Portable X-ray scanner

Abstract

The present specification discloses systems for a compact and portable X-ray transmission imaging system that is used for security inspection of small items. The system includes a housing with an X-ray tunnel for receiving an article to be inspected, a conveyor for conveying the article through the tunnel, a dual source X-ray system, with a central target, for generating two overlapping cone beams, and a two-dimensional X-ray detector system for detecting the generated dual energy X-rays.

Description

FIELD

The present specification discloses security systems for X-ray inspection, in particular portable X-ray security inspection systems having a compact profile.

BACKGROUND

X-ray security inspection machines are widely used at security checkpoints, such as those in airports, malls, courthouses, government offices, embassies, schools, and prisons. Where space is not restricted or where an X-ray security inspection machine is required on a permanent basis, the machine may be set up and configured on site, and retained there indefinitely. Such machines are provided in various sizes and specifications, depending on their intended application.

Nevertheless, as the issue of security becomes an ever greater priority, there is an increasing demand for X-ray security inspection machines which find more widespread application. In particular, there is a need for X-ray security inspection machines which may be employed in space-restricted environments and/or which are readily moveable, i.e. portable, from one location to another. Most of the currently available machines are heavy, bulky, and not readily portable. Additionally, most of the currently available security screening systems cannot be easily and efficiently deployed in outdoor environments or at temporary venues, where they are frequently needed to scan for threats inside small baggage items such as backpacks and purses. Typically, tedious and sometimes ineffective hand-searches of small items must be performed.

In most X-ray systems, a highly collimated continuous wave (CW) X-ray source is used, which covers the objects being scanned with a thin fan beam. This has an advantage that a relatively inexpensive linear array of dual-energy detectors can be used for generating the image, while providing the capability of organic/inorganic discrimination based on a thin front detector to detect lower-energy X-rays and a thicker rear detector to detect higher-energy X-rays. The ratio of low-to-high-energy X-ray signal is used for material discrimination. However, a limitation of this approach is that most of the X-rays emitted by the source are absorbed in the collimator, and the X-ray source has to be on continuously to cover the full extent of the object. This leads to a required large amount of shielding in all directions except for the forward fan beam, and also power consumption is high. Such systems therefore tend to be heavy and/or bulky, making them difficult to transport from one place to another.

Dual-energy imaging is typically used to image bags and parcels in the security industry. In X-ray baggage scanners, however, a line array of dual-layer detectors is used. The detector layer closest to an X-ray source is used to primarily detect low-energy X-rays and the detector layer furthest from the source, usually filtered by some filtration material such as copper, is used to primarily detect high-energy X-rays. A width of the detector is typically on the order of 1 mm, and a continuous X-ray source is required to scan objects 1 mm or so, at a time. With an intense X-ray source that is collimated to a fan beam, objects can be scanned at speeds of the order of 20 cm/s. All other X-rays emitted by the source need to be stopped by shielding.

There is a need, therefore, for an improved X-ray security inspection system that may be employed in space-restricted environments and/or which is readily moveable, i.e. portable, from one location to another. Also, there is a need for a security inspection system that is compact, light-weight, and can be ported to temporary and/or outdoor venues.

SUMMARY

In some embodiments, the present specification discloses a portable X-ray imaging system, comprising: a housing defining a tunnel for receiving an article to be inspected; a conveyor for conveying the article through the tunnel at a pre-defined speed; an X-ray production system positioned within said housing, comprising: a first X-ray source pulsed at a first time to generate a first electron beam along a first longitudinal axis, wherein said first X-ray source is positioned above the conveyor; a second X-ray source pulsed at a second time to generate a second electron beam along a second longitudinal axis, wherein the first and second X-ray sources are positioned opposing to each other, wherein the second time is different than the first time, and wherein said second X-ray source is positioned above the conveyor; a target positioned between the first and second X-ray sources, wherein the target has a first side facing the first X-ray source and a second side facing the second X-ray source, wherein the first electron beam strikes the first side at a first impact point to generate a first X-ray cone beam and the second electron beam strikes the second side at a second impact point to generate a second X-ray cone beam, and wherein respective apexes of the first and second X-ray cone beams are separated by a distance; and an X-ray detector system positioned within said housing and comprising at least one two-dimensional flat panel detector for generating a first image of the article corresponding to the first X-ray cone beam and a second image of the article corresponding to the second X-ray cone beam, wherein the first and second sides of the target are inclined at an angle with respect to a top surface plane of said at least one detector.

Optionally, the first X-ray source is operated at a first voltage and the second X-ray source is operated at a second voltage, and wherein the second voltage is higher than the first voltage.

Optionally, an angle of inclination of the first side relative to the top surface plane of the at least one detector ranges from 10 to 45 degrees and wherein an angle of inclination of the second side relative to the top surface plane of the at least one detector ranges from 10 to 45 degrees.

Optionally, the first and second longitudinal axes are non-collinear and lie on a single plane. Optionally, the first longitudinal axis lies at a first angle relative to the first side and the second longitudinal axis lies at a second angle relative to the second side, wherein said first and second angles are less than or equal to 10 degrees.

Optionally, the distance ranges from 1 cm to 5 cm.

Optionally, a difference between the first and second times is defined such that the first image overlaps the second image as the article is conveyed at the pre-defined speed.

Optionally, the two-dimensional flat panel detector has a first layer, a second layer of a first thickness positioned on top of said first layer, a third layer of a second thickness positioned on top of said second layer, and a fourth layer of a third thickness positioned on top of said third layer.

Optionally, a first additional layer is included between said first and second layers and a second additional layer is included between said third and fourth layers.

In some embodiments, the present specification is directed toward a portable X-ray imaging system, comprising: a housing defining a tunnel for receiving an article to be inspected; a conveyor for conveying the article through the tunnel at a pre-defined speed; an X-ray production system positioned within said housing, comprising: a first X-ray source pulsed at a first time to generate a first electron beam along a first longitudinal axis; a second X-ray source pulsed at a second time to generate a second electron beam along a second longitudinal axis, wherein the first and second X-ray sources are positioned opposing to each other, and wherein the second time is greater than the first time; a target positioned between the first and second X-ray sources, wherein the target has a first side facing the first X-ray source and a second side facing the second X-ray source, wherein the first electron beam strikes the first side at a first impact point to generate a first X-ray cone beam and the second electron beam strikes the second side at a second impact point to generate a second X-ray cone beam, and wherein respective apexes of the first and second X-ray cone beams are separated by a distance; and an X-ray detector system positioned within said housing and comprising at least one two-dimensional flat panel detector for generating a first image of the article corresponding to the first X-ray cone beam and a second image of the article corresponding to the second X-ray cone beam, wherein the first and second sides of the target are inclined at an angle with reference to a top surface plane of said at least one detector, and wherein said at least one detector has a first layer, a second layer of a first thickness positioned on top of said first layer, a third layer of a second thickness positioned on top of said second layer, and a fourth layer of a third thickness positioned on top of said third layer.

Optionally, said first layer comprises an amorphous silicon or CMOS detector substrate, said second layer comprises a color filter, said third layer comprises a first scintillator material that emits scintillation light in a first color, and said fourth layer comprises a second scintillator material that emits scintillation light in a second color.

Optionally, said first thickness is less than 1 mm but greater than 0 mm, said second thickness ranges from less than 1 mm to 3 mm, and said third thickness is less than 1 mm but greater than 0 mm.

Optionally, said first layer predominantly detects high energy X-rays while said fourth layer predominantly detects low energy X-rays.

Optionally, a first additional layer is included between said first and second layers and a second additional layer is included between said third and fourth layers.

Optionally, at least one of said first additional layer and second additional layer are fiber-optic plates.

Optionally, said second layer is deposited on said first additional layer, wherein said first additional layer comprises a fiber-optic plate.

Optionally, a first additional layer is positioned below said second layer such that said second layer is adjacent to said first additional layer and wherein the first additional layer comprises a fiber-optic plate.

Optionally, at least one of said first additional layer and second additional layer comprises a third scintillator material that emits scintillation light in a third color.

Optionally, said second layer is configured to permit two or more colors to penetrate there through.

Optionally, the first electron beam defines a first plane and the second electron beam defines a second plane and wherein the first plane and second plane are positioned proximate to each other within a range of 0 mm to 3 mm.

In some embodiments, the present specification is directed toward an X-ray transmission imaging system and method, comprising: a housing defining an X-ray tunnel for receiving an article to be inspected; a conveyor for conveying the article through the tunnel; an X-ray source for irradiating the article within the tunnel, comprising at least one source for generating X-ray beams; and an X-ray detector system, wherein the X-ray detector system comprises at least one two-dimensional flat panel detector for detecting X-ray beams.

In some embodiments, the present specification is directed toward an X-ray transmission imaging system, comprising: a housing defining an X-ray tunnel for receiving an article to be inspected; a conveyor for conveying the article through the tunnel; an X-ray source for irradiating the article within the tunnel, comprising at least one source for generating X-ray beams; and an X-ray detector system, wherein the X-ray detector system comprises at least one two-dimensional flat panel detector for detecting X-ray beams; and wherein either the source is capable of producing different X-ray energies, or the detector is capable of measuring two or more X-ray spectra simultaneously, or both.

Optionally, the X-ray source comprises at least one X-ray source that may be pulsed. Still optionally, the X-ray source pulses with alternating high and low energies.

Optionally, the X-ray source is a continuous source operated at low current. Optionally, said source is operated using a continuously changing high voltage that cyclically alternates, for example using a sinusoidal or approximately square wave, between a low voltage and a high voltage.

Optionally, the X-ray production subsystem comprises two X-ray sources operating at different voltages.

Optionally, the flat panel detector is covered with a scintillator material.

Optionally, the two-dimensional flat panel detector uses a first layer of flat panel detectors and a second layer of flat panel detectors, and wherein the first layer of flat panel detectors acts as a filter for the second layer of flat panel detectors. Still optionally, the first layer of flat panel detectors is covered with a scintillator material of a first thickness and the second layer of flat panel detectors is covered with a scintillator material of a second thickness, and wherein the first thickness is less than the second thickness. Optionally, there is additional filtration material, such as copper sheet, between the first layer and the second layer.

Optionally, the system includes a touchscreen interface to operate the system. Still optionally, the touchscreen interface comprises a bright screen.

Optionally, the system includes handles on either end of the system.

Optionally, the system comprises a gurney, wherein the system is placed over the gurney. Still optionally, the gurney is foldable.

In some embodiments, the detector system comprises a substrate layer; a color filter placed over the substrate layer; an intermediate layer of a first scintillator material placed over the color filter, the intermediate layer emitting scintillation light in a first color; and a top layer of a second scintillator layer placed over the intermediate layer, the top layer emitting scintillation light in a second color. The color filter may be patterned in multiple sections (e.g., checkerboard) that alternately permit two or more colors to penetrate, where the color filter sections are matched to one or the other of the color spectra emitted by the two scintillators.

In some embodiments, the detector system comprises: a substrate layer; a first fiber-optic plate placed over the substrate layer; a color filter placed over the first fiber optic plate; an intermediate layer of a first scintillator material placed over the color filter, the intermediate layer emitting scintillation light in a first color; a second fiber-optic plate places over the intermediate layer; and a top layer of a second scintillator layer placed over the intermediate layer, the top layer emitting scintillation light in a second color. Optionally, the fiber-optic plate can be replaced with a non-fiber-optic but transparent plate, e.g., made of glass. Optionally, the second plat may incorporate, a second (but not patterned) color filter.

The aforementioned and other embodiments of the present shall be described in greater depth in the drawings and detailed description provided below.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the present invention will be appreciated, as they become better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:

FIG. 1 illustrates an overview of a compact X-ray inspection system, in accordance to an embodiment described in the present specification;

FIG. 2 illustrates an exemplary X-ray inspection system with an X-ray source irradiating a set of 2D detectors with a wide-angle X-ray beam;

FIG. 3A illustrates a dual-energy pulsed source-ray production system, in accordance with an embodiment of the present specification;

FIG. 3B shows a target with built-in shielding and collimation, in accordance with an embodiment of the present specification;

FIG. 4 illustrates a dual-energy large area flat-panel detector system, in accordance with an embodiment of the present specification;

FIG. 5 illustrates an alternative embodiment of a detector system;

FIG. 6 illustrates an exemplary foldable wheeled-carriage used with an embodiment of the system of the present specification;

FIG. 7 is a flowchart illustrating a plurality of steps of a method of operating the X-ray inspection system of FIG. 1, in accordance with embodiments of the present specification; and,

FIG. 8 illustrates a single beam dual-energy X-ray production system, in accordance with an embodiment of the present specification.

DETAILED DESCRIPTION

The present specification discloses multiple embodiments. The following disclosure is provided in order to enable a person having ordinary skill in the art to practice the invention. Language used in this specification should not be interpreted as a general disavowal of any one specific embodiment or used to limit the claims beyond the meaning of the terms used therein. The general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the invention. Also, the terminology and phraseology used is for the purpose of describing exemplary embodiments and should not be considered limiting. Thus, the present specification is to be accorded the widest scope encompassing numerous alternatives, modifications and equivalents consistent with the principles and features disclosed. For purpose of clarity, details relating to technical material that is known in the technical fields related to the invention have not been described in detail so as not to unnecessarily obscure the present invention.

It should be noted herein that any feature or component described in association with a specific embodiment may be used and implemented with any other embodiment unless clearly indicated otherwise.

One of ordinary skill in the art would appreciate that the features described in the present application can operate on any computing platform including, but not limited to: a laptop or tablet computer; personal computer; personal data assistant; cell phone; server; embedded processor; DSP chip or specialized imaging device capable of executing programmatic instructions or code.

It should further be appreciated that the platform provides the functions described in the present application by executing a plurality of programmatic instructions, which are stored in one or more non-volatile memories, using one or more processors and presents and/or receives data through transceivers in data communication with one or more wired or wireless networks.

FIG. 1 illustrates an overview of a compact X-ray inspection system 100, in accordance with an embodiment described in the present specification. System 100 may be a portable and light-weight X-ray scanning system. System 100 may be configured to scan small baggage items such as backpacks, purses, or any other small items, in order to identify firearms, 0.5 lbs. or more of explosives, or any other concealed material that may be contraband or poses a security risk. Embodiments of system 100 include at least an X-ray source 102, a two-dimensional (2D) detector array 104, a machine housing defining an X-ray tunnel 106 for receiving an article to be inspected, and optionally, a conveyor 108 for conveying the article through the tunnel 106. In other optional embodiments, any system or mechanism for moving the object or source and detector may be used to obtain a full image of the object. X-ray source 102 may irradiate X-ray beams that are detected by detector array 104. Additionally, handles 110 may be provided at corners of system 100 to enable handling of system 100 while transporting it as a single unit from one place to another. System 100 may communicate with an interface 112 that allows monitoring of articles under inspection through images scanned by system 100 through detector array 104. Interface 112 may also enable a user to control operation of system 100. In embodiments, interface 112 may be a touchscreen interface. In embodiments, the screen of interface 112 may be back-lit brightly during its operation such that display of interface 112 is visible when system 100 is placed in an outdoor environment. In embodiments, interface 112 is the primary interface to system 100, in addition to some cabinet X-ray requirements such as emergency-off buttons and perhaps forward and backward scan buttons 114. An optional shade may be designed in case the screen of interface 112 is not bright enough for operation in direct sunlight. In embodiments, operator assist algorithms may be programmed within system 100, which help users/operators analyze cluttered images. In embodiments, automated threat detection algorithms are implemented within system 100.

Referring to FIG. 6, a foldable wheeled carriage 600 may be used, much like the gurney used in ambulances, to allow transportation of system 100. Use of carriage 600 may allow easy stowing of system 100 in a van with rails for this purpose.

X-Ray Source

A large-area dual-energy detector system may be useful when used with a cone-beam pulsed X-ray source, relative to a collimated beam X-ray source. The cone beam may cover a large section of an object under inspection with each X-ray pulse, therefore saving power and allowing high throughput. Moreover, more of the X-rays produced by such a source may be used for scanning, resulting in lower shielding requirements in all other directions, which directly translates into lower system weight.

FIG. 2 illustrates an exemplary X-ray inspection system 200 with an X-ray source 202 producing a wide-angle beam 210 and irradiating one or more detectors 204. In embodiments, X-ray source 202 is a pulsed X-ray source of between 100 kV and 300 kV with a wide-angle beam. With each X-ray pulse, beam 210 thus covers a large section of an object under inspection that may pass through a tunnel 206 over a conveyor belt 208. As a result, X-ray source 202 may utilize less power and allow high throughput, relative to conventional compact X-ray inspection systems. Additionally, the configuration of source 202 may enable more of the X-ray beams produced by source 202 to be used for scanning, resulting in lower shielding requirements in all other directions, translating into lower weight of system 200.

Typical flat-panel detectors 204 can be up to 17 by 17 inches in size, in which case two or three of them could be sufficient to image objects. For a use case where only very small objects are imaged, a single detector could be sufficient, with an appropriately sized smaller tunnel. In a preferred embodiment, a larger number of smaller panels may be employed, such as 4×6 inch panels, in which case 10 or 12 small panels may be employed. The panels would be arranged to as close as possible intercept the X-rays from the source at 90 degrees at their center, or whichever arrangement is compatible with regards to locations and dimensions of other components, such as the belt.

Most security systems require an ability of material discrimination in order to distinguish organic and inorganic items, or distinguish one item from another, inside an object under inspection. In embodiments, dual-energy images are created to meet this requirement. In one embodiment, source 202 comprises a dual-energy X-ray source. The voltage setting of source 202 may be varied to achieve dual-energy imaging. In examples, source 202 voltage settings for this type of imaging are 50-90 kVp and 100-160 kVp. In embodiments, switching sources may be employed to switch between different voltage settings of source 202. In an embodiment two closely spaced X-ray sources with different voltages may be used for dual-energy analysis. Optionally, the X-ray source is a continuous source operated at low current. Optionally, said source is operated using a continuously changing high voltage that cyclically alternates, for example using a sinusoidal or approximately square wave, between a low voltage such as 50-90 kVp and a high voltage such as 100-160 kVp.

The system of the present specification may be embodied in different shapes and sizes such that the objectives of the present invention are achieved. In an embodiment, the X-ray housing has a pyramid structure. As shown in FIG. 2, the shape can be of any shape as long as the X-rays from the source can be collimated to irradiate the full area (or most of the full area) of the detector array below. While collimation would happen close to the source using a collimator, there would still be scattered X-rays from walls of the housing, from the conveyor belt, from the detectors and from the object being inspected. Thus, the housing has to be constructed so as to (1) let the cones of X-rays that illuminate the detectors pass through without encountering too much material (with the exception of the tunnel through which the bag travels, which should be made, in the area where the X-ray beam passes through, of a reasonably X-ray transparent material, such as aluminum or carbon fiber); (2) prevent X-rays, in any other direction, from exiting the housing and causing a radiation dose to operators and others; and (3) be optimized for light weight. The housing can further be designed such that it keeps operators and/or personnel and any exposed body parts as far away as possible from any remaining sources of primary or scattered radiation, such that applicable cabinet X-ray standards are satisfied.

Referring back to FIG. 1, in an exemplary embodiment, tunnel 106 is 60 centimeters (cm) wide and 40 cm high. In an exemplary embodiment system 100 has approximate dimensions of 40 inches height, 60 inches length, and 36 inches width. In an exemplary embodiment, system 100 weighs approximately 100 lbs.

FIG. 3A illustrates a dual-energy pulsed X-ray production system 300 while FIG. 3B shows a target 312 with built-in shielding and collimation, in accordance with an embodiment of the present specification. Referring now to FIGS. 3A and 3B simultaneously, the system 300 comprises a first single-energy pulsed X-ray source 302a and a second single-energy pulsed X-ray source 302b positioned in opposition to each other. A target 312 is positioned between the first and second X-ray sources 302a, 302b. In some embodiments, the target 312 is positioned symmetrically to lie in the middle of the X-ray sources 302a, 302b. However, in alternate embodiments the target 312 is asymmetrically positioned between the two sources 302a, 302b such that the target 312 is closer to one source and commensurately farther from the other source. The target 312 comprises a first side or surface 312a facing the first X-ray source 302a and a second side or surface 312b facing the second X-ray source 302b.

In embodiments, at a time t1, the first X-ray source 302a produces a first pulsed beam 310a of electrons along a first longitudinal axis 320. The first electron beam 310a is accelerated over a high voltage V1 to strike the first side 312a of the target 312 at a first impact point 335a, producing X-rays more or less in the shape of a cone of X-rays 314a having its apex at the first impact point 335a. Similarly, at a time t2, which is later than t1, the second source 302b produces a second pulsed beam 310b of electrons along a second longitudinal axis 325. The second electron beam 310b is accelerated over a high voltage V2, where V2 is different from V1, to strike the second side 312b of the target 312 at a second impact point 335b, the second side 312b being opposite to the first side 312a, producing X-rays more or less in the shape of a cone of X-rays 314b having its apex at the second impact point 335b.

In some embodiments, the first and second longitudinal axes 320, 325 are substantially parallel and aligned to each other such that the axes 320, 325 lie in the same plane. In some embodiments, the first and second longitudinal axes 320, 325 are substantially parallel to each other but lie on different planes such that the respective planes of the two axes 320, 325 are separated by a distance of less than 3 mm and preferably not more than 1 mm. In still other embodiments, the first and second longitudinal axes 320, 325 are angled with respect to each other and may or may not lie on the same plane. In one embodiment, the axes 320, 325 are mutually inclined at angles of up to 10 degrees but lie on the same plane. In another embodiment, the axes 320, 325 are mutually inclined at angles of up to 10 degrees and lie in different respective planes that are separated by a distance of less than 3 mm and preferably not more than 1 mm.

Accordingly, it is advantageous for the two impact points 335a, 335b of the electron beams (the apexes of the X-ray cones 314a and 314b) to be an equal distance away from a plane of the detectors 330 such that they are in the same plane, within a tolerance of a few mm, for example within a tolerance of less than 3 mm, so that the images generated by the two X-ray beams 314a, 314b are “commensurate”, that is, appear to have the same magnification. Further, as shown in FIG. 3B, the X-rays 314a, 314b are generated respectively inside two wide cones 340a, 340b with their respective tips at the electron beam impact points 335a, 335b. Shielding occurs in part by the target 312 itself, so the cones 314a, 314b are limited in at least one direction by the first and second sides 312a, 312 of the target 12 that, in various embodiments, are inclined at an angle with reference to the plane of the detector 330. X-ray emitted in all directions other than the two wide cones 340a, 340b are shielded. Since the emission of X-rays from the target 312 depends only weakly on the incoming electron beam direction, the electron beams do not have to be perfectly aligned. Thus, in some embodiments, the non-collinearity of the electron beam axes 320 and 325 can be up to 10 degrees. It is advantageous, however, for each impact point (also known as a “beam spot”) 335a, 335b to not move by more than a millimeter or so in any direction during operation, since moving spots affect imaging resolution, and adversely affect image registration between the two images generated by the two X-ray cone beams 314a, 314b. Slow spot movement over time (also referred to as “drift”) is not as serious in this case since a large cone beam is used on a large-area detector as it is with a finely collimated fan beam in typical prior art applications since in prior art applications this drift leads to intensity fluctuations. In various embodiments, the size of the beam spots 335a, 335b is about 1 mm since larger spot sizes lead to lower imaging resolution.

In various embodiments, the respective planes of the first and second sides or surfaces 312a, 312b are angled or inclined with reference to the plane of the detectors 330 (positioned at the bottom relative to the target 312 and X-ray sources 302a, 302b). In various embodiments, the respective planes of the first and second sides 312 are inclined at angles ranging from 5 to 75 degrees and preferably from 10 to 45 degrees with respect to the plane of the detectors 330. In embodiments, the plane of the detectors 330 is substantially horizontal. In some embodiments, the target 312 is configured in the form of an inverted trapezoid having the first and second opposing sides 312a, 312b as shown in FIG. 3A. However, the target 312 can be of any polygonal shape as long as it includes the first and second opposing sides or surfaces 312a, 312b inclined at an angle with reference to the plane of the detectors 330 thereby allowing the X-rays 314a, 314b to be emitted in the direction of the detectors 330. As shown in FIG. 3B, the target 312 includes built-in shielding to block X-rays in all directions, other than the direction of the detectors 330, thereby requiring reduced external shielding.

In embodiments, the parts of the first and second sides 312a, 312b that are being struck by the electron beams are thin layers of tungsten which are typically of the order of having a thickness of less than 1 mm, although other materials may be employed. The remainder of the target material could be any material, for example copper or aluminum, such that the target 312 can conduct both heat and electricity. In a case where the voltages of the first and second sides 312a, 312b are not the same (in the case where the cathodes are at ground potential but the target 312 is not), an insulating material that is capable of withstanding the voltage difference between the two halves of the target 312 is used to separate the two halves of the target 312. For example, in that use scenario, each half of the target 312 may be comprised of a first layer, which may be a thin layer of tungsten on top of a second layer, which may be a layer of copper or aluminum for heat and electricity conduction.

The overall dimensions of the target 312 depend on the desired angle of inclination of the first and second sides 312a, 312b. In various embodiments, the angular inclination of the first and second sides 312a, 312b is configured: a) so that the cones of X-rays 314a, 314b are emitted downwards onto the plane of the detectors 330 at the bottom side of the system 300, b) to generate a desired width of the cone beams 314a, 314b, and c) to have a desired distance 320 between the apexes of the two cones 314a, 314b of X-rays. In the case where the target 312 is split in two halves, the overall dimensions of the target 312 are dependent on the distance required to ensure that the voltage difference between the two halves can be sustained. In a typical configuration, but not limited to such configuration, it would be advantageous for the distance 320 between cone apexes to be small, for example 0.5 to 5 cm, and for the overall size of the target 312 to be slightly larger than that, for example 1 to 7 cm in the direction of measurement of the distance ‘d’ (as shown by the double-pointed arrow 320.

In embodiments, system 300 is surrounded by an enclosure 316. Enclosure 316 may be constructed to be opaque to X-rays except for a window at the bottom of enclosure 316 to allow passage for conical beams 314a and 314b.

The voltages V1 and V2, over which electron beams 310a and 310b are accelerated, may be produced in two ways. In one embodiment, a cathode of source 302a is kept at negative voltage V1, a cathode of source 302b is kept at negative voltage V2, and target 312 is an anode kept at electrical ground potential, thus producing a positive voltage difference for each source 302a and 302b.

In another embodiment, target 312 is split into two parts in the middle in a manner that an insulating material is inserted between the two parts. The part of target 312 facing source 302a may be an anode that is kept at positive high voltage V1, while the part of target 312 facing source 302b may also be an anode that is kept at positive high voltage V2. Cathodes of both sources 302a and 302b may be kept at ground potential.

In embodiments, an object being inspected is placed below system 300, in order to be imaged by a detector (not shown) placed below the object.

In embodiments, an image I1 is taken with a large-area 2D detector (known as a flat-panel detector), such as detector array 204 of FIG. 2. The image I1 may be taken at the time t1 using the X-rays from beam 314a. Another image I2 may be taken with the same detector at the time t2 using the X-rays from beam 314b. If the object being imaged is stationary, then the two images I1 and I2 will see the object from two different vantage points, determined by a distance (d) 320 between apexes of the two conical beams 314a and 314b.

In an embodiment, between time t1 and time t2, the object to be imaged using X-ray system 300 is moved by approximately distance d 320 between the apex of conical beams 314a and 314b, in a direction defined by locations of the first source 302a and the second source 302b. This could be arranged by means of conveyor belt 208 of FIG. 2 that runs at a velocity v equal to the distance d divided by the difference between the two times (t2−t1). Therefore,v=d/(t2−t1).

Alternatively, the difference in time between successive pulses could be adjusted in such a way as to ensure that the image taken with beam 314a at time t1 perfectly overlaps the image taken with beam 314b at time t2. In that case,t2−t1=d/v.

Since the two images I1 and I2 are now aligned properly, and since they were taken with X-ray beams 314a and 314b of different source voltages V1 and V2, a single dual-energy image may be constructed from the two images I1 and I2.

System 300 may repeat this process until the entire object is imaged. Following the X-ray pulses from source 302a and source 302b at times t1 and t2, another X-ray pulse using source 302a may be generated at a time t3 and another X-ray pulse using source 302b at a time t4. The time between pulses t3 and t4, i.e., t4−t3, may be selected to match the movement of the object, as explained before.

FIG. 8 illustrates a single beam dual-energy X-ray production system 800, in accordance with an embodiment of the present specification. The system 800 comprises a grounded electron beam generator or source 805 emitting an electron beam 810 that is configured to alternately strike a first target 815a and a second target 815b. A positive high voltage HV2 is applied to the second target 815b. Through a conductive connection and resistors 820, 822 the voltage drops to ground voltage for the source 805. The conductive connection between resistors 820, 822 is also connected to the first target 815a which is maintained at a voltage HV1. Values of the resistors 820, 822 are chosen such that the voltage HV1 on the first target 815a is lower than the voltage HV2 on the second target 815b but higher than ground potential.

During operation, the electron beam 810 passes through a deflector 825. When, at a first time t1, a first low voltage v1 is applied to the deflector 825, the beam 810 deflects in the direction of the first target 815a to strike the first target 815a and produce a first X-ray cone 830a with maximum X-ray energy determined by voltage HV1. When, at a second time t2 which is greater than t1, a second low voltage v2 is applied to the deflector 825 (wherein the second voltage v2 is lower than the first voltage v1 and possibly of a sign opposite to that of voltage v1), the electron beam 810 deflects in the direction of the second target 815b to strike the second target 815b and produce a second X-ray cone 830b with maximum X-ray energy determined by voltage HV2. Accordingly, a switching dual-energy source is realized using the single electron beam generator 805. It should be appreciated that that it is much easier and faster to switch between low voltages v1 and v2 on the deflector 825 than between high voltages such as HV1 and HV2 using a switching source.

In various embodiments, the targets 815a, 815b are spaced sufficiently far apart to avoid high-voltage sparking to occur between the targets 815a, 815b due to the voltage difference (HV2−HV1) between the targets 815a, 815b. In embodiments, the distance between the two targets 815a, 815b is of the order of a few cm. In various embodiments, the targets 815a, 815b are appropriately shaped to avoid sharp protrusions which concentrate the field strength and can cause breakdown, as is known to persons of ordinary skill in the art.

In an alternate embodiment, the beam 810 is aimed exclusively at the second target 815b and no deflector is employed. The first target 815a, located closer to the electron beam generator 805, is inserted periodically into the beam 810 or on-demand by mechanical means. In one embodiment, this is accomplished by the first target 815a comprising a wheel with a conical edge, with at least one gap, rotating at a constant rate of rotation. The rotating wheel leads to situations where the electron beam 810 either strikes the conical edge of the first target 815a thereby producing X-ray cone 830a, or passes through a gap to strike the second target 815a to produce X-ray cone 830b. This embodiment has an advantage of not requiring a deflector and associated electronics, but has a disadvantage of requiring a moving target and mechanical and electrical means for moving the target.

Detector

Referring back to FIG. 2, in embodiments, 2D detector array 204 uses flat-panel detectors covered with scintillator material. Flat-panel detectors may be prepared using amorphous silicon or CMOS technologies. Embodiments of the present specification describe systems to allow dual-energy operation of detector 204 for organic/inorganic discrimination.

A large-area dual-energy detector system is useful when used with a cone-beam pulsed X-ray source or a cone-beam low-current continuous source, such as those described in various embodiments above. In order to detect X-rays, flat-panel detectors usually also have a layer of scintillator material. In some cases, there is also a fiber-optic plate, usually made of glass, between the scintillator and the detection layer, which collimates the scintillation light towards the pixels below. Examples of common scintillators include Cesium Iodide (CsI) and Gadolinium Oxi-Sulfide (GdOS). The panels are then read out pixel by pixel and the data are transferred to a computer for processing, interpretation and display.

Dual-energy images may be generated using such detectors. In one known method, dual-energy X-ray source system 300 of FIG. 3 is used. An image is acquired using one energy setting on detector 204, and sometime later a second image is acquired using a different energy setting. Typical source voltage settings for this type of imaging may be 50-90 kVp and 100-160 kVp. This technique is commonly known as “double shot”.

In an alternative known method, two flat-panel detectors are arranged on top of one another, with an additional filtration material (such as a copper sheet) between the two detectors. Low-energy X-rays are primarily detected in an upper detector, while higher-energy X-rays are capable of penetrating both the upper detector and the filtration material, to be registered in a lower detector. This technique is known as “single shot”.

FIG. 4 illustrates a dual-energy large area flat-panel detector system 400, in accordance with an embodiment of the present specification. Detector system 400 is shown in the figure through two different views—view 400a illustrates an assembled detector system 400, and view 400b depicts a disintegrated assembly of layers/components of detector system 400. In embodiments, detector system 400 may consist of four layers. A bottom layer 402 may be an amorphous silicon or CMOS detector substrate layer, a second layer 404 is a color filter, a third layer 406 may be an intermediate layer of scintillator material that emits scintillation light in one color. In an example, also used here for illustration purposes, scintillation light at layer 406 is of blue color, such as that derived from some plastic scintillators, or CsI(Na) for example. A final top layer 408 may consist of a mostly opaque screen-type scintillator that emits scintillation light in another color. In an example, also used here for illustration purposes, scintillation light at layer 408 is of red color, such as that derived from GdOS(Eu) screen material, for example. Since such screen material is usually white in appearance (i.e., it reflects all colors of optical light equally), it may also serve as a reflector for the light produced by intermediate layer 406 of scintillator material.

In various embodiments, a thickness of the top scintillator layer 408 is less than 1 mm but greater than 0 mm, a thickness of the intermediate scintillator layer 406 is of the order of less than 1 mm to 3 mm while the color filter second layer 404 is typically less than 1 mm thick but greater than 0 mm.

During operation, low-energy X-rays may be detected in the thin top scintillator layer 408, and scintillation light may be produced with the color spectrum of the type of scintillator used in that layer. For example, if top layer 408 is made of thin (0.1 to 0.5 mm, for example) red-emitting screen material, the light produced will be transmitted through intermediate layer 406 of scintillator material, and mostly detected by pixels covered by red parts of color filter 404. In embodiments, higher energy X-rays are mostly detected in the thicker intermediate layer 406 of scintillator material between top layer 408 and color filter 404. For example, if layer 406 is made, of 1 to 2 mm, of blue-emitting scintillator, this light would be detected mostly by the pixels below the blue parts of color filter 404.

FIG. 5 illustrates an alternative embodiment of a dual energy detector system 500. Detector system 500 is shown in the figure through two different views—view 500a illustrates an assembled detector system 500, and view 500b depicts a disintegrated assembly of layers/components of detector system 500. In this embodiment, a fiber-optic plate 504 is included between a detector substrate layer 502 and a color filter 506. Additionally, a fiber-optic plate 510 is included between an intermediate scintillator layer 508 and a top scintillator layer 512. Fiber optic plate 504 is usually the layer that sits directly on top of detector substrate 502 in existing detector panels and may guide scintillation light down to the detector pixels, while eliminating light going in other directions, thereby advantageously increasing spatial resolution. It may be possible to deposit color filter 506 on fiber-optic plate 504 rather than on detector substrate 502 directly, with the advantage that no special modifications need to be made to detector substrate 502, and existing flat-panel detectors may be modified to produce a dual-energy flat-panel detector 500. In embodiments, color filter layer 506 is inverted along with fiber-optic plate 504 such that color filter 506 is adjacent to detector substrate 502. In embodiments, fiber-optic plate 504 is optional, and may be omitted. In embodiments, color filter layer 506 is deposited directly on detector substrate 502. In embodiments, color filter layer 506 is a separate layer that can be inserted where most advantageous.

The second fiber-optic plate 510 may be included between the two scintillator layers 508 and 512 in order to increase filtration of the X-rays between the two scintillator layers 508 and 512. A thickness of this fiber-optic plate 510 may be adjusted to provide the desired amount of X-ray filtration. In embodiments, one or both fiber-optic plates 504 and 510 may be replaced with simple glass plates or plates of another transparent material, or with leaded-glass plates that provide higher attenuation for the same thickness, or with a plate made of a third scintillation material emitting another color of scintillation light. In an embodiment where one or both fiber-optic plates 504 and 510 are replaced with a third scintillation material, suitable modifications are made to color filter 506 in order to take advantage of the third scintillator. In an embodiment, the second plate 510 also is or incorporates a second un-patterned color filter that eliminates or reduces any undesired part of the spectral range of light emitted by the top layer 512 of scintillator.

In various embodiments, a thickness of the top scintillator layer 512 is less than 1 mm but greater than 0 mm, a thickness of the intermediate scintillator layer 508 and of the fiber-optic plates 504, 510 is on the order of less than 1 mm to 3 mm while the color filter layer 506 is typically less than 1 mm thick but greater than 0 mm.

Color Filters

Color filters (404, 506 of FIGS. 4, 5 respectively) can be manufactured that efficiently divide the optical spectrum between two colors at a selectable wavelength. In various embodiments, the color filters (404, 506 of FIGS. 4, 5 respectively) are patterned in multiple sections (for example, checkerboard) that alternately permit two or more colors to penetrate. Embodiments of color filters, such as filters 404 and 506, are known for their use in CCDs for color cameras (Bayer filters, which are usually small but have very fine resolution) and in LCD panels (which can be very large). In embodiments, inkjet printers may be used to print color filters (404, 506) directly on substrates (402, 502).

Scintillators

In embodiments, standard scintillators are used with flat-panel detectors (400, 500 of FIGS. 4, 5 respectively). These may include scintillators using Terbium-doped Gadolinium Oxysulfide, with chemical formula Gd2O2S(Tb), also called GdOS or Gadox, and Thallium-doped Cesium Iodide, with chemical formula CsI(Tl). Gadox screens may emit in the green part of a spectrum, with a maximum emission at around 550 nm. Gd2O2S(Eu) may be used, which is a variety of Gadox that is doped with Europium, and it has its emission maximum in the red part of a spectrum around 630 nm.

In other embodiments, a scintillator material used is CsI that can be either doped with Sodium (Na), in which case it emits in the blue range of the spectrum (maximum emission near 400 nm), or with Thallium (Tl) in which case it emits in the green range of the spectrum (maximum emission near 550 nm). CsI can be made in thin sheets, with columnar microstructure that guides the scintillation light towards the detector pixels.

In yet other embodiments, plastic scintillator may be used in thin sheets. A variety of different plastic scintillators are available, with emission in red, blue and green. Other examples of scintillator material that may be used include and are not limited to Calcium Tungstate CaWO4, with emission in the blue range of the spectrum at 400 nm.

With reference to FIGS. 4, 5, in an embodiment, one could implement a dual-energy flat-panel detector system 400/500, using CaWO4 screen material for top scintillator layer 408/512, emitting in blue, and measuring predominantly the lower-energy X-rays. CsI(Tl) may be used for intermediate scintillator layer 406/508, emitting mostly in green, and measuring predominantly the higher-energy X-rays. In the embodiment, color filter 404/506 may consist of blue-transmitting and green-transmitting parts.

In another embodiment, top scintillator layer 408/512 may be made of Europium-doped GdOS, emitting in red, and measuring predominantly the lower-energy X-rays. Intermediate scintillator layer 406/508 may be made using CsI(Na), emitting mostly in blue, and measuring predominantly the higher-energy X-rays. In the embodiment, color filter 404/506 may consist of red-transmitting and blue-transmitting parts.

In another embodiment, top scintillator layer 408/512 may be made of Europium-doped GdOS, emitting in red, and measuring predominantly the lower-energy X-rays. Intermediate scintillator layer 406/508 may be made using CsI(Tl), emitting mostly in green, and measuring predominantly the higher-energy X-rays. In the embodiment, color filter 404/506 may consist of red-transmitting and green-transmitting parts.

FIG. 7 is a flowchart illustrating a plurality of steps of a method of operating a portable and light-weight X-ray scanning system, in accordance with embodiments of the present specification. At step 705 the portable X-ray scanning system, such as system 100 of FIG. 1, is transported to and placed at a site for inspection, and connected to a source of power, which may either be wall-power, or produced by a generator, or produced by a battery. Optionally, an inverter may be used in combination with a battery in order to produce an alternating current power source. The system 100 may include a power switch to turn on primary power to all the subsystems. Referring now to FIGS. 1 and 7 simultaneously, at step 710 the system 100 is activated using the interface 112 so that the conveyor 108 starts moving with a predefined velocity v. At step 715, an object (such as a bag) for inspection is placed on the conveyor 108 to move the object through the inspection tunnel 106.

In accordance with embodiments, an X-ray source is a dual-energy pulsed X-ray source comprising two single-energy pulsed X-ray sources, such as the X-ray production system 300 described with reference to FIG. 3. Also, in accordance with embodiments, the 2D detector array 104 is a large-area dual-energy flat panel detector system, such as detector system 400 or 500 of FIG. 4 or 5 respectively. In alternate embodiments, the X-ray source is a continuous single-energy source while the detector array is a dual-energy detector system. In still alternate embodiments, the X-ray source is a dual-energy source while the detector array is a single-energy detector system. As the object moves through the tunnel 106, at step 720 a first X-ray source is pulsed at time t1 to produce a first beam of electrons accelerated over a first voltage V1. The first beam of electrons strikes a first angled surface or side of a target to generate a first cone beam of X-rays. At step 725, the first cone beam strikes the detector array to generate a first image of the object at a first energy corresponding to the first voltage V1.

At step 730, a second X-ray source is pulsed at time t2 (wherein t2>t1) to produce a second beam of electrons accelerated over a second voltage V2, different from the first voltage V1. The second beam of electrons strikes a second angled surface or side of the target to generate a second cone beam of X-rays. At step 735, the second cone beam strikes the detector array to generate a second image of the object at a second energy corresponding to the second voltage V2.

In embodiments V2>V1, therefore the first cone beam of X-rays is at a lower energy compared to the second cone beam. Each of the first and second cone beams of X-rays comprises a spectrum of X-ray energies, the first cone beam having more low-energy X-rays and the second cone beam having comparatively more high-energy X-rays. Accordingly, with reference to the detector system 400 of FIG. 4 for example, the thin top scintillator layer 408 predominantly measures low-energy X-rays while the thicker intermediate layer 406 predominantly measures high-energy X-rays.

In some embodiments, the target is configured and positioned as an inverted trapezoid such that the first and second angled sides of the trapezoid respectively face towards the first and second X-ray sources, the parallel sides of the trapezoid are substantially parallel to the direction of the first and second beams of electrons, and the shorter of the two parallel sides is placed downwards. However, in alternate embodiments the target can be of any polygonal shape as long as it includes the first and second angled sides that are inclined at an angle with reference to a plane of the detectors. The angle of inclination of the first and second opposing sides with reference to the plane of the detectors enable generation of the first and second cone beams such that apexes of the first and second cone beams are separated by a predefined distance d. Accordingly, the velocity v of the conveyor is determined to be v=d/(t2−t1) at step 710. This enables the first and second images to overlap. In alternate embodiments, the conveyor speed v is set for a desired throughput while the pulsation timing differential is determined as (t2−t1)=d/v to enable the first and second images to overlap.

Since the first and second images are aligned or overlap with each other, and since they were taken with first and second X-ray beams of different source voltages V1 and V2, a single dual-energy image may be constructed from the first and second images, at step 740.

The concept outlined through various embodiments of the present specification has several advantages. A major advantage is that it has very similar features to existing small baggage scanners used at checkpoints. Therefore, the concept of operation may be identical to that used for checkpoint screening. A bag is put on the conveyor belt at one end of the system, is transported through a tunnel to the other end, and an organic/inorganic color-coded image is displayed on the screen. Software may be used in embodiments of the present invention that is similar to the software used in current baggage scanner products, including existing operator-assist features.

The above examples are merely illustrative of the many applications of the system of present invention. Although only a few embodiments of the present invention have been described herein, it should be understood that the present invention might be embodied in many other specific forms without departing from the spirit or scope of the invention. Therefore, the present examples and embodiments are to be considered as illustrative and not restrictive, and the invention may be modified within the scope of the appended claims.

Claims

1. A portable X-ray imaging system, comprising: a housing defining a tunnel for receiving an article to be inspected;a conveyor for conveying the article through the tunnel at a pre-defined speed;an X-ray production system positioned within said housing, comprising: a first X-ray source pulsed at a first time to generate a first electron beam along a first longitudinal axis, wherein said first X-ray source is positioned above the conveyor;a second X-ray source pulsed at a second time to generate a second electron beam along a second longitudinal axis, wherein the first and second X-ray sources are positioned opposing to each other, wherein the second time is different than the first time, and wherein said second X-ray source is positioned above the conveyor;a target positioned between the first and second X-ray sources, wherein the target has a first side facing the first X-ray source and a second side facing the second X-ray source, wherein the first electron beam strikes the first side at a first impact point to generate a first X-ray cone beam and the second electron beam strikes the second side at a second impact point to generate a second X-ray cone beam, and wherein respective apexes of the first and second X-ray cone beams are separated by a distance; andan X-ray detector system positioned within said housing and comprising at least one two-dimensional flat panel detector for generating a first image of the article corresponding to the first X-ray cone beam and a second image of the article corresponding to the second X-ray cone beam, wherein the first and second sides of the target are inclined at an angle with respect to a top surface plane of said at least one detector and wherein the pre-defined speed of the conveyor is a function of at least the first time and the second time.

2. The system of claim 1, wherein the first X-ray source is operated at a first voltage and the second X-ray source is operated at a second voltage, and wherein the second voltage is higher than the first voltage.

3. The system of claim 1, wherein an angle of inclination of the first side relative to the top surface plane of the at least one detector ranges from 10 to 45 degrees and wherein an angle of inclination of the second side relative to the top surface plane of the at least one detector ranges from 10 to 45 degrees.

4. The system of claim 1, wherein the first and second longitudinal axes are non-collinear and lie on a single plane.

5. The system of claim 1, wherein the first longitudinal axis lies at a first angle relative to the first side and the second longitudinal axis lies at a second angle relative to the second side, wherein said first and second angles are less than or equal to 10 degrees.

6. The system of claim 1, wherein the distance ranges from 1 cm to 5 cm.

7. The system of claim 1, wherein a difference between the first and second times is defined such that the first image overlaps the second image as the article is conveyed at the pre-defined speed.

8. The system of claim 1, wherein the two-dimensional flat panel detector has a first layer, a second layer of a first thickness positioned on top of said first layer, a third layer of a second thickness positioned on top of said second layer, and a fourth layer of a third thickness positioned on top of said third layer.

9. The system of claim 8, wherein a first additional layer is included between said first and second layers and a second additional layer is included between said third and fourth layers.

10. A portable X-ray imaging system, comprising: a housing defining a tunnel for receiving an article to be inspected;a conveyor for conveying the article through the tunnel at a pre-defined speed;an X-ray production system positioned within said housing, comprising: a first X-ray source pulsed at a first time to generate a first electron beam along a first longitudinal axis;a second X-ray source pulsed at a second time to generate a second electron beam along a second longitudinal axis, wherein the first and second X-ray sources are positioned opposing to each other, and wherein the second time is greater than the first time;a target positioned between the first and second X-ray sources, wherein the target has a first side facing the first X-ray source and a second side facing the second X-ray source, wherein the first electron beam strikes the first side at a first impact point to generate a first X-ray cone beam and the second electron beam strikes the second side at a second impact point to generate a second X-ray cone beam, and wherein respective apexes of the first and second X-ray cone beams are separated by a distance; andan X-ray detector system positioned within said housing and comprising at least one two-dimensional flat panel detector for generating a first image of the article corresponding to the first X-ray cone beam and a second image of the article corresponding to the second X-ray cone beam, wherein the first and second sides of the target are inclined at an angle with reference to a top surface plane of said at least one detector, wherein said at least one detector has a first layer, a second layer of a first thickness positioned on top of said first layer, a third layer of a second thickness positioned on top of said second layer, and a fourth layer of a third thickness positioned on top of said third layer and wherein the pre-defined speed of the conveyor is a function of the first time and the second time.

11. The system of claim 10, wherein said first layer comprises an amorphous silicon or CMOS detector substrate, said second layer comprises a color filter, said third layer comprises a first scintillator material that emits scintillation light in a first color, and said fourth layer comprises a second scintillator material that emits scintillation light in a second color.

12. The system of claim 10, wherein said first thickness is less than 1 mm but greater than 0 mm, said second thickness ranges from less than 1 mm to 3 mm, and said third thickness is less than 1 mm but greater than 0 mm.

13. The system of claim 10, wherein said first layer predominantly detects high energy X-rays while said fourth layer predominantly detects low energy X-rays.

14. The system of claim 10, wherein a first additional layer is included between said first and second layers and a second additional layer is included between said third and fourth layers.

15. The system of claim 14, wherein at least one of said first additional layer and second additional layer are fiber-optic plates.

16. The system of claim 14, wherein said second layer is deposited on said first additional layer, wherein said first additional layer comprises a fiber-optic plate.

17. The system of claim 14, wherein at least one of said first additional layer and second additional layer comprises a third scintillator material that emits scintillation light in a third color.

18. The system of claim 10, wherein a first additional layer is positioned below said second layer such that said second layer is adjacent to said first additional layer and wherein the first additional layer comprises a fiber-optic plate.

19. The system of claim 10, wherein said second layer is configured to permit two or more colors to penetrate there through.

20. The system of claim 10, wherein the first electron beam defines a first plane and the second electron beam defines a second plane and wherein the first plane and second plane are positioned proximate to each other within a range of 0 mm to 3 mm.