Spectrally and spatially resolving x-ray and particle detection system

Abstract

The detection system of an x-ray or charged particle imaging system utilizes a high bandgap, direct conversion x-ray detection material. The signal of the x-ray / charged particle projection is recorded in a spatial light modulator, for example a liquid crystal (LC) light valve. The light valve is then read out by a polarized light optical microscope and a high speed camera. The camera is used to track the halo in the light valve to resolve its intensity and correlate it to the intensity of the incoming x-ray photons or charged particles. This allows for a spatially resolved imaging with an x-ray and / or charged particle spectrometer.

Description

Background Technology

[0001] X-ray microscopy and other X-ray imaging applications require the detection of X-ray photons (<500 keV) with high spatial resolution and high efficiency. This can typically be achieved with scanning systems and simple (e.g., unit-by-unit) detection systems, or with pixelated or spatially resolved full-field and more complex detection systems.

[0002] Currently, some full-field X-ray microscopy detection systems utilize optical coupling between thin scintillator detectors and charge-coupled devices (CCDs) or complementary metal-oxide-semiconductor (CMOS) cameras. Sometimes the coupling is direct. In other cases, they are coupled via optical microscopy. These setups achieve high-resolution imaging by combining the magnification of the X-ray region and / or the optical region.

[0003] Another type of full-field detection system utilizes high-bandgap, direct-conversion photoconductive detection materials. Signals from X-rays or high-energy particles (e.g., projections) are recorded in an adjacent spatial light modulator (e.g., a liquid crystal (LC) light valve (LV)). The light valve is then illuminated by an external light source, such as an optical microscope. Compared to current scintillator-optical microscope-camera detection systems, this configuration reduces light loss in the optical system.

[0004] Spectroscopic / energy-resolved detection systems are also known. Current high-resolution X-ray (including gamma-ray / charged particle) spectrometers typically include semiconductor (e.g., silicon, high-purity germanium at liquid nitrogen temperatures, and CdTe / CdZnTe) detectors and accompanying electronics for amplifying and processing electrical pulses formed by impacting individual X-ray photons or particles. The most common spectrometers have only one detector element, thus detecting only the energy and timing information of the incident X-ray photons.

[0005] Pixelated spectral-resolved detection systems are also known. Typically, they employ the same amplifiers and processing circuitry to provide spatial resolution. However, due to the small pixel size, such systems generally have low energy resolution. Furthermore, these pixelated detectors are typically expensive due to the semiconductor fabrication processes and high costs associated with their electronics. A recent development in spatially resolved spectrometers is the pnCCD X-ray camera, which can provide both high energy and spatial (48 micrometer (μm) pixel) resolution. However, since it can currently only be made from silicon, its applicable detection energy is limited to lower X-ray energies.

[0006] A common drawback of all existing spectrometers is that they can only be used to detect relatively low X-ray photon fluxes. This is because all these detectors require individual measurement of X-ray photons to detect their energy (and spatially resolve the photon's position). Each X-ray photon induces an electron pulse in the detector, and the pulse is measured individually. Therefore, higher flux rates can cause electron pulses to overlap, leading to a stacking problem that limits the maximum possible flux rate.

[0007] For existing spectrometers (primarily spatially resolved spectrometers), one approach to overcome this problem is to reduce pixel size. However, this strategy has both physical and technological limitations. Physically, smaller pixels will have greater crosstalk due to pixel coupling, and more importantly, they will have weighted potential crosstalk, where X-rays deposited in one pixel region induce signals at multiple adjacent pixels. Therefore, it will soon reach the limit where smaller pixels do not significantly reduce the count. Technologically, smaller pixels are also limited by processing technology and associated costs. Summary of the Invention

[0008] The disclosed embodiments relate to spectrometers, particularly spatially resolved particle (e.g., X-ray or charged particle) spectrometers. Current methods typically involve optically measuring and counting X-ray photons or other particles over a much smaller area. Therefore, they can be detected at higher rates and with higher spatial resolution.

[0009] More specifically, in one example, a liquid crystal light valve spatial light modulator is coupled to a high-resolution microscope objective. A high-speed camera is then used to resolve X-ray or particle interaction events within a very small area. Although camera readout may be slow, the event rate of impacting a single pixel or region is low due to the small area, even at high throughput. Therefore, this setup can achieve detection at a higher throughput than current X-ray spectrometers. With high-resolution detection, spatial resolution can also be very high. Furthermore, there is a wide selection of direct conversion detector materials available, which can be customized to meet the requirements of specific use cases to detect higher energies and / or higher resolution.

[0010] Current detection and related imaging systems employ photoconductive X-ray detectors and high-speed cameras. The X-ray detectors have a sandwich structure, comprising a direct-conversion X-ray detection photoconductor layer and a spatial light modulator (e.g., a liquid crystal (LC) light valve). During operation, X-ray photons generate electron-hole pairs in the photoconductor layer, thereby locally altering the electric field. This local field causes local reorientation of the liquid crystal in adjacent liquid crystal films, which will appear as spots of light. Over time, these spots exhibit a halo effect: growing and then dissipating. By tracking the halo over time using a camera, the position and energy of the incident photons / particles can be determined, thus obtaining spectral and spatial resolution.

[0011] Generally, according to one aspect, the present invention is characterized by a spectral and spatially resolved particle detection system. The system includes a photoconductivity detector, an optical microscope for reading out the photoconductivity detector, and a camera coupled to the photoconductivity detector via the optical microscope. A computer system (e.g., a dedicated computer such as a graphics processing unit (GPU), application-specific integrated circuit (ASIC), field-programmable array (FPGA), general-purpose computer, or some combination of these or other computer systems) is then used to acquire images generated by the camera and track the photoconductivity detector's response to particles over time.

[0012] Typically, computer systems track halos in photoconductor detectors induced by particles to determine the energy of the particles and their position on the detector.

[0013] Typically, computer systems determine the energy of particles by referencing an energy / intensity map that correlates the maximum spot intensity or integrated spot intensity or the fitted intensity map with the energy of the received particles.

[0014] Typically, the interval between consecutive frames captured by the camera is smaller than the relaxation time of the photoconductivity detector. In this way, the halo of the light spot on the photoconductivity detector can be tracked to determine its position and intensity.

[0015] Therefore, for many applications, the camera will be a high-speed camera. For example, in many implementations, the interval between consecutive frames captured by the camera is less than 1 millisecond.

[0016] In one application, the photoconductivity detector was used to detect X-rays.

[0017] In the current embodiment, the photoconductive X-ray detector includes a liquid crystal light valve and a photoconductive detector layer. The photoconductive detector layer may contain bismuth, lead, mercury, tellurium, selenium, or thallium. Furthermore, the optical microscope may be a polarizing microscope, which reads out the photoconductive detector via transmission or reflection.

[0018] Typically, according to another aspect, the present invention is characterized by a particle detection method comprising: converting a particle into an electron-hole pair in a photoconductive detector; reading out the photoconductive detector using a camera coupled to the photoconductive detector; and processing an image generated by the camera and tracking the response of the photoconductive detector to the particle over time to determine the position and energy of the particle.

[0019] Image processing is performed in computer systems, such as dedicated computers, graphics processing units (GPUs), application-specific integrated circuits (ASICs), field-programmable arrays (FPGAs), general-purpose computers, or some combination of these or other computer systems. A computer system can even be part of a camera.

[0020] Typically, according to another aspect, the present invention is characterized by an imaging system. This system includes a stage system for holding an object, a photoconductive detector for detecting particles from the object, and a camera coupled to the photoconductive detector via an optical microscope. A computer system then acquires images generated by the camera and tracks the response of the photoconductive detector to particles over time to image the object and determine the energy of the particles.

[0021] Generally, according to another aspect, the present invention is characterized by a method for calibrating a particle detection system. The method includes generating particles with known energies, converting the particles into electron-hole pairs in a photoconductivity detector, reading out the photoconductivity detector using a camera, processing an image generated by the camera and tracking a light spot generated by the particles received by the photoconductivity detector, and determining the relationship between the light spot and the energy of the particles.

[0022] The above and other features of the invention, including various novel structural details and combinations of components, as well as other advantages, will now be described in more detail with reference to the accompanying drawings and pointed out in the claims. It should be understood that the specific methods and apparatus embodying the invention are shown by way of illustration and not as limitations thereof. The principles and features of the invention can be used in various and many embodiments without departing from the scope of the invention. Attached Figure Description

[0023] In the accompanying drawings, reference numerals refer to the same parts in different views. The drawings are not necessarily drawn to scale; rather, the focus is on illustrating the principles of the invention. In the figures:

[0024] Figure 1AThis is a schematic diagram of an X-ray microscope employing a detection system according to the present invention;

[0025] Figure 1B This is a schematic diagram of an X-ray fluorescence (XRF) microscope using a detection system according to the present invention;

[0026] Figure 2 This is a side cross-sectional view of a photoconductive X-ray detector;

[0027] Figure 3 yes Figure 2 A schematic diagram of the equivalent circuit of the photoconductive X-ray detector in the image;

[0028] Figure 4A and Figure 4B Two X-ray detection systems are shown, one for a polarized optical microscope operating in reflection mode and the other in transmission mode.

[0029] Figure 5 This is a plot of the time factor changing over time (in seconds), showing the temporal evolution of a light spot, where τ d =10ms and τ c =10ms;

[0030] Figure 6 This is a schematic diagram showing the halos associated with various detected X-rays or particles across the range of photoconductive X-ray detectors;

[0031] Figure 7 This is a flowchart illustrating a method for calibrating a testing system;

[0032] Figure 8 This is a flowchart illustrating a method for generating energy-resolved images or projections using a detection system;

[0033] Figure 9 A side cross-sectional view of a vertically aligned LC cell 80 without an external field is shown, illustrating how molecules are firmly anchored at the two boundaries and how molecules in the main region are forced to follow the same orientation. It also shows how the orientation of molecules near the charge q bends when the charge q approaches the LC cell.

[0034] Figure 10A and Figure 10B Simulation results for a 1 μm thick LC vertical alignment cell are shown. The parameters used in the simulation are: K = 10 pN, ε || =15, ε ⊥ =3, z0=10nm, q=60e;

[0035] Figure 11A and Figure 11BSimulation results for a 1 μm thick LC planar cell are shown. The parameters used in the simulation are: K = 10 pN, ε || =15, ε ⊥ =3, z0=10nm, q=62e.(a)n y (b)n z ;

[0036] Figure 12A and Figure 12B Simulation results for a 1 μm thick LC vertically aligned cell exhibiting flexural electrical effect are shown. The parameters used in the simulation are: K = 10 pN, e1 - e3 = 12.2 pC / m, e1 + e3 = 40.0 pC / m, ε || =15, ε ⊥ =3, z0=10nm, q=33e;

[0037] Figure 13A and Figure 13B Simulation results for a 1 μm thick LC vertical alignment box containing a biased z-direction field are shown. The parameters used in the simulation are: K = 10 pN, ε || =15, ε ⊥ =3, z0=10nm, q=95e, E z0 =0.5V / m; and

[0038] Figure 14A and Figure 14B Simulation results for a 1 μm thick LC vertically aligned cell with flexural electrical effect and biased z-direction field are presented. The parameters used in the simulation are: K = 10 pN, e1 - e3 = 12.2 pC / me1 + e3 = 40.0 pC / m, ε || =15, ε ⊥ =3, z0=10nm, 0=25e, E z0 =0.5V / m. Detailed Implementation

[0039] The invention will now be described more fully with reference to the accompanying drawings, in which exemplary embodiments of the invention are illustrated. However, the invention may be embodied in many different forms and should not be construed as limited to the embodiments described herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art.

[0040] As used herein, the term “and / or” includes any and all combinations of one or more of the related listed items. Furthermore, unless expressly stated otherwise, the singular form and the articles “a,” “an,” and “the” should also include the plural form. It should be further understood that when the terms include, comprise, including, and / or comprising are used in this specification, they specify the presence of the stated feature, integer, step, operation, element, and / or component, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or combinations thereof. Furthermore, it is understood that when an element (including a component or subsystem) is referred to and / or shown as connected to or coupled to another element, it may be directly connected to or coupled to the other element, or there may be intermediate elements present.

[0041] Unless otherwise defined, all terms used herein (including technical and scientific terms) have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. It should be further understood that terms (e.g., those defined in common dictionaries) should be interpreted as having a meaning consistent with that meaning in the relevant technical context, and should not be interpreted in an idealized or overly formal sense unless explicitly defined herein.

[0042] Figure 1A This is a schematic diagram of an X-ray CT microscopy system 200-1 applicable to a spectral and spatially resolved X-ray detection system 100.

[0043] Nevertheless, the present invention is also applicable to the detection of other particles, including charged particle analysis systems and non-microscopic systems.

[0044] The X-ray CT microscopy system 200-1 typically includes an X-ray imaging system having an X-ray source system 202 that generates a multicolor or possibly monochromatic X-ray beam 205, and a stage system 210 with an object holder 212 for holding and positioning an object 214 so that the object 214 can be scanned in a fixed beam. A spectral and spatially resolved X-ray detection system 100 detects the X-ray beam 205 after it has been modulated by the object 214. A base 207, such as a platform or optical stage, provides a stable foundation for the X-ray CT microscopy system 200-1.

[0045] Generally, the stage system 210 is capable of positioning and rotating the object 214 within the X-ray beam 205. Therefore, the stage system 210 typically includes a precise triaxial stage 250 that translates and positions the object very precisely along the x, y, and z axes, but with a relatively small range of motion. This allows the region of interest of the object 214 to lie within the X-ray beam 205. The triaxial stage 250 is mounted on a theta stage 252, which rotates the object 214 about the y-axis within the beam. The theta stage 252 is in turn mounted on a base 207.

[0046] In some embodiments, source system 202 is typically a synchrotron X-ray radiation source, or alternatively a “laboratory X-ray source.” As used herein, a “laboratory X-ray source” is any suitable X-ray source that is not a synchrotron X-ray radiation source. Laboratory X-ray source 202 may be an X-ray tube in which electrons are accelerated in a vacuum by an electric field and injected into a target metal, emitting X-rays as the electrons decelerate within the metal. Typically, depending on the type of metal target used, these sources produce a continuous spectrum of background X-rays and intensity spikes at specific energies derived from characteristic lines from a selected target. Furthermore, the X-ray beam is divergent and lacks spatial and temporal coherence.

[0047] In one example, source 202 is a rotating anode type or microfocusing source with a tungsten target. Targets of molybdenum, gold, platinum, silver, or copper can also be used. Preferably, a transmission structure is used in which the electron beam strikes the thin target from the back side. X-rays emitted from the other side of the target serve as X-ray beam 205.

[0048] Preferably, the x-ray beam generated by source 202 is modulated to suppress unwanted energy or radiation wavelengths. For example, unwanted wavelengths present in the beam can be eliminated or attenuated by using an energy filter (designed to select the desired x-ray wavelength range (bandwidth)) held in filter wheel 260. Modulation is also typically provided by collimators or condensers and / or x-ray lenses (such as zone plate lenses).

[0049] When object 214 is exposed to X-ray beam 205, the X-ray photons transmitted through the object form a modulated X-ray beam, which is received by the spectral and spatially resolved X-ray detection system 100. In some other examples, zone plate objective X-ray lenses are used to form an image on the spectral and spatially resolved X-ray detection system 100.

[0050] Typically, a magnified projection image of object 214 is formed on the spectral and spatially resolved X-ray detection system 100. The magnification is equal to the inverse ratio of the source-to-object distance 302 to the source-to-detector distance 304.

[0051] The X-ray source system 202 and the spectral and spatially resolved X-ray detection system 100 are sometimes mounted on their respective z-axis stages. For example, in the illustrated example, the X-ray source system 202 is mounted on the base 207 via the source stage 254, and the spectral and spatially resolved X-ray detection system 100 is mounted on the base 207 via the detector stage 256. In practice, the source stage 255 and detector stage 256 are low-precision, high-travel-range stages that allow the X-ray source system 202 and the spectral and spatially resolved X-ray detection system 100 to move into place, typically very close to the object, during object scanning, and then retract to allow removal of the object from the stage system 210, loading of a new object onto the stage system 210, and / or repositioning of the object on the stage system 210.

[0052] According to the present invention, the spectral and spatially resolved x-ray detection system 100 typically includes a photoconductive x-ray detector 12 and a camera 110. The camera is typically a high-speed camera because it captures images quickly enough to resolve halos induced in the photoconductive x-ray detector 12 associated with the detection of individual x-ray photons or other particles. Therefore, the camera typically captures images at a rate of 60 frames per second or higher. Preferably, its rate is equal to or greater than 120 frames per second, preferably greater than 500 frames per second, or even 1000 frames per second.

[0053] The operation of the X-ray CT microscopy system 200-1 and the scanning of the object 214 are controlled by a computer system 224, which typically includes an image processor subsystem and a controller subsystem. The computer system is used to set the bias voltage of the photoconductive X-ray detector 12 and read out the optical images detected by the camera 110 of the spectral and spatially resolved X-ray detection system 100.

[0054] According to the present invention, the computer system 224 obtains the image generated by the camera 110 and tracks the halo induced by X-ray photons or other particles detected in the photoconductive X-ray detector 12 over time, thereby determining the position and energy of the photons on the photoconductive X-ray detector 12, and thus obtaining spectral and spatial resolution.

[0055] Specifically, computer system 224 determines the energy of photons or other particles by referencing energy / intensity map 310 (e.g., lookup table, LUT). This map correlates the maximum spot intensity or integrated spot intensity, or the fit of the intensity map, with the energy of the receiving particles.

[0056] It should be noted that the computer system does not have to be a single device. For example, a single-board computer or microcontroller can be used as the control system of the X-ray CT microscopy system 200-1. A separate computer can be used to process the images generated by the camera to generate spatially and spectrally resolved images or projections of objects, and / or tomographic reconstructions of objects based on various projections. In fact, camera images can be stored and then processed or reprocessed to generate object images and reconstructions. Therefore, a dedicated computer, such as a graphics processing unit (GPU), an application-specific integrated circuit (ASIC), a field-programmable array (FPGA), a general-purpose computer, or some combination of these or other computing systems, will be included as part of the computer system 224 to process images. Furthermore, these computing systems can also be integrated within the housing of the camera 110.

[0057] For reconstruction, the image processor of the computer system uses CT reconstruction algorithms to combine projected images of the object to create 3D tomographic volume information of the object. The reconstruction algorithm can be analytical, where convolution or frequency domain filtering of the projected data is combined with backprojection onto a reconstruction mesh. Alternatively, the reconstruction algorithm can be iterative, where techniques from numerical linear algebra or optimization theory are used to solve a discretized version of the projection process, which may include modeling of the physical properties of the imaging system.

[0058] Figure 1B This is a schematic diagram of the X-ray fluorescence XRF microscopy system 200-2. The spectral and spatially resolved X-ray detection system 100 is also applicable to this system.

[0059] X-ray fluorescence (XRF) microscopes 200-2 typically include an X-ray imaging system having an excitation system 204 that generates a high-energy beam 206 to induce X-ray fluorescence in elements of an object 214. For example, the high-energy beam 206 can be an X-ray beam or an electron beam.

[0060] Typically, X-ray optics, such as objectives or reflective elements (e.g., multicapillary optics or Kumakhov lenses), are used to capture fluorescent X-rays from an object and form an image on the detection system.

[0061] Here, the stage system 210 is capable of positioning and rotating the object 214 within the high-energy beam 206. Therefore, the stage system 210 will typically also include a precision three (3) or more axis stage for translating and positioning the object.

[0062] Figure 2A basic setup for a possible photoconductive X-ray detector 12 that can be used to implement the present invention is shown. The detector is typically used to detect X-rays as well as other particles, including charged particles. It includes an electro-optic spatial light modulator, such as an LC light valve disposed on one side of the photoconductive assembly 60.

[0063] The photoconductive component 60 includes a glass substrate 70 (on which a transparent conductive indium tin oxide (ITO) electrode 72 is disposed) and a photoconductor layer 74 with a thickness of approximately 5 to 2000 μm.

[0064] The electro-optic modulator 62 includes a liquid crystal (LC) cell 80. The LC cell 80 includes a pair of alignment layers 82 and 84. A spacer 86 acting between the alignment layers maintains a uniform separation between the alignment layers 82 and 84, thereby defining an LC cavity 88.

[0065] Nematic liquid crystal material 90 fills LC cavity 88. Top ITO electrode 92 covers alignment layer 84, while glass substrate 94 covers ITO electrode 92. ITO electrode 92 and glass substrate 94 are suspended on photoconductive component 60 to facilitate connection of potential source V (bias) between ITO electrodes 93 and 72, respectively.

[0066] The LC cell 80 is constructed in a similar manner to the self-standing LC cell, but the LC cell is defined by a glass substrate 94 and a photoconductor layer 74, rather than by a pair of glass substrates.

[0067] Epoxy resin 96 extends between ITO electrodes 92 and 72 to fill the area between the air-separated ITO electrodes and suppress breakdown of the LC cell 80 when a potential is applied to the ITO electrodes. Epoxy resin 96 also seals the electro-optic modulator 62 to suppress delamination of the layers forming the photoconductive X-ray detector 12.

[0068] In the past, amorphous selenium (a-Se) was mostly used as the photoconductor layer 74. On the other hand, the photoconductor layer 74 is made of a material that simultaneously possesses a high bandgap and high resistivity. The advantage of a high bandgap photoconductor is that it typically produces a much smaller leakage current under high voltage bias. This is important for reducing detector noise. Furthermore, it simplifies and stabilizes the performance of the LC light valve. It also simplifies the analysis and modeling of the detector system.

[0069] For example, the photoconductor layer 74 can be a single crystal bismuth silicate (Bi). 12 SiO 20 (BSO) or (chemically similar) bismuth germanate crystals Bi 12 GeO 20 (BGO) or Bismuth titanate Bi 12 TiO 20(BTO). In other examples, the high bandgap material of the photoconductor layer 74 is a lead-based high bandgap perovskite material, such as lead methylammonium halide (MAPbX3), lead formamidinium halide (FAPbX3), and lead cesium halide (CsPbX3).

[0070] There are several advantages to choosing these photoconductors. All of these materials contain high atomic number (Z) materials (e.g., bismuth Z = 83, lead Z = 82). This will enhance X-ray absorption because the photoelectric cross-section (absorbed power) is approximately equal to the Z value. 4 In proportion to the target X-ray energy range, photoelectric interactions dominate (i.e., X-rays from a few keV to several hundred keV), and other types of interactions, such as Compton scattering, are preferred. Furthermore, these high-Z materials typically have higher densities than low-Z materials such as a-Se. However, this would be a relatively small effect compared to the Z difference.

[0071] Several other examples of high bandgap materials for photoconductor layer 74 are: ZnTe, ZnSe, HgS, TlBr, HgI2, and mercury halide materials such as Hg2I2, Hg2Br2, and Hg2Cl2. (See U.S. Patent Application Publication No. 2016 / 0216384, entitled "Detection of nuclear radiation via mercurous halides," by H. Chen, J.-S. Kim, F. Jin, and S. Trivedi.)

[0072] Figure 3 The effective circuit of the X-ray light valve detector system is shown. As shown, the photoconductive component 60 can be composed of capacitors C connected in parallel. P It is represented by multiple current sources and charge sources. The LC box 80 can consist of capacitors C all connected in parallel. lc Resistor R lc And current source representation.

[0073] In a steady state without X-ray radiation, if an appropriate LC conductivity is chosen (not too small or too large, but larger than that of a photoconductor), its voltage drop will be very low or close to zero because the LC cell is very thin and has a higher conductivity than that of a photoconductor.

[0074] Another important aspect of selecting LC conductivity is setting the charge relaxation time τ of the device. c The charge drift time in the photoconductor layer 74 is on the order of nanoseconds to microseconds, depending on the material of the photoconductor layer and the bias voltage (V). (偏置) This is typically greater than the interval τ between consecutive image frames. f (1 / τ fThe frame rate is much shorter. After the charges drift to LC cell 80, they will dissipate relatively slowly; this dissipation time depends primarily on the charge relaxation time of LC cell 80. The LC layer relaxation time is... (ε r σ and LC represent the dielectric constant and conductivity, respectively.

[0075] In normal operation, τ f It should be at least greater than τ c The length is several times shorter to ensure that the charge-generated light spots are captured across multiple frames, allowing for estimation of the evolution of these spots (halos). Therefore, τ is typically... f <τ c / 2, preferably τ f <τ c / 4. Basically, the LC box 80 acts as both a charge reset resistor and a capacitor for this purpose.

[0076] Generally speaking, for fast imaging, τ f Less than 1 millisecond (msec), but for slow imaging, it can be longer than several seconds. Furthermore, τ can be controlled by doping the LC cell 80 to increase its conductivity and / or by changing its temperature. c .

[0077] Generally, crystalline photoconductors are preferred over amorphous materials such as a-Se. The advantage of crystalline materials is that they typically possess better material properties (higher electromobility, less charge trapping, etc.). The main advantage of amorphous materials is that they can be fabricated in large areas in a cost-effective manner. However, for the high-resolution detectors described here, the detectors are typically small in area, so this advantage is not particularly important for the current system.

[0078] In the past, some LC spacers used microrod spacers (such as those used in the LCD display industry), but such spacers are suitable for low-resolution and large-area LC light valve applications.

[0079] The proposed X-ray detection system is designed for high-resolution, small-area LC light valve applications. This allows the microrods / microspheres to be visible. Therefore, some embodiments use edge spacers 86 at the edges of the LC cavity 88. Preferably, the thickness of the spacers 86, and thus the thickness of the LC cavity 88, is 1 μm or less (for high spatial resolution and a large dose dynamic range).

[0080] Polarized reflective microscopy is a technique suitable for examining surfaces containing structures that alter the polarization state during reflection and / or light propagation. For example, this method allows for the easy inspection of structural grains in ore samples, many metal alloys, and thin films, along with LC thin films. The illumination wavefront encounters a polarizer located in a vertical illuminator before the beamsplitter that guides the light into the objective lens. The linearly polarized light wave is focused onto the sample / mirror surface and reflected back to the objective lens. After exiting the objective aperture as a parallel wavefront beam, the light is projected onto a second polarizer (analyzer) oriented 90 degrees relative to the first polarizer. Only the depolarized wavefront passes through the analyzer to reach the tubular lens.

[0081] Figure 4A and 4B Two possible spectral and spatially resolved X-ray detection systems 100 based on polarized optical microscopy are shown, which are capable of reading out the photoconductive X-ray detector 12 via camera 110.

[0082] More specifically, each spectral and spatially resolved X-ray detection system 100 typically includes a photoconductive X-ray detector 12 and a camera 110, which are coupled to each other via a polarized optical microscope 102. The photoconductive X-ray detector 12 reflects the X-ray projection of the X-ray beam 205 in its electro-optic modulator 62. The polarized optical microscope 102 then illuminates the electro-optic modulator 62 with polarized light for detection by the camera 110.

[0083] exist Figure 4A In this process, the incident X-ray beam (or charged particle beam) 205 is received in the photoconductor layer 74 of the photoconductor assembly 60. The generated charge forms an image in the electro-optic modulator 62 of the photoconductor X-ray detector 12.

[0084] Simultaneously, an external light source 130 generates light. Examples of external light sources include light-emitting diodes (LEDs), laser diodes, and filter incandescent lamps. If the light is not polarized from the source, it is polarized in polarizer 114 and then reflected by a 50 / 50 beam splitter 112. The light is focused onto an electro-optic modulator 62 via objective lens 113. The image recorded in the electro-optic modulator 62 is reflected in the polarization rotation of the LC cell 80. This light is reflected by a reflective layer 76 between the photoconductor layer 74 and the electro-optic modulator 62. This reflective layer can be a dichroic thin-film mirror. In other cases, the photoconductor layer 74 may reflect wavelengths of light from the external light source 130, making the reflective layer 76 unnecessary.

[0085] A portion of the reflected light passes through beam splitter 112 to reach second polarizer 118, which serves as an analyzer. Tubular lens 116 forms an image on camera 110.

[0086] exist Figure 4B In this configuration, the external light source 130 and the beam splitter 112 are located upstream of the photoconductive X-ray detector 12. The external light is then transmitted through the photoconductive X-ray detector 12.

[0087] Figure 4B The transmission version shown is relatively simple, but it prevents the spectral and spatially resolved X-ray detection system 100 from approaching an X-ray or charged particle beam source. Figure 4A In the reflection mode shown, an additional reflective layer (or dielectric mirror) 76 is placed on the photoconductive assembly 60. This reflective layer (or dielectric mirror) 76 reflects the detection light back to the optical microscope system to detect polarization changes in the LC layer. The properties of the dielectric mirror should not interfere with charge transport in the device. Therefore, a metallic mirror is not desirable.

[0088] exist Figure 4B In another configuration of the mid-transmission mode, the analyzer can be placed directly behind the glass substrate supporting the transparent electrodes (or integrated with the glass substrate to create a thin analyzer with transparent electrodes). This directly converts the polarization changes in the LC layer into changes in light intensity before the light enters the optical microscope. The advantage of this approach is that expensive and difficult-to-obtain high-NA polarization objectives can be replaced with ordinary high numerical aperture (NA) objectives.

[0089] The lenses in the example diagram are not limited to microscope objectives.

[0090] The spectral and spatially resolved X-ray detection system 100 operates as follows to resolve X-ray photons or other particles spectrally and spatially. After each X-ray photon (or charged particle) strikes and is absorbed by the photoconductor layer 74, it forms numerous electron-hole pairs. These processes are very fast and can be effectively considered instantaneous. The next event is the drift of electrons and holes to opposite sides of the interface between the photoconductor 74, the ITO electrode 72, and the LC cell 80 of the electro-optic modulator 62. We use... This represents the longest possible charge drift time, where D is the photoconductor thickness, μ is the mobility of slower charge carriers, and V is the bias voltage. For fairly good detector materials, τ dr At least on the microsecond scale, which can be considered instantaneous in most applications. After the charge drifts into the LC cell 80 (acting as a lossy capacitor), there are two relevant timescales: the charge relaxation time τ. c Due to the large changes in the conductivity of liquid crystals (doping and temperature), the charge relaxation time τ c It may change significantly; pointing to the vector relaxation time τ d The reaction rate of LC molecules to torque (applied by a change in electric field) can be several milliseconds. A simple model considering only charge relaxation time and director relaxation time can be written as:

[0091]

[0092] Where q is the charge drift to the LC layer; a is a scaling factor that depends on the device configuration details; the two terms on the right of the exponential decay constitute the time factor; and I(t) is the total signal strength.

[0093] Figure 5 The model signal pulse illustrates the evolution of the total intensity of a photon deposition (showing only the time factor), where τ d =10ms and τ c =10ms.

[0094] Figure 6 The image data of one frame captured by camera 110 at time t=0 is shown. A series of spots B1-B7 at different halo stages can be seen within the range of LC cell 80. We assume that the system is uniform and that the time constant is the same at different locations. The three halos on the right side of the frame are only to illustrate the evolution of the time factor of each signal spot. The amount of charge deposited on each spot may be different and is a function of the energy of the particles that produce the spot.

[0095] In any case, by tracking the position of each spot and resolving the size of the halo of the spot and thus the energy of the photons or particles that produce the halo, a spatially resolved and spectral (energy) resolved image of object 214 is generated.

[0096] calibration

[0097] Figure 7 This is a flowchart illustrating the method for calibrating an imaging spectrometer.

[0098] For calibration, photons or particles of known energy are generated in step 510. Calibration requires some energy value. In one example, the x-ray source system 202 is a set of radioactive isotopes with the correct energy peak within the detection energy range. In another example, the x-ray source system 202 is a tunable synchrotron monochromatic source.

[0099] The flux requirement is low, therefore the relaxation time of LC cell 80 is... (ε r =ε || ,ε ⊥ and σ = σ || ,σ ⊥ Within the LC alignment, individual X-ray deposition events are sparsely distributed, such as... Figure 6 As shown. In step 512, the camera captures and stores numerous images of the LC box.

[0100] Next, in step 514, the image is processed to identify and track the location and intensity of individual spots across multiple frames during the LC layer relaxation time. In step 516, any overlapping spots are discarded. The conductivity σ of the LC layer is selected to set the charge relaxation time to cover several frames (a longer relaxation time is used for higher spectral resolution; a shorter relaxation time is used for faster imaging).

[0101] In step 518, the relative intensity of the light spot is determined. This can be achieved by obtaining the maximum light spot intensity, summing the intensities over several frames, or curve fitting the time-varying intensity.

[0102] Then, in step 520, all event intensity values ​​of each spot are aggregated to form a histogram.

[0103] The peak value in the histogram is related to the corresponding input X-ray photon energy in step 522.

[0104] The above steps can be repeated using different radioactive isotopes, for example, to produce different input X-ray energies.

[0105] Then, in step 524, an energy / intensity map 310 is generated, such as a lookup table (LUT), to convert the spot intensity into input energy (through appropriate interpolation).

[0106] The above calibration can be repeated under several different bias and imaging conditions. These conditions include bias amplitude and frequency, as well as transit suppression conditions.

[0107] Figure 8 It describes how to use calibration to generate spectral and spatially resolved images or projections of object 214.

[0108] Now, system 200 is used for actual spectral imaging, but the energy of the x-ray (charged particle) source 202 is unknown.

[0109] Based on spectral accuracy requirements, the device can operate under a certain degree of packing conditions, meaning it can still function even when event spots slightly overlap. However, the average spatial photon density / distribution should still be much lower than the system's optical resolution. Packing suppression and correction algorithms can be applied to push the operating flux level above the calibrated flux.

[0110] In any case, in step 610, an X-ray beam 205 is generated to illuminate the object 214.

[0111] In step 612, camera 110 generates an image of the electro-optic modulator 62 of photoconductive X-ray detector 12. In step 614, the image is processed to determine the location and center of the halo induced by the received photons or charged particles.

[0112] In step 616, the images are processed using the same method as in step 518 to determine the relative intensity of each spot on multiple images.

[0113] Finally, in step 618, a lookup table (LUT) is used to correlate the spot intensity with the input energy.

[0114] Steps 614–618 are performed in a computer system, but the computer system does not need to be a single device. For example, the computer system can be a general-purpose computer where the images have already been downloaded and stored, or it can be performed as an add-on to the imaging camera in a dedicated ASIC / FPGA module or in a GPU. The advantage of such a module is that it speeds up processing and reduces the amount of data and storage required.

[0115] analyze

[0116] The effect of point charge on LC cell:

[0117] Figure 9 The effect of point charges near the thin LC cell is shown to illustrate the working principle of the device. The LC cell 80 is aligned by two alignment layers 84 and 82, one above and one below. Different alignment orientations can be used, and we will discuss two possible alignment methods below. When X-ray photons are absorbed by the direct detection material of the photoconductor layer 74, it generates many electron-hole pairs. Under the influence of an external bias electric field, electrons (or holes in the case of reverse polarization) drift toward the LC cell. Depending on the conductivity of the LC and the alignment layers, the charge will remain near the interface for a finite amount of time. Here, we only consider the static case where the charge remains near the interface of the vertically aligned LC cell 80 to illustrate the working principle of the LCLV X-ray spectrometer. The charge q is placed near the interface at a distance z0. This is a more easily considered case because it has cylindrical symmetry along the z-axis.

[0118] This illustrates the case where LC molecules tend to align with an external electric field under positive dielectric anisotropy. We assume that the charge is very small and the directional field distortion is small, so the electric field formed by the charge within the LC cell is approximately equal to that without any directional field distortion. We also assume that the dielectric properties on both sides of the boundary are not significantly different, so we can neglect the effect of the charge. This may not be valid since LC is an anisotropic liquid, but it may be approximately valid for the case of small dielectric anisotropy. The following calculations are for illustrative purposes only (because the uncertainties in material properties are large, this is only to guide our understanding), we have

[0119]

[0120] Where E ρ It is a field along the radial direction, E z It is the field along the z-axis. And r 2 =ρ2 (z+z0) 2 r is the distance from the charge, ρ is the radial distance of the cylinder, and z0 is the distance from the point charge to the LC interface; ε || and ε ⊥ These are the parallel and perpendicular dielectric constants of the LC layer. We will use a constant approximation to attempt to solve for the pointing vector field using this charge. For n x and n y We have

[0121]

[0122] Where K is the average of the three elastic constants and ε a =ε || -ε ⊥ Represented in cylindrical coordinates, we have

[0123]

[0124] Figure 10A Simulation results are shown for a typical case of vertical alignment with an LC layer thickness d = 1 μm (parameters used in the simulation are: K = 10 pN, ε || =15,ε ⊥ =3, z0=10nm, q=60e). For small tilts, n ρ It is the same as the pointing angle θ (in radians) relative to the z-axis. It can be proven that, for optical detection, neglecting diffraction effects, the optical signal is the same as ∫θ 2 It is proportional to dz, where the integral is along the z-axis along the detection light propagation. Figure 10B The integration results are shown.

[0125] We can see that the effect of a point charge is very small, concentrated near the charge location. This is partly because, in the lectin equation, the force term is proportional to the square of the electric field. This nonlinear dependence will cause the signal output to depend nonlinearly on the charge deposition, which is undesirable. However, the situation is improved by incorporating the flexural electric effect (which has a term that depends linearly on the electric field), a problem we will discuss below. Furthermore, the LC lectin tilt is shaped like a ring. This is because at the point directly above the charge, the electric field points upwards, and the tilt is zero. The ring is about one-tenth of a micrometer in size, and we would not be able to see the dark area at the center if observed with visible light.

[0126] For planar alignment, we can similarly solve the partial differential equation (PDE) for the pointing vector. However, it is a three-dimensional (3D) PDE, and therefore more complex. For a perfectly aligned planar box with similar geometry, we have an electric field (when there is no charge, the pointing vector is aligned along the x-axis).

[0127]

[0128] Figure 11A and 11B Simulation results are shown for a typical case of planar alignment with an LC layer thickness d = 1 μm (parameters used in the simulation are: K = 10 pN, ε || =15,ε ⊥ =3, z0=10nm, q=62e). Note that negative and positive values ​​of n are equivalent. Under observation light, the effect of director distortion is quite complex. In these simulations, we have not considered the flexural electrical effect, which can be very strong in a small range. Therefore, these simulation results are only used to illustrate the results of planar alignment.

[0129] For the liquid crystal pointer distortion problem we consider here, the flexural effect can be strong over a small area due to the highly concentrated and non-uniform electric field. We will explain the effect of vertical alignment below. Since there are many uncertainties in the measured flexural coefficients e1 and e3 in all published results, our results are only for illustrative purposes. Actual measurements and calibrations for individual LC cells are required.

[0130] Including the flexural effect, the director equation becomes:

[0131]

[0132] To simplify the problem, we only consider the case of perpendicular alignment. Due to the cylindrical symmetry, we can rewrite it in cylindrical coordinates:

[0133]

[0134] as well as

[0135]

[0136] Figure 12A and 12B Simulation results of a 1 μm thick LC vertical alignment cell with flexural electrical effect are shown (parameters used in the simulation: K = 10 pN, ε || =15, ε ⊥=3, z0 = 10 nm, q = 33e, e1-e3 = 12.2 pC / m, e1+e3 = 40.0 pC / m). Due to the linear dependence of the electric field in the flexural terms, they are more advantageous than the square terms in the case of low charge, which is our main interest. Therefore, it greatly corrects the results. However, due to the many uncertainties in the values ​​of e1 and e3, our calculations only show this effect. According to the values ​​we chose (e1-e3 = 12.2 pC / m, e1+e3 = 40.0 pC / m), the flexural effect makes the LC detector more sensitive to small charge deposition. In addition, this effect makes it more linear in the case of small charge deposition. However, the range of director tilt becomes larger.

[0137] In the above discussion, we assumed that the externally applied electric field along the z-axis was zero. This is achieved when the external bias voltage on the light valve (a photoconductor in series with the LC layer) is low and the LC layer has relatively high conductivity (e.g., by doping the LC with an ionic salt). In other cases, when the external bias voltage is high and the LC does not have high conductivity, there is usually a voltage drop across the LC layer, and the bias field generated by the external field along the z-axis is mostly much larger than the field generated by the point charge. In this case, the director equation becomes...

[0138]

[0139] When E z0 >>E z At times, in most locations, we have

[0140]

[0141] Similarly, the force term now depends linearly on the E-field induced by the point charge.

[0142] Figure 13A and 13B It shows the bias field E z0 Simulation results of a 1μm thick LC vertical alignment cell with a voltage of 0.5V / μm (parameters used in the simulation are: K = 10pN, ε || =15,ε ⊥ =3, z0 = 10nm, q = 95e). It becomes more sensitive compared to no bias field. However, the area affected by the charge increases to cover a larger region. Since the bias field is adjustable, this can be used as one of the adjustable parameters to control the LCLV. If both flexural current and bias field are included in the calculation, the effects are combined (in the case of flexural effect and large bias voltage field, the equation depends linearly on the charge-induced field; when both effects are strong, the effects can be linearly combined).

[0143] Figure 14A and 14BThe calculation results, which include both effects, are shown (the parameters used in the simulation are: K = 10pN, ε || =15, ε ⊥ =3, z0=10nm, q=25e, e1-e3=12.2pC / m, e1+e3=40.0pC / m,E z0 =0.5V / m). This is very similar to the case where only the flexural effect exists. However, different combinations can occur for different material parameters.

[0144] LC box sensitivity

[0145] Several factors affect the sensitivity of point charge detection using an LC cell, such as light intensity and various alignment factors. Theoretically, the ultimate factor is the director field tilt caused by thermal noise.

[0146] As can be seen (PG de Gennes and J. Prost, The Physics of Liquid Crystals. Clarendon Press, 1995), for a blocky liquid crystal (LC) with its director aligned along the z-direction,

[0147]

[0148] For our simplified case, K3 = K α =K. and therefore We have

[0149]

[0150] In our case, due to the constraints of two boundaries, q in the z-direction is discretized. Where m is an integer. q ρ It is the wave number in the xy plane and ρ 2 =x 2 +y 2 For each q z Pattern, θ m z-dependency Where m is an integer, and this satisfies the boundary condition θ m (0)=θ m (d) = 0.

[0151] To obtain each q z The theta fluctuation of the model, we will apply to q ρ from arrive Integrate, where L is the lateral size of the liquid crystal layer, and ρ bThis is the feature size in the xy direction (set to 0.5 μm in all examples).

[0152]

[0153] We have Ω = πL 2 d; factor This makes the result dimensionless; factor Two-dimensional integral dq representing wavenumber ρ dq ρ (πL 2 It's the area. Indicates q x q y (Convert the sum to an integral). If the lateral size of the LC layer is much larger than the thickness of the LC layer, we have

[0154]

[0155] This expression shows that the lowest mode, m=1, induces the maximum director tilt, which we can consider as a typical fluctuation amplitude. Insert the value K=10pN,ε used in the example above. a =12, d=1μm, for characteristic size ρ b =0.5μm, we have: if E z0 =0.5V / μm And if there is no external bias field

[0156] To obtain the device's sensitivity to a single point charge, we will use the radius ρ b Integral induction of direction vector tilt on cylinder The mean was compared with the heat-induced mean tilt.

[0157]

[0158] For the value K = 10pN used in the example above, ε a =12, d=1μm, for characteristic size ρ b =0.5μm, we have: if E z0 =0.5V / μm And if there is no external bias field

[0159]

[0160] This shows the sensitivity of the 1μm LC cell, including the various effects considered (when biased, K = 10pN, e1-e3 = 12.2pC / m, e1+e3 = 40pC / m, ε || =15,ε ⊥=3, z0=10nm,E z0 =0.5V / μm). Without a bias field and flexural effects, the direct tilt is highly localized, and calculations will produce a tilt that violates the approximation |n| << 1, so no results are listed. We can see that the vertically aligned LC cell is quite sensitive to charge (25e roughly corresponds to the energy resolution of a 75 to 200eV detector).

[0161] Spectral resolution

[0162] Several factors can increase spectral width (and reduce energy resolution):

[0163] ●As mentioned above, the director tilt is caused by the fundamental thermal noise.

[0164] ● A limited number of detection photons. While we can provide fairly high intensity illumination, this can become a significant source of noise when the camera exposure time is short.

[0165] ● Camera noise.

[0166] ● Depending on the deposition depth, some electrons or holes are absorbed by the photoconductor. If this absorption is excessive, we will obtain a very large peak width extension. Therefore, one of the most important criteria for selecting a suitable photoconductor is its mu-tau product (mobility-lifetime product), which, together with the bias voltage and thickness of the photoconductor, determines the absorption of charge carriers. Fortunately, for most detection tasks, our resolution requirements necessitate that the photoconductor be relatively thin. Therefore, with a moderate mu-tau product and bias voltage, we should be able to obtain fairly high collection efficiency.

[0167] While the invention has been specifically shown and described with reference to preferred embodiments thereof, those skilled in the art will understand that various changes to the form and details of the invention may be made without departing from the scope of the invention as defined in the appended claims.

Claims

1. A spectral and spatially resolved X-ray and / or charged particle detection system, comprising: Photoconductivity detector; An optical microscope is used to read out the photoconductivity detector; The camera is coupled to the photoconductive detector via the optical microscope; as well as A computer system configured to acquire images captured by the camera and track over time a light spot generated as a response of the photoconductive detector to x-ray photons and / or charged particles, and to determine the energy of the x-ray photons or charged particles based on the relationship between the light spot and the energy of the x-ray photons or charged particles.

2. The detection system as described in claim 1, wherein, The response is characterized as a halo in the photoconductive detector induced by the x-ray photons or charged particles, to distinguish the energy of the x-ray photons or charged particles and their position on the photoconductive detector.

3. The detection system as described in claim 1 or 2, wherein, The computer system determines the energy of the X-ray photons or charged particles by referencing an energy / intensity map that correlates the maximum spot intensity with the energy of the received X-ray photons or charged particles.

4. The detection system as described in any one of claims 1-2, wherein, The interval between consecutive frames captured by the camera is less than the relaxation time of the photoconductive detector.

5. The detection system as described in claim 4, wherein, The interval between consecutive frames captured by the camera is less than 1 millisecond.

6. The detection system as described in any one of claims 1-2 or 5, wherein, The photoconductivity detector detects X-ray photons.

7. The detection system as described in any one of claims 1-2 or 5, wherein, The photoconductive X-ray / charged particle detector consists of a liquid crystal light valve and a photoconductive detector layer.

8. The detection system as described in claim 7, wherein, The photoconductive detector layer includes bismuth, lead, mercury, tellurium, selenium, or thallium.

9. The detection system as described in any one of claims 1-2, 5, or 8, wherein, The optical microscope in question is a polarizing microscope.

10. The detection system according to any one of claims 1-2, 5, or 8, wherein, The optical microscope reads out the photoconductivity detector in a transmission manner.

11. The detection system according to any one of claims 1-2, 5, or 8, wherein, The optical microscope reads out the photoconductivity detector by reflection.

12. A particle detection method, comprising: In a photoconductive detector, X-ray photons and / or charged particles are converted into electron-hole pairs; The photoconductivity detector is read out using a camera coupled to the photoconductivity detector; as well as The images captured by the camera are processed and the light spot generated as a response of the photoconductive detector to X-ray photons or charged particles is tracked over time to determine the position and energy of the X-ray photons or charged particles based on the relationship between the light spot and the energy of the X-ray photons or charged particles.

13. The method according to claim 12, using the system of any one of claims 1-11.

14. An imaging system, comprising: Stage system for holding objects; as well as The detection system according to any one of claims 1-11.

15. The system according to claim 14, wherein, The photoconductivity detector is the detection system according to any one of claims 1-11.

16. A method for calibrating an X-ray photon or charged particle detection system, comprising: It produces X-ray photons or charged particles with known energies; In a photoconductive detector, particles are converted into electron-hole pairs; The photoconductivity detector is read out using a camera; as well as The image generated by the camera is processed and the light spot generated by the X-ray photons or charged particles received by the photoconductive detector is tracked, and the relationship between the light spot and the energy of the X-ray photons or charged particles is determined.

17. The method according to claim 16, using the detection system according to any one of claims 1-11.

Images