised of copper and measures from about 50 to about 250 mm in length, from about 5 to about 20 mm in inner diameter, and from about 15 to about 30 mm in outer diameter, while outer tube38is comprised of either copper, an alloy of nickel, copper, and manganese (e.g., MONEL 400 alloy), or stainless steel and measures from about 50 to about 250 mm in length, from about 15 to about 30 mm in inner diameter, and from about 17 to about 35 mm in outer diameter. In this preferred embodiment, target44is prepared from a circular piece of tungsten measuring from about 5 to about 10 mm in diameter and from about 0.5 to about 3 mm in total thickness.

Shield26of the improved x-ray producing device10of the present invention is located around the drift tube20and has one or more openings for forming an x-ray beam having a pre-selected cross section.

The surface configuration or shape of shield26is not limited. The surface may be curved in two or three dimensions. For example, the surface may have a spherical shape. Alternatively, the surface may be curved along a first axis and straight along a second axis which is orthogonal to the first axis (e.g., cylindrical), curved in two dimensions with different radii in the two directions, or a surface with variable curvature over its area.

In one contemplated embodiment, which is best shown inFIG. 6, shield26has a cylindrical surface configuration or shape. In this embodiment, shield26has an opening50, which facilitates the formation of a conical x-ray beam having a substantially circular cross section from x-rays emitted from target28.

In another contemplated embodiment, which is best shown inFIG. 7, shield26has a double truncated cylindrical surface configuration or shape with a regular octagonal cross section that approximates to a sphere. As in the previous embodiment shown inFIG. 6, the shield26shown inFIG. 7also has an opening50, which facilitates the formation of a conical x-ray beam with a substantially circular cross section from x-rays emitted from target28.

As noted above, the one or more openings in shield26allow for the formation of an x-ray beam having a pre-selected cross section. Preferably, the pre-selected cross section of the x-ray beam is either a circular or rectangular cross section. In another preferred embodiment, the x-ray beam emanating from the one or more openings in shield26is in the form of long thin rectangles that approximate a line.

Materials suitable for use in making shield26include, but are not limited to, lead, lead alloys, steel, steel alloys, tungsten and tungsten alloys, with preferred materials being lead and tungsten alloys.

In a preferred embodiment, shield26comprises a cylindrical inner core prepared from a tungsten alloy. The inner core has an inner diameter ranging from about 15 to about 30 mm, an outer diameter ranging from about 100 to about 200 mm, and a length ranging from about 100 to about 400 mm, and is encased in a lead cylinder having an inner diameter ranging from about 100 to about 200 mm, an outer diameter ranging from about 250 to about 700 mm, and a length ranging from about 250 to about 700 mm. Two tungsten alloy plates, each measuring from about 100 to about 300 mm in length, from about 100 to about 300 mm in width, and from about 10 to about 30 mm in total thickness, are used to structure an opening in shield26that serves to form a narrow x-ray beam from the emitted x-rays that has a long, narrow, rectangular cross section. More specifically, identical wedges are engraved or machined into a surface of each tungsten alloy plate, and the plates assembled together with the machined surfaces facing inward thereby forming a wedge-shaped slot. The resulting assembly is then inserted into and affixed to shield26so that the apex of the wedge-shaped slot is located next to target28at the second end24of the drift tube20, while the side opposite the apex is located at a surface of shield26.

Target28of the improved x-ray producing device10of the present invention is located at the second end24of the drift tube20and substantially at the center of shield26, and produces x-rays from electrons striking the surface of the target28. As will be readily evident to those skilled in the art, the inventive x-ray producing device10operates in a transmission mode because x-rays are emitted from a side of the target28opposite from the electron collision.

The target28basically comprises an element having an atomic number greater than 72 and in a preferred embodiment is a transmission target.

In a more preferred embodiment, the target28comprises a material having good vacuum characteristics and the ability to withstand high heat and electron bombardment, and more particularly comprises a tungsten “button” having a diameter ranging from about 4 to about 10 mm, and a total thickness ranging from about 0.5 to about 4.0 mm. The tungsten “button” is brazed onto a copper disk having a diameter ranging from about 15 to about 25 mm, and a total thickness ranging from about 8 to about 20 mm. The copper disk with brazed tungsten “button” is brazed onto the second end24of the drift tube20.

As noted above, by way of the present invention it has been discovered that an x-ray target may be placed closer to and substantially in the center of a shield adopting, for example, a spherical or substantially spherical geometry, by placing the x-ray target at the end of a drift tube. As a result, the weight and cost of the shielding is minimized.