Patent Publication Number: US-2023152475-A1

Title: Wavelength-Shifting Sheet-Coupled Scintillation Detectors

Description:
CROSS-REFERENCE 
     The present application is a continuation application of U.S. Pat. Application No. 17/061,340, entitled “Wavelength-Shifting Sheet-Coupled Scintillation Detectors” and filed on Oct. 1, 2020, which is a continuation application of U.S. Pat. Application No. 16/382,973, of the same title, filed on Apr. 12, 2019, and issued as U.S. Pat. No. 10,830,911 on Nov. 10, 2020, which relies on, for priority, U.S. Pat. Provisional Application No. 62/687,550, entitled “Wavelength-Shifting Sheet Scintillation Detectors” and filed on Jun. 20, 2018. All of the above referenced applications are herein incorporated by reference in their entirety. 
     In addition, the present specification relates to U.S. Pat. Application No. 16/242,163, filed on Jan. 8, 2019, which is a continuation of U.S. Pat. Application No. 15/490,787, entitled “Spectral Discrimination using Wavelength-Shifting Fiber-Coupled Scintillation Detectors”, filed on Apr. 18, 2017, which, in turn, is a divisional application of U.S. Pat. No. 9,658,343 (the “‘343 patent”), of the same title filed on Feb. 23, 2016 and issued on May 23, 2017. The ‘343 patent is a continuation of U.S. Pat. No. 9,285,488 (the ‘488 patent), of the same title, filed on Feb. 4, 2013, and issued on Mar. 15, 2016. The ‘488 patent, in turn, claims priority from the following applications: 
     U.S. Pat. Provisional Application No. 61/607,066, entitled “X-Ray Inspection using Wavelength-Shifting Fiber-Coupled Detectors”, filed on Mar. 6, 2012. 
     U.S. Pat. Provisional Application No. 61/598,521, entitled “Distributed X-Ray Scintillation Detector with Wavelength-Shifted Fiber Readout”, and filed on Feb. 14, 2012. 
     U.S. Pat. Provisional Application No. 61/598,576, entitled “X-Ray Inspection Using Wavelength-Shifting Fiber-Coupled Detectors”, and filed on Feb. 14, 2012. 
     The above-mentioned applications are incorporated herein by reference in their entirety. 
    
    
     FIELD 
     The present specification relates generally to wavelength-shifting scintillation detectors and more specifically to a wavelength-shifting sheet detector for use in a flying spot transmission imaging system. 
     BACKGROUND 
     Fiber-coupled scintillation detectors of radiation and particles have been employed over the course of the past 30 years. In some cases, the scintillator is pixelated, consisting of discrete scintillator elements, and in other cases, other stratagems are employed (such as orthogonally crossed coupling fibers) in order to provide spatial resolution. Examples of fiber-coupled scintillation detectors are provided by U.S. Pat. Nos. 6,078,052 (to DiFilippo) and 7,326,933 (to Katagiri et al.), both of which are incorporated herein by reference. The detectors described by both DiFilippo and Katagiri et al. employ wavelength-shifting fibers (WSF) such that light reemitted by the core material of the fiber may be conducted, with low attenuation, to photo-detectors disposed at a convenient location, often distant from the scintillator itself. Spatial resolution is of particular value in applications such as neutron imaging. Spatial resolution is also paramount in the Fermi Large Area Space Telescope (formerly, GLAST) where a high-efficiency segmented scintillation detector employs WSF readout for detection of high-energy cosmic rays, as described in Moiseev, et al., High efficiency plastic scintillator detector with wavelength-shifting fiber readout for the GLAST Large Area Telescope,  Nucl. Instr. Meth. Phys. Res. A , vol. 583, pp. 372-81 (2007), which is incorporated herein by reference. 
     A conventional scintillation detector  100  is shown in a side cross-section in  FIG.  1 A  and in a front cross-section in  FIG.  1 B . An example of such a detector is described in U.S. Pat. No. 5,302,817 (to Yokota), and is incorporated herein by reference. Typically, a light-tight box  102  is lined with scintillating screens  103  where incident X-ray radiation  101  is converted to scintillation light, typically in the UV, visible, or longer wavelength, portions of the electromagnetic (EM) spectrum. Large-photocathode-area photomultiplier tubes (PMTs)  105  are coupled to receive scintillation light via portholes  108 . One problem lies in that a fraction of the scintillation light originating within the screen is transmitted from the screen into the enclosed volume. The remaining scintillation light is lost in the screen material. Scintillating screens  103  are designed to maximize the fraction of emitted light, which is tantamount to ensuring a large transmission coefficient T for the interface between screen  103  and the medium (typically air) filling the detector volume. 
     However, in a conventional backscatter detector of the sort depicted in  FIGS.  1 A and  1 B , the scintillation screens  103  should also serve as good reflectors because scintillation light, once emitted into the volume of box  102 , typically needs multiple reflections until it reaches a photodetector  105 . So, the reflection coefficient R of the screen surface should also be large, however, since the sum of T and R is constrained to be unity, both T and R cannot be maximized simultaneously, and a compromise must be struck. As a result, the light collection efficiency of the conventional backscatter detector is inherently low, with only a few percent of the generated scintillation light collected into the photo detectors. Poor light collection can possibly create a secondary quantum sink and increase image noise. The light collection efficiency can be improved by increasing the sensitive area of the photo-detectors which is not only costly, but also adds weight and size. A conventional backscatter (BX) detector assembly with photomultiplier tubes (PMT) power supplies weighs typically between 3 and 4 g/cm 2 . The light box is typically designed with an aspect ratio of 1:10 for height to thickness, in order to minimize the number of reflections from the internal surfaces. For typical sizes required for transmission detectors in handheld applications, the light box would be 2-3″ in thickness, with additional thickness requires for PMT mounting. In addition to size and light collection efficiency, conventional light box detectors are inherently non-uniform in response for application as a transmission detector. Response across the screen in the locations where the PMT is located show a significant drop in response due to the lack of scintillator material in the back of the light box in these locations. 
     Detectors used in transmission imaging with a handheld flying spot X-ray scanning system may be constructed from materials which are far more thin and rugged than traditional flat panel detectors. For transmission X-ray detection with a handheld scanning system, the detector may not be rigidly attached to the body of the system. In this case, the detector is required to be the same size as the object being imaged, in order to intercept the flying spot beam across the area of interest. In such imaging configurations, the detector response may not be corrected or calibrated due to the non-uniform and non-repeatable illumination of the detector by the source. In this case, any non-uniformity in X-ray sensitivity will be displayed directly in the final images. 
     Portable hand-held scanners currently utilize low-profile, light-weight Wave-Shifting Fiber (WSF) X-ray detectors in order to generate transmission X-ray images. In general, WSF detector technology enables a low profile, rugged and large area detection of a flying spot x-ray beam. By way of background, wavelength shifting fibers consist of a core with relatively high refractive index, surrounded by one or more cladding layers of lower refractive index. The core contains wavelength-shifting material, also referred to as dye. Scintillation light which enters the fiber is absorbed by the dye which, in turn, emits light with a longer wavelength. The longer wavelength light is emitted isotropically in the fiber material. Total internal reflection traps a fraction of that light and conducts it over long distances with relatively low loss. This is possible, as described with reference to  FIG.  2   , because the absorption  204  and emission  202  wavelength ranges of the dye effectively do not overlap so that the wavelength-shifted light is not reabsorbed. The captured fraction is determined by the ratio of the refractive indices at surfaces of the fiber. An additional advantage of WSF is that the wavelength shifting can bring the scintillation light  206  into the sensitive wavelength range of the photo detector (PMT, silicon photomultiplier (SiPM), or Multiple-Pixel Photon-Counter (MPPC), or otherwise). 
       FIG.  3 A  illustrates a known X-ray detector comprising WSF and scintillator layers, which is disclosed in co-pending U.S. Pat. Application No. 15/490,787, assigned to the Applicant of the present specification, which is herein incorporated by reference. U.S. Pat. No 9,285,488 and 9,658,343, also assigned to the Applicant of the present specification, are herein incorporated by reference in their entirety. As shown, a layer of closely spaced parallel wavelength-shifting fibers  300  is sandwiched between two layers  303  of composite scintillating screen. One commonly used scintillator material is europium-doped barium fluorochloride (BaFCI:Eu), although other scintillators, such as BaFI:Eu, or other lanthanide-doped barium mixed halides (including, by way of further example, BaBrI:Eu and BaCsI:Eu), are also used. Composite scintillator  303  is structurally supported by exterior layers  304  of plastic, or other material, providing mechanical support. Optical contact between the fiber cladding  301  and the composite scintillator  303  is established by filling the voids with index-matching material  305  of suitable refractive index which is transparent to the scintillation light. The refractive index of the filling material is chosen to optimize the collection of primary light photons into the WSF and the capture of wavelength-shifted photons in the fiber. Filling material  305  may be optical grease or optical epoxy, for example. 
     Upon incidence of X-ray photons, scintillation light emitted by scintillator  303  is coupled via cladding  301  into core  307  of the respective fibers, down-shifted in frequency (i.e., red-shifted) and propagated to one or more photo-detectors, whereby the photo-detectors convert the light from the fiber cores  307  into a current. The current is integrated for an interval of time, typically in the range of 1-12 µs, to obtain the signal strength for each pixel. Integration of the detector signal may be performed by an integrating circuit (not shown), such as an integrating pre-amplifier, for example. The useful stopping power of the detector can be increased by combining multiple layers of WSF  300  thereby increasing the depth of scintillator material  303  along the path of the incident radiation.  FIG.  3 B  illustrates a cross-sectional view of a typical WSF detector. As shown, a ribbon of WSF is sandwiched between scintillator screens  303 . The fiber ends are bundled, cut and polished. The exit surface is mounted to a PMT. This guarantees efficient light collection. In order to minimize the number of PMT’s, the fibers can also be bent into a U-shape, and bundled at one end of the detector. 
     One of ordinary skill in the art understands that the visibility of an artifact is a function of its size. Extended as well as abruptly changing artifacts are highly visible in a noisy background. For a WSF detector, a change in the efficiency of a single fiber (for instance a single point defect in a fiber) results in an extended and abrupt line defect. Such defects are highly visible as the defect extends across the length of the detector. In addition, non-uniformity may occur during or as a result of the manufacturing process of the WSF fiber (such as, but not limited to, cable bending, fiber bundling, and output coupling/polishing). 
     To overcome the challenges of achieving uniform response, a WSF detector must be manufactured in such a way that maintains the fiber position and bending uniformly across the full detector. Any variations in the spacing or bending can lead to non-uniformity in the detector response. As a result, wavelength-shifting fibers must be physically held by mechanical fixture across the full surface of the detector, which may number in the hundreds. The fiber threading the handling constitutes a manufacturing challenge which adds cost and drops final quality and yield. Thus, there is a need for a WS detector configuration for use in a flying spot transmission imaging system with improved spatial uniformity and reduced cost for materials and manufacturing. 
     SUMMARY 
     The following embodiments and aspects thereof are described and illustrated in conjunction with systems, tools and methods, which are meant to be exemplary and illustrative, and not limiting in scope. The present application discloses numerous embodiments. 
     The present specification discloses an X-ray detector comprising: at least one scintillator screen configured to absorb incident X rays and emit corresponding light rays; a wavelength shifting sheet (WSS) optically coupled with the at least one scintillator screen and configured to collect and spectrum shift the light rays, wherein the WSS has at least one edge; at least one wavelength shifting fiber (WSF) optically coupled with the at least one edge of the WSS and configured to collect the spectrum shifted light rays and spectrum shift the collected spectrum shifted light rays to generate twice-spectrum-shifted light rays; and a photodetector optically coupled to the WSF and configured to receive and detect the twice-spectrum-shifted light rays. 
     Optionally, the WSS comprises a first and a second surface wherein the at least one scintillator screen at least partially covers the first surface and a second scintillator screen at least partially covers the second surface. Optionally, the first surface is coplanar to the second surface. 
     Optionally, the at least one WSF is physically coupled with at least a portion of the edge of the WSS. 
     Optionally, the photodetector is a photomultiplier tube (PMT). 
     Optionally, the X-ray detector further comprises a reflector material covering the WSF to improve the collection of the spectrum shifted light rays. Optionally, the reflector material comprises at least one of a diffuse reflector or a specular reflector material. 
     Optionally, the at least one scintillator screen comprises a material having an optical absorption length and wherein a thickness of the at least one scintillator screen is less than the optical absorption length. 
     Optionally, the at least one scintillator screen comprises BaFCI:Eu. 
     Optionally, the X-ray detector further comprises a spatially varying attenuating material inserted between the at least one scintillator screen and the WSS, wherein the spatially varying attenuating material is configured to correct a non-uniformity in detection by the photodetector. Optionally, the spatially varying attenuating material comprises a plastic substrate printed sheet with absorbing ink on a surface of the plastic substrate printed sheet. 
     Optionally, the at least one scintillator screen is coupled with the WSS by placing the at least one scintillator screen over a surface of the WSS and wherein the at least one scintillator screen at least partially covers the surface. 
     The present specification also discloses an X-ray detector configured to detect X-rays, the detector comprising: at least one scintillator screen configured to absorb incident X rays and emit light rays based on the absorbed incident X-rays; a first wavelength shifting sheet (WSS1) coupled with the at least one scintillator screen configured to shift spectra of the light rays, wherein the WSS1 comprises at least one edge; a second wavelength shifting sheet (WSS2) coupled with at the least one edge of said WSS1 and configured to collect the spectra shifted light rays; and a photodetector configured to detect the collected spectra shifted light rays, wherein the photodetector is in optical communication with the WSS2. 
     Optionally, the WSS1 comprises a first and a second surface, wherein the at least one scintillator screen partially covers the first surface and a second scintillator screen partially covers the second surface. Optionally, the first surface is coplanar to the second surface. 
     Optionally, the photodetector is a photomultiplier tube (PMT). 
     Optionally, the X-ray detector further comprises a reflector material covering the WSS2 to improve the collection of the spectra shifted light rays. Optionally, the reflector material comprises at least one of a diffuse reflector or a specular reflector material. 
     Optionally, the at least one scintillator screen comprises a material having an optical absorption length and wherein a thickness of the scintillator screen is less than the optical absorption length. 
     Optionally, the at least one scintillator screen is made of BaFCI:Eu. 
     Optionally, the X-ray detector further comprises a spatially varying attenuating material inserted between the scintillator screen and the WSS, wherein the spatially varying attenuating material is configured to correct non-uniformity in detection photodetector. 
     Optionally, the spatially varying attenuating material comprises a plastic substrate printed sheet with absorbing ink on a surface of the plastic substrate printed sheet. 
     The aforementioned and other embodiments of the present specification 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 specification will be further appreciated, as they become better understood by reference to the detailed description when considered in connection with the accompanying drawings: 
         FIG.  1 A  shows a side cross-sectional view of a “box-type” prior art scintillation detector; 
         FIG.  1 B  shows a front cross-sectional view of a “box-type” prior art scintillation detector; 
         FIG.  2    depicts spectral relationships among scintillation light and typical wavelength-shifting fiber absorption and emission spectra as known in the prior art; 
         FIG.  3 A  illustrates a prior art X-ray detector comprising WSF and scintillator layers; 
         FIG.  3 B  is a cross-sectional view of a prior art wavelength-shifting fiber (WSF) based detector; 
         FIG.  4 A  is a diagrammatical representation of a WSF detector panel; 
         FIG.  4 B  illustrates a diagrammatical front and side view of the WSF fibers coupled in a 4x4 array and being held by a mechanical fixture; 
         FIG.  4 C  is an illustration of the WSF detector panel described with respect to  FIG.  4 A ; 
         FIG.  4 D  is a plot illustrating fiber-fiber response uniformity across the prior art detector panel shown in  FIG.  4 A ; 
         FIG.  5    is a pictorial representation of a bundled end of 1 mm wave-shifting fibers, which have been cut and polished for coupling to a PMT surface; 
         FIG.  6 A  illustrates a wavelength shifting sheet (WSS) detector, in accordance with an embodiment of the present specification; 
         FIG.  6 B  is a side view of the WSS detector shown in  FIG.  6 A ; 
         FIG.  6 C  is a diagrammatical representation of a WS detector, showing light shifted by a second WS material in a direction perpendicular to an edge of a WS detector, in accordance with an embodiment of the present specification; 
         FIG.  6 D  is a pictorial representation of an image obtained by placing a scintillator screen close to the edge of the WSS; 
         FIG.  7 A  illustrates a diagrammatical side view of a WSS X-ray detector, in accordance with an embodiment of the present specification; 
         FIG.  7 B  illustrates a diagrammatical front view of the WSS X-ray detector shown in  FIG.  7 A ; 
         FIG.  7 C  illustrates a diagrammatical side view of a WSS X-ray detector, wherein the thickness of the second WS material is increased, in accordance with an embodiment of the present specification; 
         FIG.  7 D  is a block diagram of a segmented WS sheet of a WSS X-ray detector, in accordance with an embodiment of the present specification; 
         FIG.  7 E  illustrates a plurality of channels milled into the surface of a WS sheet, in accordance with an embodiment of the present specification; 
         FIG.  7 F  is a block diagram illustrating a front view of  FIG.  7 E , showing channels milled into the surface of a WS sheet, in accordance with an embodiment of the present specification; 
         FIG.  7 G  illustrates a two-step spectral overlap in a segmented WSS detector, in accordance with an embodiment of the present specification; 
         FIG.  8 A  is a graph depicting an absorption and emission spectra of BaFCI:Eu in the wavelength shifting sheet layer of the detector shown in  FIG.  7 A ; 
         FIG.  8 B  is a graph depicting an absorption and emission spectra of BaFCI:Eu in the wavelength shifting fiber layer of the detector shown in  FIG.  7 A ; 
         FIG.  9 A  illustrates light collection or acceptance cones in a wavelength shifting fiber; 
         FIG.  9 B  illustrates light loss cones in a wavelength shifting sheet; 
         FIG.  9 C  is a diagrammatical representation of light collection cones in a rectangular WS fiber; 
         FIG.  9 D  is a graphical representation of the light capture efficiencies of a WS Sheet and a WS fiber made of materials having the same refractive index; 
         FIG.  10 A  illustrates a WSS detector comprising a wavelength shifting sheet coupled with a wavelength shifting fiber (WSF) wrapped around four edges of the sheet, in accordance with an embodiment of the present specification; 
         FIG.  10 B  illustrates image data obtained from the detector shown in  FIG.  10 A ; 
         FIG.  10 C  illustrates the image response of the WSS detector shown in  FIG.  10 A ; 
         FIG.  10 D  illustrates a log-linear cross-cut of the signal response shown in  FIG.  10 C ; 
         FIG.  11 A  is a block diagram depicting an attenuating material inserted between WSS and a scintillator screen, in accordance with an embodiment of the present specification; 
         FIG.  11 B  is an image showing the response uniformity of the WSS detector, in accordance with an embodiment of the present specification; 
         FIG.  11 C  is an image showing a compensating pattern of the attenuating material used in the WSS detector shown in  FIG.  11 A ; 
         FIG.  12 A  is an image showing signal response from a wavelength shifting fiber edge coupled WSS detector; 
         FIG.  12 B  is a graph showing the signal response of  FIG.  12 A , from a horizontal cross-section through a center region of the WSS detector; 
         FIG.  12 C  is a graph depicting a wavelength shifting sheet detector response with a variable number of fibers used for light collection at the edge of the wavelength-shifting sheet, in accordance with an embodiment of the present specification; 
         FIG.  12 D  shows a plot depicting a WSS detector response with variable number of fibers used for light collection at the edge of the WS sheet, in accordance with an embodiment of the present specification; 
         FIG.  13 A  is a diagrammatical representation of a rectangular WSS detector having WS fibers in direct communication with/coupled to four sides of a wavelength shifting sheet enabling a four side readout as used in a transmission detection mode, in accordance with an embodiment of the present specification; 
         FIG.  13 B  is a diagrammatical representation of a rectangular WSS detector having WS fibers in direct communication with/couple to two sides of a wavelength shifting sheet, enabling a two side readout as used in a transmission detection mode, in accordance with an embodiment of the present specification; 
         FIG.  13 C  is a diagrammatical representation of a rectangular WSS detector having WS fibers in direct communication with/coupled to one side of a wavelength shifting sheet, enabling a single side readout as used in a transmission detection mode, in accordance with an embodiment of the present specification; 
         FIG.  14 A  is a diagrammatical representation of a rectangular WSS detector having WS fibers in direct communication with/coupled to four sides of a wavelength shifting sheet, enabling a four side as used in a backscatter detection mode, in accordance with an embodiment of the present specification; 
         FIG.  14 B  is a diagrammatical representation of a rectangular WSS detector having WS fibers in direct communication with/coupled to two sides of a wavelength shifting sheet, enabling a two side readout as used in a backscatter detection mode, in accordance with an embodiment of the present specification; 
         FIG.  14 C  is a diagrammatical representation of a rectangular WSS detector having WS fibers in direct communication with/couple to one side of a wavelength shifting sheet, enabling a single side readout as used in a backscatter detection mode, in accordance with an embodiment of the present specification; 
         FIG.  14 D  is a diagrammatical representation of a rectangular WSS detector being used in a backscatter detection mode, having eight WS fibers placed within a plurality of channels cut into a wavelength shifting sheet, in accordance with an embodiment of the present specification; 
         FIG.  15 A  is a side view of a WSS detector comprising a plurality of channels for holding WS fibers, in accordance with an embodiment of the present specification; 
         FIG.  15 B  is a top-down view of the WSS detector shown in  FIG.  15 A , comprising a plurality of channels for holding WS fibers, in accordance with an embodiment of the present specification; 
         FIG.  15 C  is a diagrammatical representation of a WSS detector array with a segmented, pixelated WS sheet comprising a plurality of channels for holding WS fibers, in accordance with an embodiment of the present specification; 
         FIG.  15 D  is a top view of the WSS detector array of  FIG.  15 C , showing two WS fibers placed within channels cut into the segmented WS sheet, in accordance with an embodiment of the present specification; 
         FIG.  16    illustrates a curved WSS detector, in accordance with an embodiment of the present specification; 
         FIG.  17 A  is a diagrammatic representation of a WSS detector as used in a transmission mode with a flying spot X-ray imager, in an embodiment of the present specification; 
         FIG.  17 B  is a flow diagram representing the steps of using an WSS detector in a transmission mode with a flying spot X-ray imager, in accordance with an embodiment of the present specification; 
         FIG.  17 C  is a diagrammatic representation of a WSS detector as used in a backscatter mode with a flying spot X-ray imager, in an embodiment of the present specification; 
         FIG.  17 D  is a flow diagram representing the steps of using an WSS detector in a backscatter mode with a flying spot X-ray imager, in accordance with an embodiment of the present specification; 
         FIG.  17 E  is a diagrammatic representation of a multi-energy WSS detector as used in a transmission mode with a flying spot X-ray imager, in an embodiment of the present specification; 
         FIG.  17 F  is a flow diagram representing the steps of using the multi-energy WSS detector shown in  FIG.  17 E , in a transmission mode with a flying spot X-ray imager, in accordance with an embodiment of the present specification; 
         FIG.  17 G  is a diagrammatic representation of a vertical WSS detector and a ground-level WSS detector as used in a transmission mode with a flying spot X-ray imager for scanning a cargo object, in an embodiment of the present specification; 
         FIG.  17 H  is a flow diagram representing the steps of using the vertical WSS detector and a ground-level WSS detector shown in  FIG.  17 G  in a transmission mode with a flying spot X-ray imager for scanning a cargo, in accordance with an embodiment of the present specification; 
         FIG.  17 I  illustrates a WSS detector panel placed underneath a car to collect transmission radiation emitted by a small portable scanner being used to scan a boot of the car, in accordance with an embodiment of the present specification; 
         FIG.  17 J  illustrates explosives hidden approximately two feet under the boot of the car shown in  FIG.  171   ; 
         FIG.  17 K  illustrates a transmission image obtained by the WSS detector panel placed under the car as shown in  FIG.  17 I , in accordance with an embodiment of the present specification; 
         FIG.  17 L  illustrates a perspective view of the transmission detector panel shown in  FIG.  17 I , in accordance with an embodiment of the present specification; 
         FIG.  17 M  illustrates a top view of the transmission detector panel shown in  FIG.  17 I , in accordance with an embodiment of the present specification; 
         FIG.  17 N  illustrates an exploded view of a plurality of components of the transmission detector panel shown in  FIG.  17 L , in accordance with an embodiment of the present specification; 
         FIG.  17 O  illustrates a WSS detector panel placed to collect backscatter radiations emitted by a small portable scanner being used to scan a car, in accordance with an embodiment of the present specification; 
         FIG.  17 P  illustrates explosives hidden approximately two feet under the boot of the car shown in  FIG.  17 O ; 
         FIG.  17 Q  illustrates a backscatter image obtained by a built-in detector of the scanner of  FIG.  17 O , in accordance with an embodiment of the present specification; 
         FIG.  17 R  illustrates a backscatter image obtained by the scanner and the detector panel of  FIG.  17 O , in accordance with an embodiment of the present specification; 
         FIG.  18 A  illustrates a portable detector for handheld imaging, as used in a transmission mode, in accordance with an embodiment of the present specification; 
         FIG.  18 B  illustrates a portable detector for handheld imaging, as used in a forward scatter mode, in accordance with an embodiment of the present specification; 
         FIG.  18 C  illustrates a portable detector for handheld imaging, as used in a back scatter mode, in accordance with an embodiment of the present specification; 
         FIG.  18 D  is a flowchart illustrating the steps of imaging a target in a forward scatter mode by using a WSS detector panel and a handheld imaging system, in accordance with an embodiment of the present specification; 
         FIG.  19    is a table conveying exemplary dimensions and perimeter fractions of a plurality of WSS detectors in various modes of operation, in embodiments of the present specification. 
         FIG.  20    illustrates a diagrammatical representation of an object being scanned by a portable scanner and a WSS detector panel, in accordance with an embodiment of the present specification; 
         FIG.  21 A  illustrates a standard shipping box comprising an explosive object not visible with the naked eye; 
         FIG.  21 B  illustrates an image of the shipping box obtained by using a portable scanning system and a WSS detector panel of the present specification operating in a backscatter mode, in accordance with an embodiment of the present specification; 
         FIG.  21 C  illustrates an image of the shipping box obtained by using a portable scanning system and a WSS detector panel of the present specification operating in a transmission mode, in accordance with an embodiment of the present specification; 
         FIG.  22 A  illustrates explosives hidden in a concrete block; 
         FIG.  22 B  illustrates an image of the concrete block shown in  FIG.  22 A , obtained by using a portable scanning system and a WSS detector panel of the present specification operating in a backscatter mode, in accordance with an embodiment of the present specification; and 
         FIG.  22 C  illustrates an image of the concrete block shown in  FIG.  22 A , obtained by using a portable scanning system and a WSS detector panel of the present specification operating in a transmission mode, in accordance with an embodiment of the present specification. 
     
    
    
     DETAILED DESCRIPTION 
     In an embodiment, the present specification discloses an X-ray detector for a flying spot transmission imaging system, wherein the detector enables improved spatial uniformity and reduced cost for materials and manufacturing. 
     Definitions 
     The term “image” shall refer to any unidimensional or multidimensional representation, whether in tangible or otherwise perceptible form, or otherwise, whereby a value of some characteristic (such as fractional transmitted intensity through a column of an inspected object traversed by an incident beam, in the case of X-ray transmission imaging) is associated with each of a plurality of locations (or, vectors in a Euclidean space, typically R2) corresponding to dimensional coordinates of an object in physical space, though not necessarily mapped one-to-one thereonto. An image may comprise an array of numbers in a computer memory or holographic medium. Similarly, “imaging” refers to the rendering of a stated physical characteristic in terms of one or more images. 
     Terms of spatial relation, such as “above,” “below,” “upper,” “lower,” and the like, may be used herein for ease of description to describe the relationship of one element to another as shown in the figures. It will be understood that such terms of spatial relation are intended to encompass different orientations of the apparatus in use or operation in addition to the orientation described and/or depicted in the figures. 
     Where an element is described as being “on,” “connected to,” or “coupled to” another element, it may be directly on, connected or coupled to the other element, or, alternatively, one or more intervening elements may be present, unless otherwise specified. 
     For purposes of the present description, and in any appended claims, the term “thickness,” as applied to a scintillation detector, shall represent the mean extent of the detector in a dimension along, or parallel to, a centroid of the field of view of the detector. The term area, as applied to a detector, or, equivalently, the term “active area” shall refer to the size of the detector measured in a plane transverse to centroid of all propagation vectors of radiation within the field of view of the detector. 
     As used herein, and in any appended claims, the term “large-area detector” shall refer to any single detector, or to any detector module, subtending an opening angle of at least 30° in each of two orthogonal transverse directions as viewed from a point on an object undergoing inspection, equivalently, characterized by a spatial angle of at least π steradians. 
     A “conveyance” shall be any device characterized by a platform borne on ground-contacting members such as wheels, tracks, treads, skids, etc., used for transporting equipment from one location to another. 
     The terminology used herein is for the purpose of describing particular embodiments and is not intended to be limiting. In the description and claims of the application, each of the words “comprise” “include” and “have”, and forms thereof, are not necessarily limited to members in a list with which the words may be associated. 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. The singular forms “a,” “an,” and “the,” are intended to include the plural forms as well. 
     The present specification is directed towards multiple embodiments. The following disclosure is provided in order to enable a person having ordinary skill in the art to practice the specification. 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 specification. 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 specification have not been described in detail so as not to unnecessarily obscure the present specification. 
     The X-ray detector disclosed in the present specification comprises a screen fabricated from a scintillator material, such as phosphor, that is optically coupled, in optical contact or in physical communication with a wavelength-shifting sheet (WSS), which shifts light absorbed from the scintillator screen. The wavelength shifting sheet is coupled to a wavelength shifting fiber or sheet at the edge of the wavelength shifting sheet that is configured to collect a plurality of first shifted rays. The rays collected from the edge are transmitted through to a photodetector, such as a photo multiplier tube (PMT). 
     In various embodiments, the use of a Wave Shifting Sheet (WSS) as the primary material for flying spot transmission X-ray detection reduces or eliminates objectionable, highly visible defects which are inherent in X-ray detectors implemented with solely Wave Shifting Fibers (WSF). In addition, the WSS detector can be fabricated using low cost plastic materials assembled in a simple manufacturing process. The WSS detector of the present specification can be used in conjunction with any flying spot x-ray system as a transmission, backscatter and forward scatter detector, and may also include multiple layers for materials discrimination. 
       FIG.  4 A  illustrates a diagrammatical representation of a WSF detector panel.  FIG.  4 B  illustrates a diagrammatical front and side view of the WSF fibers coupled in a 4x4 array and being held by a mechanical fixture. As shown in  FIGS.  4 A and  4 B , a plurality of WSF fibers  402  are coupled with a scintillator screen  408  such that the fibers  402  are positioned between the scintillator screen  408  and a photomultiplier tube (PMT)  406 . The fibers  402  held by a mechanical support  404  at the edges and fibers  402  are in contact with each other without having any space therebetween. In order to reduce the number of fibers required in the detector by half, a single fiber loops back from the mechanical support  404  to the PMT  406  causing said fibers to bend as shown in  FIG.  4 A . In various embodiments, fiber ends may be trimmed and polished to improve optical transmission and response uniformity. Optical grease may be used to improve the optical transmission from the fiber to the PMT.  FIG.  4 C  illustrates the WSF detector panel of  FIG.  4 A , in accordance with an embodiment of the present specification. The detector panel is preferably made of off-the-shelf (OTS) materials, and the constituent WSF fibers are held together without any spacing between them. 
       FIG.  4 D  illustrates a graph  410  illustrating the fiber response uniformity of the detector panel shown in  FIGS.  4 A and  4 B . An example of the uniformity across the detector (from fiber to fiber) is plotted in  FIG.  4 D . The greyscale plot  410  depicts the signal across the detector. In this case, variability is created due to the groups of bends in the fiber at the ends of the detector as shown in  FIG.  4 A . The variability is enough to create artifacts which are visible during imaging, cannot be corrected or calibrated, and could be objectionable to the user during typical imaging. Plot  410  also depicts the output light intensity as a function of input X-ray beam position as measured across the detector. The light response output has 80% variability as calculated by the difference between the maximum and minimum intensity divided by the mean. 
     Conventionally, wave-shifting fibers are cut and their ends are polished for coupling to a PMT surface.  FIG.  5    shows the bundled end of 1 mm wave-shifting fibers  502  which have been cut and polished for coupling to a PMT surface. Any non-uniformity in the polishing of the bundle from fiber to fiber will lead to light loss in an individual fiber. The light loss will create a line defect across the entire image detected by the PMT. 
     Wavelength Shifting Sheet (WSS) Detector Structure 
       FIG.  6 A  illustrates a Wave Shifting Sheet (WSS) X-ray detector, in accordance with an embodiment of the present specification.  FIG.  6 B  is a side view of the WSS detector shown in  FIG.  6 A . Referring to both  FIGS.  6 A and  6 B , WSS detector  600  comprises a sheet  602  fabricated from a wavelength shifting material and at least partially covered by a scintillator screen  604  on at least a portion of, or the entirety of, a first surface  602   a  of a first side or at least a portion of, or the entirety of, a second surface  602   b  of a second side, or preferably both the first surface  602   a  of the first side and the second surface  602   b  of the second side, of sheet  602 , as shown in  FIGS.  6 A and  6 B . In various embodiments, the sheet  602  is a contiguous WSS sheet, and is characterized by a single solid media with single thickness throughout its surface. In an embodiment, sheet  602  is not comprised of a solid ribbon or a ribboned bundle of wavelength shifting fibers. In an embodiment, sheet  602  has a smaller aspect ratio compared to a conventional WS ribbon. In an embodiment, the first surface  602   a  of the first side or the second surface  602   b  of the second side, or preferably both the first surface  602   a  of the first side and the second surface  602   b  of the second side of sheet  602 , is smooth and the surfaces are coplanar with minimal defects in order to maintain total internal reflection and minimize the loss of light. In various embodiments, defects may include any surface imperfections such as scratches, pits, surface particles, fibers or surface bumps/burrs; as well as defects interior to the sheet including bubbles, captured particles or fibers. In embodiments, the surfaces of the sheet  602  are polished and free of scratches, pits, or imperfections. In an embodiment, the surfaces are polished to less than 0.5 µm RMS roughness with less than 10 isolated defects of size up to 100 µm in size in a total area of 1 cm 2 , less than 3 isolated defects of size ranging from 100 µm to 500 µm in size in a total area of 1 cm 2 , and no defects are larger than 500 µm. 
     Typically, in a ribboned bundle of WS fibers, the individual fibers have an aspect ratio of 1 mm x 2 mm which is constrained to fit the area of a typical PMT. In an embodiment, for a horizontal detector sheet, a typical circular fiber diameter is 1 mm. 
     In the WS sheet of the present specification, the aspect ratio may be constrained to a thickness ranging from 1 mm to 10 mm and a width ranging from 2 mm to 3,000 mm. In an embodiment, the thickness of the WS sheet  602  ranges from 2 mm to 10 mm and the width ranges from 2 mm to 5 mm in order to capture a majority of the light exiting the sheet. 
     In an embodiment, the scintillator screen  604  is a phosphor screen. In an embodiment, the border edge  606  of the wavelength shifting material sheet  602  comprises a wavelength shifting fiber(s)  607  which, in turn is coupled to a photodetector, such as a photomultiplier tube (PMT)  608 . In an embodiment, a second WS material sheet is used in place of the WS fiber(s)  607 . Thus, in an embodiment, the edge  606  of the wavelength shifting material sheet  602  may comprise a second wavelength shifting material or a wavelength shifting fiber(s), which may be different from the material of WS sheet  602 . During operation of the X-ray detector  600 , the scintillator screen  604  absorbs any incident X-rays and emits corresponding light rays which are then shifted by the sheet  602 . The shifted light is collected by the wavelength shifting (WS) fiber  607  provided at the edge  606 . The shifted light is then shifted again by the WS fiber  607  and transmitted through to the PMT  608  for subsequent detection. Since, in this embodiment, only two fibers (one from a first side and one from a second side) enter the PMT  608 , a required PMT area is less than that required with prior art detectors. 
     In an embodiment, the second WS material used in edge  606  of the WSS  602  has a refractive index of 1.5 or greater to improve a capture efficiency of total internal reflection of light which is generated by fluorescence inside the material. In various embodiments, the WS material used in edge  606  absorbs light from the WS sheet  602  and fluoresces at a longer wavelength, and efficiently trapping fluorescent light. In embodiments, the WSS detectors of the present specification are manufacturing using polyvinyl butyral for sheet  602  and polystyrene (n=1.6) with PMMA cladding for the wavelength shifting fiber  607 . 
       FIG.  6 C  is a diagrammatical representation of a WS detector, showing light shifted by a second WS material in a direction perpendicular to an edge of a WS detector, in accordance with an embodiment of the present specification. In various embodiments, a light ray  650  which is shifted by the sheet  602  is absorbed by the second WS material of edge  606  at a location  606   a , and re-directed in a direction  651  perpendicular to a line  652  that is normal to location  606   a  as shown in  FIG.  6 C . The WS sheet  602  may have an irregular shaped edge, however the light ray redirection is always normal to the edge location at which the light ray is received after it is redirected by sheet  602 . 
     In various embodiments, the highest capture efficiency for the WS sheet  602  occurs when the media surrounding the sheet  602  has an index of 1 (air), and the index of the sheet  602  is as high as possible. Hence, in embodiments, the WSS detector  600  is obtained by placing a scintillator screen  604  over a wavelength shifting sheet  602  to maintain an air gap between the scintillator  604  and the WSS  602 , and as such, does not require an embedding or molding process, thereby decreasing the cost of manufacture. If an adhesive is used to couple the sheet scintillator  604  and the WS sheet  602 , a low-index adhesive is beneficial, because, as is known, the total fraction of fluorescent light collected by an infinite WS sheet without scattering corresponds to the following formula: 
     
       
         
           
             F= 
             
               
                 
                   
                     1 
                     − 
                     
                       
                         
                           n 
                           1 
                         
                         
                             
                           2 
                         
                       
                       
                         
                           n 
                           2 
                         
                         
                             
                           2 
                         
                       
                     
                   
                 
               
             
           
         
       
     
      where n 1  is the index of refraction of the surrounding media and n 2  is the index of refraction of the WS sheet medium. Hence, in some embodiments, small regions of adhesive material may be used to attach the scintillator screen, with the contact area remaining as small as possible. 
     In various embodiments, the scintillator screen  604  does not cover the entire surface of the WS sheet  602 . In embodiments, a scintillator screen  604  smaller than the WS sheet  602  may be coupled with the WS sheet  602  in such a manner that the scintillator screen  604  is not in contact with the WS sheet near the edges  606 , in order to improve uniformity of the image obtained by using the detector  600 . If the scintillator screen  604  is placed close to the edge of the WSS  602 , there is a greater amount of direct exposure to the edge fiber(s). The scintillator screen  604  absorbs any incident X-rays and emits corresponding light rays which are then shifted by the sheet  602  and then absorbed by the WS fiber  607  at the edge  606  of the sheet  602  for a second shift. If scintillation screen  604  is placed close to the edge of the WSS  602 , the photons from light emitted due to scintillation are so close to the fiber(s), that they are directly absorbed by the WS fiber  607 , without being shifted by the WS sheet  602 . 
       FIG.  6 D  is a pictorial representation of an image obtained by placing a scintillator screen close to the edge of the WSS. Bright regions  630  at the edge region are caused by direct illumination (not WSS shifted) scintillator light striking the WSF. Hence, in various embodiments, the scintillator screen  604  is of a different size than WSS  602 . In an embodiment, the scintillator screen  604  is either cut smaller than the WSS, or an opaque material is applied along the edges of the scintillator screen to block out a portion of the scintillator screen edge. 
     Since the light transmitted to the PMT  608  exits from a narrow region  610  along the edge of the WSS detector  600 , in an embodiment, the PMT  608  may be included in the same enclosure as the WSS detector  600 , or may be coupled through a clear fiber optic cable to an external PMT (not shown in the  FIG.  6 A ) having an area equal to or larger than the exit area of the cable. In an embodiment, having a coupling with a cable from both ends of the detector to minimize cable absorption losses, an external PMT having an area equal to approximately 20 mm 2  is used for a total WS sheet thickness of 3 mm to 5 mm and a width of an edge border WS fiber at approximately 2 mm, for absorption of 90% of the incident light. 
     In various embodiments, the areal density of the top and bottom scintillating screen  604  can be optimized for maximum x-ray absorption. 
       FIG.  7 A  illustrates a diagrammatical side view of a WSS X-ray detector, in accordance with an embodiment of the present specification.  FIG.  7 B  illustrates a diagrammatical front view of the WSS X-ray detector shown in  FIG.  7 A . As shown a first set of incident X rays  702  are absorbed by a top scintillator layer  704  and the corresponding emitted light  703  enters the WSS layer  706 , which in turn shifts the light  703  a first time and transmits the light outward, to its edges, in all directions such that the light  703  is received, absorbed, and then shifted a second time by WS fibers  708 . A second set of incident X rays  710  which pass through the top scintillator layer  704  and the WSS layer  706 , are absorbed by a bottom scintillator layer  712  and the corresponding light re-enters the WSS layer  706 , which in turn shifts the light and transmits the light outward, to its edges, in all directions such that the light is also absorbed by WS sheet or fibers  708 . The shifted light absorbed by the WS sheet or fibers  708  and shifted again are transmitted along the length of the WS sheet or fibers and subsequently transmitted into a photo multiplier tube (PMT)  714  for detection. In an embodiment, a second WS material sheet is preferably used in place of the WS fibers  708 . 
     An advantage of the second shift of light by the WS sheet or fibers  708  is that the photons are concentrated into a smaller exit face area. With a single shift as used in most currently available detectors, the PMT area is proportional to the width of the detector multiplied by its thickness. A typical detector width is 400 mm and a typical thickness of a WS fiber is 1 mm, leading to a required PMT area of 400 mm 2 . By using a WS sheet instead of fiber and thus incorporating a second shift, the PMT area needed reduces to the thickness of the WS sheet multiplied by the width of the second shift region or edge region. A typical sheet thickness is 2 mm and a typical width of the second WS material  708  is 2 mm, which equals a required PMT area of 4 mm 2 , with a significant reduction by a factor of 100X. The area reduction factor is limited by both a light trapping efficiency and a fundamental physical limit which prevents light concentration where the total entropy is reduced. 
     In an embodiment, the second WS material  708  covers the entire face of the first WS sheet  706 . Further in an embodiment, the efficiency of the light collection in the WS sheet  708  is improved by increasing the thickness of the second WS material  708 .  FIG.  7 C  illustrates a diagrammatical side view of a WSS X-ray detector, wherein the thickness of the second WS material  708  is increased, in accordance with an embodiment of the present specification. As shown in  FIG.  7 C , the additional thickness captures light  705  which exits the second WS material  708  at an angle equal to the critical angle of the material. In an embodiment where the width and thickness of the second WS material  708  is ‘W’ and ‘T’, respectively, the optimized value of ‘T’ for capturing of 90% of the light emitted by WS sheet  706  is: 
     
       
         
           
             T2 
             = 
             T1 
             + 
             2 
             ⋅ 
             cos 
             
               
                 
                   α 
                   c 
                 
               
             
           
         
       
     
      where ‘αc’ is the critical angle for first WS sheet  706 , and sin(αc) =1/n; where ‘n’ is the refractive index for second WS material  708 , assuming air is surrounding second WS material  708 . In embodiments where the second WS material  708  is fiber, the light capture efficiency is dependent on the shape of the fiber, and a greater capture efficiency may be obtained by using a square cross section of WS fiber  708 . 
     In an embodiment, a reflector material  716  is provided around all exposed edges of WS fiber or sheet to improve X ray absorption and transmission to the PMT  714 . The reflector material  716  may be composed of a diffuse reflector (paint or tape) or a specular reflector material (metallic). 
     In various embodiments, the thickness of the top scintillator layer  704  does not exceed the optical absorption length of the scintillator material, as that may lead to absorption of X-rays which do not contribute to the signal detected by PMT  714 . As is known, the optical path length in a scintillator screen  704  is limited by absorption of light scattered in the phosphor. Hence, X-rays which are absorbed near the top of the scintillator screen  704 , emit light which is absorbed before exiting the bottom of the scintillator screen  704  and entering the WSS  706 . 
     The scintillator material emits visible light, preferably in the UV portion of the spectrum in order to maintain efficient energy transfer. In an embodiment, Europium-doped barium fluorochloride (BaFCI:Eu) is used as the scintillator material. In other embodiments various other suitable scintillator materials, such as, but not limited to Gadolinium Oxysulfide, and Cesium Iodide may be used. 
     There are advantages to collecting light shifted by the WS sheets with a wavelength-shifting fiber  607  used at the edges of the sheet  602 , as opposed to using a second wavelength shifting sheet, including the following:
     WS fibers are not directly illuminated in the active area of the image, and thus variations in individual fibers response (output coupling, bend loss, fiber defects) affect only large regions of the image and not lines across the image;   WS fibers enable more efficient collection around the corners of the detector; and,   WS fibers are flexible and the coupling cable can be extended past the edge of the active area of the detector to enable remote coupling to the PMT at a large distance from the detector (up to 1 meter).   

     In an embodiment, the WS sheet of the detector as described with reference to  FIGS.  7 A,  7 B, and  7 C  is optically segmented into pixels using a mechanical milling process.  FIG.  7 D  is a block diagram of a segmented WSS of a WSS X-ray detector array, in accordance with an embodiment of the present specification. WSS  720  comprises a plurality of segments  722  formed by milling grooves  732  into WSS  720 , bound by WS fibers  724  at the edges. In various embodiments, the grooves have a thickness ranging from 0.25 mm to 1 mm and a depth ranging from 50% to 90% of the total sheet thickness are cut into WS sheet having a minimum thickness of 0.5 mm. In various embodiments, the milled regions  732  (dead region) are made as thin as possible to maintain detector efficiency.  FIG.  7 E  illustrates grooves  732  milled into the surface of a WS sheet  720 , in accordance with an embodiment of the present specification.  FIG.  7 F  is a block diagram illustrating a front view of grooves milled into the surface of a WS sheet, in accordance with an embodiment of the present specification. As shown in  FIGS.  7 D,  7 E and  7 F , WS sheet  720  is divided into a plurality of segments  722  by grooves  732 . In embodiments, the grooves  732  may be filled with reflector material such as, but not limited to, 3M ESR tape, Teflon tape, or white TiO 2  paint to prevent cross talk of optical photons between pixels while also improving detector efficiency. To maintain the mechanical structure of the WSS  720  when the grooves are placed into the WS material, the sheet may be adhesively bonded to a light-weight and thin mechanical support  734 . In embodiments, the top of the mechanical support  734  is coated with a reflector material  736  to reflect scintillator light into the sheet. The minimum pixel size, which in an embodiment is 0.5 mm is limited by the machining of the grooves. If the pixel size is approximately the same as the groove width, the dead region in the pixel will be large and the detector will not function. The minimum pixel size may be approximately 2 mm, for a 0.5 mm groove width. As described with reference to  FIGS.  7 A and  7 D , the segmented pixelated WS sheet is covered with a scintillator layer for converting incident X rays into light rays which are collected by a WS fiber provided at the edge of the WSS detector and fed to a PMT.  FIG.  7 G  illustrates a two-step spectral overlap in a WSS detector having a segmented WSS, in accordance with an embodiment of the present specification. Plot  750  illustrates absorption and emission plots of both a scintillator and WSS while plot  752  illustrates absorption and emission plots of both the WSS and WSF. Plot  750  demonstrates the light spectrum overlap between the emission spectra of the scintillator and the absorption of the WSS. Plot  752  demonstrates the spectral overlap between the WSS and the WS fiber. Regions where the two curves do not overlap represent inefficiency in the light coupling between the detector layers. 
       FIG.  8 A  is a graph depicting an absorption and emission spectra of BaFCI:Eu in the WSS layer of the detector shown in  FIG.  7 A .  FIG.  8 B  is a graph depicting an absorption and emission spectra of BaFCI:Eu in the WS fiber layer of the detector shown in  FIG.  7 A . As show in  FIG.  8 A , scintillator material BaFCI:Eu has an emission spectrum peak near 390 nm. Light is absorbed in the WSS layer and emitted at the shifted wavelength of 400 nm. 
     With reference to  FIGS.  7 A and  7 B  light  703  travels in the WSS detector through either diffuse reflection from the surface of the WS sheet  706  or the scintillator layer  704 , or through total internal reflection (TIR) from the WSS surface. All light rays which are emitted in the WS sheet  706  at an angle which is larger than the critical angle of the WS material are captured by TIR and propagated to the edge of the sheet  706 . Hence the light collection efficiency of the WSS detector is dependent on the index of refraction of the WS sheet  706 . 
     In an embodiment, in order to improve the TIR angle, the index of refraction of the scintillator material  704  maybe decreased while the index of the WSS layer  706  may be increased, as the critical angle  
     
       
         
           
             
               θ 
               c 
             
             = 
             arcsin 
             
               
                 
                   
                     
                       n 
                       2 
                     
                   
                   
                     
                       n 
                       1 
                     
                   
                 
               
             
               
             ; 
           
         
       
     
     where n 1  denotes the refractive index of the WSS material  706  and n 2  denotes the refractive index of air if no adhesive is used to attach the scintillator  704  to the WSS. For example, if typical plastics such as acrylic/PMMA are used in the detector, having a refractive index n1=1.4 and wherein the refractive index of air is equal to 1, the critical angle θ c ,can be calculated as being equal to 40°. 
     It is advantageous to use WS sheets for obtaining X ray detectors as provided by the present specification instead of using WS fibers, as the light collection in WS sheets is more efficient than that in WS fibers.  FIG.  9 A  illustrates light collection cones in a WS fiber. In a WS fiber  902 , light rays which fall within two light collecting cones  904  are transmitted via TIR along the length of the fiber  902 .  FIG.  9 B  illustrates light loss cones in a WS sheet. In sheet  906 , light is collected for all rays except for light lost due to rays which fall within two light loss cones  908 , leading to a much wider volume of acceptance angles. 
       FIG.  9 C  is a diagrammatical representation of light collection cones in a rectangular WS fiber. As shown in  FIG.  9 C  rectangular fiber  910  comprises four light collecting cones  912 , one cone corresponding to each of the four surface of fiber  910 . 
     The solid angle for a single light loss cone is 
     
       
         
           
             
               Ω 
               
                 cone 
               
             
             = 
             2 
             π 
             
               
                 1 
                 − 
                 
                   
                     1 
                     − 
                     
                       1 
                       
                         
                           n 
                           2 
                         
                       
                     
                   
                 
               
             
           
         
       
     
     Thus the capture efficiency for the rectangular WS fiber  910  is: 
     
       
         
           
             
               F 
               
                 rect 
               
             
             = 
             
               
                 
                   
                     4 
                     π 
                     − 
                     4 
                     ⋅ 
                     
                       Ω 
                       
                         cone 
                       
                     
                   
                 
               
               
                 4 
                 π 
               
             
             = 
             
               
                 2 
                 ⋅ 
                 
                   
                     1 
                     − 
                     
                       1 
                       
                         
                           n 
                           2 
                         
                       
                     
                   
                 
               
             
             − 
             1 
           
         
       
     
     Assuming that an infinite WS sheet has two faces, and thus two light loss cones have a capture efficiency of: 
     
       
         
           
             
               F 
               
                 sheet 
               
             
             = 
             
               
                 
                   
                     1 
                     − 
                     
                       1 
                       
                         
                           n 
                           2 
                         
                       
                     
                   
                 
               
             
           
         
       
     
     Hence, it can be seen that the light capture efficiency in WS sheets is greater than that in WS fibers.  FIG.  9 D  is a graphical representation of the light capture efficiencies of a WS Sheet  920  and a WS fiber  930  made of materials having the same refractive index. As, demonstrated by the plots  920  and  930 , WS sheets provide better light capturing efficiencies. 
     Referring to  FIGS.  7 A and  7 B  again, light will be attenuated in the WSS material  706  which contains a dye material, leading to a uniform decrease in the signal intensity away from the X-ray source absorption point. The dye material is used to absorb light emitted from the scintillator and emit light that transmits to the edge of the WS sheet  706 . In various embodiments, any commercially available wavelength shifting paint may be used as the dye material. In order to minimize absorption, the WSS  706  material is kept as thin as possible for creating a shortest length path for the light  703  to reach to the edge of the sheet. Conversely, the WSS  706  material thickness is increased to minimize the number of reflections from the surface of the sheet. Depending on the attenuation length of light in WSS  706  material and the absorption loss in the scintillator layer  704 , the optimal thickness of the WSS  706  changes. For example, the thickness of a WSS  706  material may range from 2 mm to 10 mm, with an attenuation length of approximately 3.5 cm. 
     In edge region, the light  703  is absorbed and re-emitted for collection by the PMT  714 . In an embodiment, in order to improve coupling efficiency and uniformity, the edge WSS or WSF  708  material is in contact with the WSS  706  material around four sides of the detector; and the width of the edge WSS or WSF  708  material is increased to an optimal level for better absorption of light from the WSS  706 . In embodiments, the width of the edge WSS or WSF  708  material is determined by measuring the optical attenuation length at the wavelength of emission light from the WSS  706 , and selecting a length with an attenuation greater than 90%. In various embodiment, the width of the edge WSS or WSF  708  material ranges from 2 mm to 5 mm. In an embodiment, the width of the edge WSS or WSF  708  material is 4 mm. 
       FIG.  10 A  illustrates a WSS detector comprising a wavelength shifting sheet  1002  coupled with a wavelength shifting fiber (WSF) wrapped around/couple to/in direct communication with four edges of the sheet, in accordance with an embodiment of the present specification. In other embodiments, the WSF may be wrapped around/coupled to/in direct communication with one, two, or three edges/sides of the WS sheet  1002 . In the embodiment shown in  FIG.  10 A , the WS sheet has a dimension of 12″ x 12″ and the aggregate WSF fiber at the edges comprises a bundle of six WS fibers. The corners of the of the WS sheet  1002  have been rounded for uniform bending of the WSF fiber with a fillet having an approximate diameter of 0.75”. 
     In an embodiment, the detector  1000  is constructed using a plastic (PVB) WSS material coated with a dye for absorbing light in the UV spectrum and emitting light in the blue spectrum, and having a width ranging from 50 mm to 800 mm, height ranging from 50 mm to 2,500 mm and thickness ranging from 2 mm to 10 mm; wherein all edges of the sheet are diamond milled. 
     In an embodiment, a scintillator screen of BaFCI:Eu having a density ranging from 40 mg/cm2 to 250 mg/cm2 is coupled with the front and back faces of WSS detector  1000 . In an embodiment a reflector tape  1004  is used to attach the WSS  1002  with the WSF, as well as to attach the scintillator screen to the WSS. A PMT  1006  is coupled with the detector  1000  for signal detection. In other embodiments, the WS sheet may be circular, oval, or have an irregular shape with at least one edge; wherein, the WSF is wrapped around at least a portion of the edge. 
       FIG.  10 B  illustrates image data obtained from the detector shown in  FIG.  10 A . The image is a transmission image of a HDPE phantom with resolution blocks demonstrating steel penetration. Penetration thru ⅜″ thick steel is demonstrated in the outset portion  1030  of the image  1040 . The WSS detector  1000  as described in the present specification has a predictable and repeatable non-uniform response.  FIG.  10 C  illustrates the gain uniformity of the image response of the WSS detector shown in  FIG.  10 A . As shown, excluding the edge region, the detector has a smooth varying gain with a variability being greater than 10%.  FIG.  10 D  illustrates a log-linear cross-cut of the signal response shown in  FIG.  10 C . 
     When coupled with a second WS material on all four sides, the non-uniformity of the detected image obtained by using the WSS detector as described above, is symmetric and lowest in the center of the detector panel with a weak change due to attenuation in the second WS material sheet. In an embodiment, the non-uniformity is corrected by inserting a spatially varying attenuating material between the scintillator screen and the WSS.  FIG.  11 A  is a block diagram depicting an attenuating material inserted between WSS and a scintillator screen, in accordance with an embodiment of the present specification. Referring to  FIG.  11 A  attenuating material  1102  is inserted between WSS  1104  and a scintillator screen  1106  in the form of a compensating sheet. The attenuating material  1102  may be applied in the form of a printed sheet with absorbing ink on the surface. For a given set of materials and sizes of detector, the non-uniformity and compensation pattern are fixed and repeatable from detector to detector. The compensating sheet made of the attenuating material  1102  is printed with a pattern that attenuates high response regions of the detector panel. In various embodiments, the attenuating material  1102  is a spatially varying attenuating material, used to smooth out the repeatable gain variations in the WS sheet. Repeatable variations may be caused by the geometry of the WSS, by the light cross-shading in the detector gain. In embodiments, by inserting the compensating sheet with attenuation of light emitted from the scintillator, the gain variation can be corrected. 
       FIG.  11 B  illustrates an image showing the response uniformity of the WSS detector, in accordance with an embodiment of the present specification.  FIG.  11 C  illustrates a compensating pattern of the attenuating material  1102  used in the WSS detector shown in  FIG.  11 A . In various embodiments, the spatially attenuating material  1102  has the following characteristics:
     Dynamic range &amp; Extinction coefficient: For maximum absorption, the attenuating material has a large extinction coefficient with the ability to absorb  100   x  the total light output from the scintillator in 1-2 mm thickness (µ=5 cm-1). The spectral absorption is neutral (flat) across the range from 350 nm to 600 nm in order to cover the full range of scintillators commonly used. The minimum absorption cuts the output of the scintillator by &lt;6-7% of the total output.   Low cost: &lt;100 \$ for 16″ x 16″ part, substrate plastic or other readily available material.   Printed/Patterned: ability to change absorption with printing resolution of at least 0.5 mm, using dithered printing or greyscale.   Thin: total attenuator material is less than 1-2 mm thick in order to add a minimum of thickness to the WSS detector.   

     In various embodiments, techniques such as but not limited to laser printed pattern on acetate film by using either greyscale pattern or dithered printing; printed absorber such as: screen printing, inkjet printing, transfer print, print inks on acetate or other clear substrate, or directly print absorber on the WSS; absorbing sheet with varying thickness; textured sheet or textured surface of WSS such as bead blast, sanded, chemically roughened, or direct transfer thru mold, or plastic film or directly textured WSS surface; printed scintillator material; and variable dye concentration WSS such as spatially patterned surface printed dye and resin on the WS sheet may be used. 
     As described above with reference to  FIG.  7 A , in various embodiments, it is advantageous collect of X rays with WS fibers as opposed to a second WS material sheet.  FIG.  12 A  illustrates an image showing signal response from a WS fiber edge coupled WSS detector.  FIG.  12 B  illustrates a graphical depiction of a cross cut of the signal response shown in  FIG.  12 A , horizontally across the center region of the WSS detector. In various embodiments, the response of a WSS detector coupled with four sided WSF collection shows an improvement in uniformity. The uniformity improves from a 100X variation in a single sided WSS to 2.6X variation from edge to center of the detector in a WSS detector coupled with four sided WSF collection. In addition, variations are smooth and correctable with an absorbing filter. In embodiments, the number of fibers coupled with the edges of a WSS detector may be increased in order to improve the total amount of light absorbed.  FIG.  12 C  illustrates additional WS Fibers  1202  arranged at the edge of WSS  1204  and a scintillator screen  1206 , in accordance with an embodiment of the present specification. In various embodiments, inserting additional fibers increases the total amount of light collected at the edge of the sheet  1204 . 
       FIG.  12 D  shows a plot depicting a WSS detector response with variable number of fibers used for light collection at the edge of the WS sheet, in accordance with an embodiment of the present specification. Plot  1222  depicts response from a WSS detector having 2, 4 and 6 WS Fibers coupled with the edges. Plots  1224 ,  1226  and  1228  depict response from WSS detectors coupled along a single edge with 2, 4 and 6 WS fibers respectively for feeding collected X-rays to a PMT. As shown by the plots, additional fibers add to the amount of material which can absorb light at the edge of the sheet, and in embodiments, each additional two fibers adds 1 mm of thickness to the absorbing region thickness. 
       FIG.  13 A  is a diagrammatical representation of a rectangular WSS detector having WS fibers in direct communication with/coupled to four sides of a wavelength shifting sheet enabling a four side readout as used in a transmission detection mode, in accordance with an embodiment of the present specification. WSS detector  1340  comprises a scintillator screen  1342  partially covering a WS sheet  1344 , which is in turn coupled with WS Fibers  1346  on all four sides for transmitting shifted light rays into a PMT  1348 . In an embodiment, the scintillator screen  1342  is an 80 mg/cm2 sheet and is applied on both sides (top and bottom) of the WS sheet  1344  which has a thickness of 4 mm with corners rounded at 0.75” ROC. In an embodiment, the WS fibers  1346  are provided in the form a cable having a diameter of 1 mm, and the PMT  1348  has a diameter of 8 mm. 
       FIG.  13 B  is a diagrammatical representation of a rectangular WSS detector having WS fibers in direct communication with/couple to two sides of a wavelength shifting sheet, enabling a two side readout as used in a transmission detection mode, in accordance with an embodiment of the present specification. WSS detector  1350  comprises a scintillator screen  1352  partially covering a WS sheet  1354 , which is in turn coupled with two WS fiber cables  1356  on two sides (as shown in  FIG.  13 B ) for transmitting shifted light rays into a PMT  1358 . In an embodiment, the remaining two sides of the WS sheet  1354  are covered with a reflector material  1359  to improve X-ray absorption. 
       FIG.  13 C  is a diagrammatical representation of a rectangular WSS detector having WS fibers in direct communication with/coupled to one side of a wavelength shifting sheet, enabling a single side readout as used in a transmission detection mode, in accordance with an embodiment of the present specification. WSS detector  1370  comprises a scintillator screen  1372  partially covering a WS sheet  1374 , which is in turn coupled with one WS fiber cable  1376  on one sides (as shown in  FIG.  13 C ) for transmitting shifted light rays into a PMT  1378 . In an embodiment, the remaining three sides of the WS sheet  1374  are covered with a reflector material  1379  to improve X-ray absorption. 
       FIG.  14 A  is a diagrammatical representation of a rectangular WSS detector having WS fibers in direct communication with/coupled to four sides of a wavelength shifting sheet, enabling a four side as used in a backscatter detection mode, in accordance with an embodiment of the present specification. WSS detector  1440  comprises a scintillator screen  1442  partially covering a WS sheet  1444 , which is in turn coupled with WS fibers  1446  on all four sides (as shown in  FIG.  14 A ) for transmitting shifted light rays into a PMT  1448 . In embodiments, the WS fibers  1446  may be provided in the form of a single cable. 
       FIG.  14 B  is a diagrammatical representation of a rectangular WSS detector having WS fibers in direct communication with/coupled to two sides of a wavelength shifting sheet, enabling a two side readout as used in a backscatter detection mode, in accordance with an embodiment of the present specification. WSS detector  1450  comprises a scintillator screen  1452  partially covering a WS sheet  1454 , which is in turn coupled with two WS fiber cables  1456  on two sides (as shown in  FIG.  14 B ) for transmitting shifted light rays into a PMT  1458 . In an embodiment, the remaining two sides of the WS sheet  1454  are covered with a reflector material  1459  to improve X-ray absorption. In an embodiment, the WS fibers  1446  may be provided as two cables. 
       FIG.  14 C  is a diagrammatical representation of a rectangular WSS detector having WS fibers in direct communication with/coupled to one side of a wavelength shifting sheet, enabling a single side readout as used in a backscatter detection mode, in accordance with an embodiment of the present specification. WSS detector  1470  comprises a scintillator screen  1472  partially covering a WS sheet  1474 , which is in turn coupled with one WS fiber cable  1476  on one sides (as shown in  FIG.  14 C ) for transmitting shifted light rays into a PMT  1478 . In an embodiment, the remaining three sides of the WS sheet  1474  are covered with a reflector material  1479  to improve X-ray absorption. 
     In an embodiment, a WSS detector operating in backscatter mode comprises a plurality of channels cut into a WS sheet wherein WS fibers are positioned within said channels instead of along one or more sides of the WS sheet.  FIG.  14 D  is a diagrammatical representation of a rectangular WSS detector having discrete WS fibers placed within a plurality of corresponding channels cut into the WS sheet, as used in a backscatter detection mode, in accordance with an embodiment of the present specification. WSS detector  1480  comprises a scintillator screen  1482  partially covering a WS sheet  1484 , comprising channels, each channel housing a WS fiber  1486  (as shown in  FIG.  14 D ) for transmitting shifted light rays into a PMT  1488 . In embodiments, eight channels and thus, eight WS fibers  1486  are employed. In an embodiment, a reflector material  1490  is provided around all exposed edges of WS sheet  1484  to improve X ray absorption and transmission to the PMT  1488 . 
       FIG.  15 A  is a side view of a backscatter WSS detector comprising a plurality of channels for housing WS fibers, in accordance with an embodiment of the present specification.  FIG.  15 B  is a top view of the backscatter WSS detector comprising a plurality of channels for holding WS fibers, in accordance with an embodiment of the present specification. The placement of WS fibers in channels cut within the surface of the WS sheet is advantageous as a smaller length of WS fiber may be required, compared to embodiments where the WS fiber is placed along the sides of the WS sheet. In an embodiment, the spacing between the channels ranges between 5 mm to 20 mm; and for a 20 cm x 20 cm x 0.5 cm sheet with channels at 1 cm spacing, approximately (20 cm * 20 channels) + 20 cm = 420 cm of WS fiber is required; in comparison, (2*(20+20))*(8 cables) = 640 cm of WS fiber being required for an embodiment where the WS fiber is placed along the sides of a WS sheet of the same dimensions. 
     Co-pending U.S. Pat. Application No. 16/242,163, of the same Applicant of the present specification, entitled “Spectral Discrimination using Wavelength-Shifting Fiber-Coupled Scintillation Detectors” is herein incorporated by reference in its entirety. In addition, U.S. Pat. Application No. 15/490,787, entitled “Spectral Discrimination using Wavelength-Shifting Fiber-Coupled Scintillation Detectors”, filed on Apr. 18, 2017, which, in turn, is a divisional application of U.S. Pat. No. 9,658,343 (the “‘343 patent”), of the same title filed on Feb. 23, 2016 and issued on May 23, 2017 are also incorporated by reference herein in their entirety. Also, U.S. Pat. No. 9,285,488 (the ‘488 patent), of the same title, filed on Feb. 4, 2013, and issued on Mar. 15, 2016 and any priority applications thereof are herein incorporated by reference in their entirety. The embodiments described in the present specification are more cost-effective as they do not require that individual fibers are separated into low and high resolution, with each needing separate PMTs, which would increase the overall length of the fiber. Thus, the wavelength-shifting sheet scintillation detectors of the present specification are lower cost owing to both the elimination of a fiber bundle and manufacturing complexity due to the handling, cutting and polishing of the fiber bundles included therein. 
     As shown in  FIGS.  15 A and  15 B , a first set of incident X rays  1502  are absorbed by a top scintillator layer  1504 . The corresponding emitted light  1503  enters a WS sheet  1506 , which in turn shifts the light a first time, such that the shifted light  1507  is received, absorbed, and shifted a second time by WS fibers  1508  placed within channels  1509  formed within the WS sheet  1506 . A second set of incident X rays  1510  which pass through the top scintillator layer  1504  and the WSS  1506 , are absorbed by a bottom scintillator layer  1512  whereby the corresponding emitted light re-enters the WSS  1506 , which in turn shifts the light  1515  a first time, such that the shifted light is received, absorbed, and shifted a second time by WS fibers  1511  placed within channels  1513  formed within the WS sheet  1506 . The light shifted by the WS fibers  1508 ,  1511  is transmitted along the length of the WS fibers  1508 ,  1511  and subsequently transmitted to a photo multiplier tube (PMT)  1514  for detection. In embodiments, a reflector material  1516  is provided around all exposed edges of WS sheet  1506  to improve X ray absorption and transmission to the PMT  1514 . The reflector material  1516  may be composed of a diffuse reflector (paint or tape) or a specular reflector material (metallic) such as, but not limited, to Teflon tape, 3M enhanced specular reflector (ESR) Tape and Silver Metalized Polyester. 
     In an embodiment, along with having channels housing wavelength shifting fibers as described with reference to  FIGS.  15 A and  15 B , the sheet of a WSS detector in accordance with an embodiment of the present specification, may be optically segmented into pixels using a mechanical milling process.  FIG.  15 C  is a diagrammatical representation of a cross-section of a WSS detector array with a segmented pixelated WS sheet comprising a plurality of channels for holding WS fibers, in accordance with an embodiment of the present specification.  FIG.  15 D  is a top view of the WSS detector having two WS fibers placed within channels cut into the segmented WS sheet, in accordance with an embodiment of the present specification. As shown in  FIG.  15 C , a scintillator layer  1580  converts incident X rays  1582  from a flying spot X ray beam  1581  to scintillation light  1584  which is typically in the ultraviolet (UV) wavelength range of approximately 400 nm. The light  1584  then falls upon a segmented WS sheet  1586  comprising optically isolated pixels ranging in size from approximately 2 mm to 5 mm. As shown in  FIG.  15 C  the WSS  1586  comprises a plurality of channels  1590  containing WSF  1592 . In an embodiment, the segments  1588  and the WSF  1592  are made perpendicular to each other. The WSS  1586  converts the UV light  1584  to blue light having a wavelength of approximately 425 nm which then strikes WSF  1592 , which in turn absorbs the blue light and converts it to green light having a wavelength of approximately 475 nm. The green light is detected by a PMT connected to the WSF  1592 . 
     Light from the pixelated WSS  1586  is multiplexed at the transition from WSS  1586  to WSF  1592 . In an embodiment, a reflective material coating is inserted between the WSS  1586  and the WSF  1592  for preventing light from exiting the WSS  1586 . In an embodiment, the channels  1590  containing the WSF  1592  are not covered by the reflective coating. In an embodiment, the reflective coating may be patterned so that only light from the selected channel may exit the WSS. In this manner, pixels may be grouped to share a signal on a common fiber, if the X-ray beam spot  1581  does not simultaneously illuminate two pixels from the same segment. 
     In various embodiments, a reflective coating may be patterned onto the edge of the WSS  1586  or directly onto the WS fiber  1592 , so that only light from specific channels may enter the fiber.  FIG.  15 C  illustrates a reflector deposited on the edge of the WSS  1586  everywhere except at the openings for the channels. Light from the fibers exits at the PMT. Hence, in various embodiments, light is either reflected or transmitted to the WSF  1592  using the optical reflector coating. Thus, pixels of the same multiplexed group can transmit light from the WSS  1586  to the WSF  1592 . 
     In some imaging applications, for example non-destructive pipeline inspection, it is important to maintain proximity from the object being imaged to the detector in order to prevent a degradation of the spatial resolution of the recorded image. Hence, in an embodiment, the WSS detector is made of a curved WS sheet.  FIG.  16    illustrates a curved WSS detector, in accordance with an embodiment of the present specification. As shown, a curved scintillator screen  1602  at least partially covers a curved WS sheet  1604  having a WSF cable coupled along the edges for enhancing the uniformity of the detector. 
     In an embodiment, a typical thickness of a WSS detector is 6 mm, wherein two scintillator screens having a thickness of 1 mm each, a WSS having a thickness of 2 mm and a cover having a thickness of 3 mm is employed. In an embodiment, the WSS detector has an area greater than 432 mm*432 mm; weight less than 2.5 kg; and a bezel thickness less than 4 mm. In an embodiment, the WSS detector operates in a temperature range of -20C to 50C, and a humidity range of 20% to 80%. 
     In an embodiment, the WSS detector of the present specification may be converted into a multi-energy detector. The multi-energy WSS detector may be obtained by employing a layered structure that includes a high energy and a low energy WSS detector. A filter such as, but not limited to, a sheet of copper may be inserted between two stacked WSS (high and low energy) detectors to obtain a multi-energy WSS detector. 
     In another embodiment, the WSS detector described in the present specification may be converted to a flexible X-ray detector by using WSS and/or edge collection WSF/WSS made of flexible materials such as, but not limited to silicone based materials. In embodiments, where the photodetector available for coupling with the WSS detector is larger than the thickness of the active area of the WSS detector, the PMT may be removed from the WSS detector package and coupled with the detector via a clear fiber optic cable. 
     Wavelength Shifting Sheet (WSS) Detector Implementation 
     In various embodiments, the WSS detector described above may be implemented as a transmission detector, a forward scatter detector, and a backscatter detector, depending on the placement of the detector with respect to an imager being used in conjunction with the detector. In embodiments, the detector of the present specification is implemented in an imaging system having an enclosure and housing that is built around the imaging system, and that is separate from the imaging system. 
       FIG.  17 A  is a diagrammatic representation of a WSS detector as used in a transmission mode with a flying spot X-ray imager, in an embodiment of the present specification. A flying spot X-ray imager  1702  irradiates an object  1704  being inspected with a flying spot beam  1706 , as shown in the FIG. A WSS detector  1708  placed behind the object  1704  (such that the object  1704  is between the imager  1702  and the detector  1708 ), receives a transmission beam  1710  comprising X rays that are transmitted through the object  1704 . The beam  1710  is absorbed by a scintillator layer (not shown in the FIG.) and shifted by one or more WS materials of the WSS detector  1708  as explained in the preceding sections. The shifted beam is transmitted to a photomultiplier tube  1712  for detection which in turn transmits the detected data to a data acquisition system (not shown in the FIG.) of the imager  1702  for processing. 
       FIG.  17 B  is a flow diagram representing the steps of using an WSS detector in a transmission mode with a flying spot X-ray imager, in accordance with an embodiment of the present specification. At step  1701  an object under inspection is irradiated with a flying spot X-rays, emanating from an X-ray imager. At step  1703  a transmission beam comprising X rays that are transmitted through the object are received by a WSS detector placed behind the object (such that the object is positioned between the imager and the detector). At step  1705 , the transmission beam is absorbed by a scintillator layer of the detector and emitted as corresponding light rays. At step  1707 , the emitted light rays are shifted by one or more WS materials of the WSS detector. At step  1709 , the shifted light rays are transmitted to a PMT for detection. At step  1711 , the shifted light rays are converted to electrical signals by the photomultiplier tube. At  1713 , the electrical signals are transmitted to a data acquisition system for processing. 
       FIG.  17 C  is a diagrammatic representation of a WSS detector as used in a backscatter mode with a flying spot X-ray imager, in an embodiment of the present specification. A flying spot X-ray imager  1702  irradiates an object  1704  being inspected with a flying spot beam  1706 , as shown in the FIG. A WSS detector  1708  placed before the object  1704  (such that the detector  1708  is between the imager  1702  and the object  1704 ), receives a backscatter beam  1720  comprising X rays that are backscattered by the object  1704 . The beam  1720  is absorbed by a scintillator layer (not shown in the FIG.) and shifted by one or more WS materials of the WSS detector  1708  as explained in the preceding sections. The shifted beam is transmitted to a photomultiplier tube  1712  for detection which in turn transmits the detected data to a data acquisition system (not shown in the FIG.) of the imager  1702  for processing. In the embodiment shown in  FIG.  17 B , the WSS detector  1708  is implemented as an auxiliary backscatter detector while the imager  1702  comprises a primary backscatter detector  1722  integrated with the imager  1702 . In embodiments, the auxiliary detector provides additional detection areas, capturing more of the scattered X-ray photons and thus boosting the contrast to noise ratio (CNR) of the imaging system. 
       FIG.  17 D  is a flow diagram representing the steps of using a WSS detector in a backscatter mode with a flying spot X-ray imager, in accordance with an embodiment of the present specification. At step  1721  an object being inspected is irradiated with a flying spot X-rays, emanating from an X-ray imager. At step  1723  a backscatter beam comprising X rays that are backscattered by the object are received by a WSS detector placed before the object (such that the detector is positioned between the imager and the object). At step  1725 , the backscatter beam is absorbed by a scintillator layer of the detector and emitted as corresponding light rays. At step  1727 , the emitted light rays are shifted by one or more WS materials of the WSS detector. At step  1729 , the shifted light rays are transmitted to a PMT for detection. At step  1731 , the shifted light rays are converted to electrical signals by the photomultiplier tube. At  1733 , the electrical signals are transmitted to a data acquisition system for processing. 
       FIG.  17 E  is a diagrammatic representation of a multi-energy WSS detector as used in a transmission mode with a flying spot X-ray imager, in an embodiment of the present specification. A flying spot X-ray imager  1702  irradiates an object  1704  being inspected with a flying spot beam  1706 , as shown in the FIG. A multi-energy WSS detector  1730  comprising a layered structure that includes a high energy  1732  and a low energy  1734  WSS detector, with a filter  1736  such as, but not limited to, a sheet of copper inserted between the two stacked WSS (high and low energy) detectors is placed behind the object  1704  (such that the object  1704  is between the imager  1702  and the detector  1730 ), receives a transmission beam  1710  comprising X rays that are transmitted through the object  1704 . The beam  1710  is shifted by each of the high energy  1732  and the low energy  1734  WSS detectors and the shifted beams are transmitted to photomultiplier tubes  1738  and  1740  respectively for detection, which in turn transmit the detected data to a data acquisition system (not shown in the FIG.) of the imager  1702  for processing. The filter  1736  provides a shift in the energy of the transmission beam  1710 , preferentially removing more low energy X-rays and shifting the beam spectrum to a higher energy level. As a result, in various embodiments the first detector layer is a Low Energy (LE) detector  1734  and the second post filter  1736  layer is a High Energy (HE) detector  1732 . 
     Multi-energy WSS detectors are described in U.S. Pat. No. 9,285,488 entitled “X-ray inspection using wavelength-shifting fiber-coupled scintillation detectors”, assigned to the Applicant of the present specification, and is herein incorporated by reference in its entirety. Multi-energy detectors work by separating the signals from both the front and back layers of scintillators, which allows the front layer to give a measure of the low-energy component of each pixel while the back layer gives a measure of the high-energy components. Putting a layer of absorbing material between the front and back scintillators is a standard methodology to enhance the difference between low and high energy components, which is typically done with a Sc-WSF detector. The Sc-WSF detector makes practical a dual-energy detector consisting of a layer of Sc-WSF, such as BaFCl-WSF, on top of a plastic scintillator detector; the BaFCl is sensitive to the low-energy x-rays and not the high-energy x-rays, while the plastic detector is sensitive to the high-energy x-rays and very insensitive to low energy x-rays. An alternative and potentially more effective material discriminator can be made by using more than two independent layers of Sc-WSF, with separate readouts for each layer. A passive absorber, such as an appropriate thickness of copper, can be inserted after the top Sc-WSF to enhance dual energy application, as is practiced with segmented detectors. Alternatively, the middle scintillator can be used as an active absorbing layer. The measurement of three independent parameters allows one to get a measure of both the average atomic number of the traversed materials and the extent of beam hardening as well. 
       FIG.  17 F  is a flow diagram representing the steps of using the multi-energy WSS detector shown in  FIG.  17 E , in a transmission mode with a flying spot X-ray imager, in accordance with an embodiment of the present specification. At step  1741  an object being inspected is irradiated with a flying spot X-rays, emanating from an X-ray imager. At step  1743  a transmission beam comprising X rays that are transmitted through the object are received by the multi-energy WSS detector comprising a high energy (HE) and a low energy (LE) WSS material layer, with a filter inserted between the two stacked WSS layers, placed behind the object (such that the object is positioned between the imager and the detector). At step  1745 , the transmission beam is absorbed by a scintillator layer of the detector and emitted as corresponding light rays. At step  1747 , the emitted HE light rays are shifted by the HE WSS material layer of the WSS detector and are transmitted to a first PMT coupled with the HE WSS material layer. At step  1749 , the emitted LE light rays are shifted by the LE WSS material layer of the WSS detector and are transmitted to a second PMT coupled with the LE WSS material layer. At step  1751 , the shifted light rays are converted to electrical signals by the first and the second PMTs. At  1753 , the electrical signals are transmitted to a data acquisition system for processing. 
       FIG.  17 G  is a diagrammatic representation of a vertical WSS detector and a ground-level WSS detector as used in a transmission mode with a flying spot X-ray imager for scanning a cargo object, in an embodiment of the present specification. A flying spot X-ray imager  1752  irradiates a cargo object  1754  being inspected with a flying spot beam  1756 , as shown in the FIG. A WSS vertical detector  1758  and a WSS ground detector  1760  are arranged perpendicular to each other, as shown in  FIG.  17 G  for scanning the cargo object  1754 . A WSS vertical detector  1758  and a WSS ground detector  1760  are placed behind the object  1754  (such that the object  1754  is between the imager  1752  and the detectors  1758 ,  1760 ), receive a transmission beam  1770  comprising X-rays that are transmitted through the object  1754 . The beam  1770  is absorbed by a scintillator layer (not shown in the FIG.) and shifted by one or more WS materials of the WSS detectors as explained in the preceding sections. The shifted beam is transmitted to a photomultiplier tube  1772  for detection which in turn transmits the detected data to a data acquisition system (not shown in the FIG.) of the imager  1752  for processing. WSS vertical detector  1758  may have multiple detector segments in a direction transverse to the direction of transmission beam  1770  and substantially along the direction of relative motion between inspected cargo object  1754  and transmission beam  1770  so as to provide an indication of skewness or lateral shift of the detectors with respect to the beam. 
       FIG.  17 H  is a flow diagram representing the steps of using the vertical WSS detector and a ground-level WSS detector shown in  FIG.  17 G  in a transmission mode with a flying spot X-ray imager for scanning a cargo, in accordance with an embodiment of the present specification. At step  1761  the cargo being inspected is irradiated with a flying spot X-rays, emanating from an X-ray imager. At step  1763  a transmission beam comprising X rays that are transmitted through the cargo are received by the WSS vertical detector and the WSS ground detector arranged perpendicular to each, and placed behind the cargo (such that the cargo is positioned between the imager and the detector). At step  1765 , the transmission beam is absorbed by scintillator layers of the WSS vertical detector and the WSS ground detector and are emitted as corresponding light rays. At step  1767 , the emitted light rays are shifted by one or more WS materials of the WSS vertical detector and the WSS ground detector. At step  1769 , the shifted light rays are transmitted to one or more PMTs coupled with the WSS vertical detector and the WSS ground detector for detection. At step  1771 , the shifted light rays are converted to electrical signals by the one or more PMTs. At  1773 , the electrical signals are transmitted to a data acquisition system for processing. 
     In embodiments, the detector of the present specification may be used in an integrated mode or an accessory mode with respect to an imaging system. The embodiments described above are representative of implementation in an integrated mode. In embodiments, in accessory mode, the WSS detector of the present specification further includes an enclosure and housing is built around the detection system that is separate from an imaging system where the imaging system includes a radiation source. The detection system enclosure comprises at least a handle and is powered by a power source built within the enclosure or is powered by an external power source. In an embodiment, the WSS detection system being used in an accessory mode is self-powered and wireless. In an embodiment, the detected radiation converted to electrical signals by a PMT of the WSS detection system is conveyed to the imaging system for processing via a shielded cable in an analog form. In another embodiment, the analog electrical signals are converted to digital signals and conveyed to the imaging system wirelessly. In an embodiment, the detected radiation may also be conveyed to the imaging system as light signals, wherein the PMT electronics for processing the light signals is provided in the imaging system, thereby reducing the size of the WSS detector system significantly by removing all PMT electronic components from the detection system. An advantage of using the WSS detector as a small portable accessory to the imaging system is that the position of the detector with respect to an object being scanned can be varied easily, thereby optimizing scan coverage of the object. 
     In some embodiments, the scanning system being used in conjunction with the WSS detector of the present specification comprises a plurality of channels for coupling with one or more detectors. The channels may be configured via a user interface to enable simultaneous coupling with more than one WSS detectors. In embodiments, the detector-type is user configurable. For example, a system may have a built-in backscatter detector configuration and two additional detector channels that may be user configured. The images captured by each of said detectors may either be processed separately or may be summed. 
     During the use of the WSS detector in a transmission mode, there is a wide range around the detector’s WSS panel, where the imaging is apparent through forward scatter interaction of X-rays irradiating the object being scanned. Hence, by using the WSS detector in an accessory mode, users may position the detector in desirable orientations with respect to the object. 
       FIG.  17 I  illustrates a detector panel placed under a car to collect transmission radiations emitted by a small portable scanner being used to scan a boot of the car, in accordance with an embodiment of the present specification. As shown, detector panel  1782  is placed under a boot  1784  of the car to collect the transmission radiation emitted by the portable scanner  1786  which is being used to scan the boot  1784 .  FIG.  17 J  illustrates explosives hidden approximately two feet under the boot  1784  of the car shown in  FIG.  17 I . The explosives include explosive stimulants i.e. 5 lbs of sugar  1788  and 500 ml liquid  1790  and a gun  1792 .  FIG.  28 C  illustrates a transmission image  1794  obtained by the detector panel placed under the car as shown in  FIG.  17 I , in accordance with an embodiment of the present specification. As can be seen in  FIG.  17 K , the transmission image  1794  clearly shows the metal gun  1792  while the organic explosives  1788 ,  1790  become nearly transparent in the image. Hence even though, when used as a transmission detector, the active size of the detector panel  1782  may become limiting, the forward scatter generated by the scanner  1786  in the vicinity of the detector  1782  is also detected, thereby making the effective field of view larger than the detector area. 
       FIG.  17 L  illustrates a top perspective view of a prototype of the transmission detector panel shown in  FIG.  17 I , in accordance with an embodiment of the present specification.  FIG.  17 M  illustrates a top view of a prototype of the transmission detector panel shown in  FIG.  17 I , in accordance with an embodiment of the present specification.  FIG.  17 N  illustrates a close up view of the components of the transmission detector panel shown in  FIG.  17 L , in accordance with an embodiment of the present specification. Referring to  FIGS.  17 L,  17 M and  17 N  transmission detector panel  1795  comprises WS sheet  1796  coated with a top layer of a scintillator material  1797 , and is bounded by WS fiber cables  1798 . During a detection operation, the scintillator material  1797  absorbs the transmission X rays falling on the detector panel  1795  and converts said rays to light rays, which in turn are shifted by the WSS  1796  and WS fiber  1798  and fed to a PMT  1799  powered by a power supply  1791 . The WS fiber cables  1798  are placed in cable retainers  1787  and are coupled with the PMT  1799  via fiber cable guide  1789  and fiber collet  1793 . 
       FIG.  17 O  illustrates a WSS detector panel placed to collect backscatter radiation emitted by a small portable scanner being used to scan a car, in accordance with an embodiment of the present specification. As shown, detector panel 17102 is placed to collect the backscatter radiation emitted by the portable scanner 17104 which is being used to scan the boot 17106 of a car.  FIG.  17 P  illustrates explosives 17108 hidden approximately two feet under the boot 17106.  FIG.  17 Q  illustrates a backscatter image 17110 obtained by a built-in detector of the scanner of  FIG.  17 O , in accordance with an embodiment of the present specification. The backscatter image 17110 obtained by the scanner 17104, which is obtained using the in-built detectors (not shown in the FIG.) of said scanner 17104 does not show the hidden explosive 17108 (shown in  FIG.  17 P ).  FIG.  17 R  illustrates a backscatter image 17112 obtained by the scanner and the detector panel of  FIG.  17 O , in accordance with an embodiment of the present specification. As can be seen in  FIG.  17 R , the backscatter image 17112 clearly shows the explosive stimulant 17108 hidden within the boot 17106. The spatial resolution of the backscatter image 17112 is governed by the scanning beam spot size, however, the beam penetration and SNR is greatly enhanced as compared to the backscatter image 17110. 
       FIG.  18 A  illustrates a portable detector for handheld imaging, as used in a transmission mode, in accordance with an embodiment of the present specification.  FIG.  18 B  illustrates a portable detector for handheld imaging, being used in a forward scatter mode, in accordance with an embodiment of the present specification.  FIG.  18 C  illustrates a portable detector for handheld imaging, as used in backscatter mode, in accordance with an embodiment of the present specification. Referring to  FIGS.  18 A,  18 B and  18 C , in various embodiments, in order to operate portable WSS detector  1806  for handheld imaging in transmission, forward scatter or backscatter mode, an operator  1802  is required to position the detector  1806  with respect to the target being scanned  1808 , energize a hand-held imaging system  1804 , translate the imaging system  1804  across the region of the target  1802  and collect the scattered and transmission data for review. Referring to  FIG.  18 A , transmitted photons are detected by the detector  1806 . Referring to  FIG.  18 B , the detector panel  1806  is placed on a side opposite a side of the hand held imaging system  1804  with respect to the target  1808 , or, in some embodiments, at right angles to the target  1808 , such that when the target  1802  is irradiated with X rays from the imaging system  1804  the forward scatter from the target  1802  is detected by the WSS detector panel  1806 . Referring to  FIG.  18 C  backscattered photons are detected by the hand-held imaging system  1804 . 
       FIG.  18 D  is a flowchart illustrating the steps of imaging a target in a forward scatter mode by using a WSS detector panel and a handheld imaging system, in accordance with an embodiment of the present specification. At step  1810  a WSS detector panel is placed on a side opposite a side of a hand held imaging system with respect to the target object being scanned. In various embodiments, the detector panel is placed in close proximity to the target as shown in  FIG.  18 B . At step  1812 , the target is irradiated with X rays from the hand held imaging system. At step  1814 , forward scatter from the target is captured and detected by the detector panel. In various embodiments, the detected forward scatter is processed by an image processing system coupled with the detector panel to produce a scanned image of the target object. 
       FIG.  19    is a table conveying the dimensions and perimeter fractions of a plurality of WSS detectors, in an embodiment of the resent specification. The perimeter fraction is the amount of the edge where the WSF cable is in contact with the WSS. Row  1902  illustrates the dimension and perimeter fraction of a handheld transmission WSS detector, wherein the length ranges from 8 to 16 inches, width ranges from 8 to 24 inches, and perimeter fraction ranges from 50% to 100%. Row  1904  illustrates the dimension and perimeter fraction of a handheld backscatter WSS detector, wherein the length and width ranges from 6 to 12 inches, and perimeter fraction ranges from 25% to 50%. Row  1906  illustrates the dimension and perimeter fraction of a cargo backscatter WSS detector, wherein the length ranges from 40 to 80 inches, width ranges from 15 to 40 inches, and perimeter fraction ranges from 50% to 100%. Row  1908  illustrates the dimension and perimeter fraction of a cargo transmission WSS detector, wherein the length ranges from 80 to 150 inches, width ranges from 6 to 12 inches, and perimeter fraction ranges from 50% to 100%. 
     Hence, the present specification provides a WSS detector for use in a flying spot transmission imaging system with improved spatial uniformity. The cost for materials and manufacturing the WSS is less as compared to prior art detectors as: off-the-shelf scintillating screens may be used in place of specialized molded parts; there is no requirement for fiber handling, bundling or polishing; and adhesives are not required in any of the active optical paths, greatly reducing the chances for yield failure due to bubbles or voids. 
     Since, backscatter systems have imaging limitations in certain applications where Transmission imaging is advantageous, the WSS detector of the present specification is implemented as a Transmission Scan Panel that can be used with any commercially available handheld scanning system such as, but not limited to, the MINI Z scanning system, to provide a simultaneous secondary image. As described above, the detector of the present specification provides a transmission image when placed directly behind an item being scanned, or an additional backscatter image when placed on the near side of the object being scanned. The secondary image is displayed next to the standard Backscatter image on a display screen coupled with the MINI Z scanning system. In various embodiments, the Transmission Scan Panel is portable, light-weight, and connects to handheld scanning system with a simple, single cable connection. 
     In an embodiment, some exemplary physical attributes of the WSS detector panel of the present specification are:
     Active Imaging Area: 16 in x 22 in (41 cm x 56 cm)   Weight: 10 pounds (4.5 kg)   Cable length: Up to 30 feet (9 m) total, using 3 individual 10 foot (3 m) cables connected in series.   

     In an embodiment, some exemplary performance specification of the WSS detector panel of the present specification are:
     Nominal Penetration: up to 20 mm steel   Nominal Resolution: 0.5 mm at 0 cm standoff; 3 mm at 10 cm standoff; and 4 mm at 25 cm standoff   

     In an embodiment, the WSS detector panel of the present specification includes a manual gain adjust reduce/eliminate image noise. 
       FIG.  20    illustrates a diagrammatical representation of an object  2002  being scanned by a portable scanner  2004  and a WSS detector panel  2006 , in accordance with an embodiment of the present specification. In an embodiment, where the object  2002  is a wire-resolution phantom object, a standoff distance  2008  between the object  2002  and the imaging system  2004  is 0 cm to obtain a 1.5 mm wire resolution; 3 mm to obtain a 10 cm wire resolution; and 5 mm to obtain a 25 cm wire resolution. 
       FIG.  21 A  illustrates a standard shipping box  2102  comprising an explosive object not visible to naked eye.  FIG.  21 B  illustrates an image  2104  of the shipping box obtained by using a portable scanning system and a WSS detector panel of the present specification operating in a backscatter mode, in accordance with an embodiment of the present specification. As shown in  FIG.  21 B  the explosive object which is a pipe bomb is not visible in the scanning image  2104  obtained by operating the WSS detector panel in a backscatter mode.  FIG.  21 C  illustrates an image  2106  of the shipping box obtained by using a portable scanning system and a WSS detector panel of the present specification operating in a transmission mode, in accordance with an embodiment of the present specification. As shown in  FIG.  21 C  the pipe bomb  2108  is clearly visible in the scanning image  2106  obtained by operating the WSS detector panel in a transmission mode. 
       FIG.  22 A  illustrates explosives hidden in a concrete block. As can be seen in the figure, a soda can IED  2202  and a steel pipe bomb  2204  are hidden in a concrete block  2206 , with approximately 1.5 inches thick walls.  FIG.  22 B  illustrates an image  2210  of the concrete block shown in  FIG.  22 A , obtained by using a portable scanning system and a WSS detector panel of the present specification operating in a backscatter mode, in accordance with an embodiment of the present specification. As shown in  FIG.  22 B  the explosive objects are not visible in the scanning image  2210  obtained by operating the WSS detector panel in a backscatter mode.  FIG.  22 C  illustrates an image  2212  of the concrete block shown in  FIG.  22 A , obtained by using a portable scanning system and a WSS detector panel of the present specification operating in a transmission mode, in accordance with an embodiment of the present specification. As shown in  FIG.  22 C  the soda can IED  2202  and a steel pipe bomb  2204  are clearly visible in the scanning image  2212  obtained by operating the WSS detector panel in a transmission mode. 
     The above examples are merely illustrative of the many applications of the system and method of present specification. Although only a few embodiments of the present specification have been described herein, it should be understood that the present specification might be embodied in many other specific forms without departing from the spirit or scope of the specification. Therefore, the present examples and embodiments are to be considered as illustrative and not restrictive, and the specification may be modified within the scope of the appended claims.