Patent Publication Number: US-2021173104-A1

Title: Radiation detector with subpixels operating in different modes

Description:
TECHNICAL FIELD 
     The disclosure herein relates to a radiation detector, in particular to a radiation detector having subpixels operating in different modes. 
     BACKGROUND 
     A radiation detector is a device that measures a property of a radiation. Examples of the property may include a spatial distribution of the intensity, phase, and polarization of the radiation. The radiation may be one that has interacted with a subject. For example, the radiation measured by the radiation detector may be a radiation that has penetrated or reflected from the subject. The radiation may be an electromagnetic radiation such as infrared light, visible light, ultraviolet light, X-ray or γ-ray. The radiation may be of other types such as α-rays and β-rays. 
     One type of radiation detectors is based on interaction between the radiation and a semiconductor. For example, a radiation detector of this type may have a semiconductor layer that absorbs the radiation and generate charge carriers (e.g., electrons and holes) and circuitry for detecting the charge carriers. 
     SUMMARY 
     Disclosed herein is a radiation detector, comprising: a pixel comprising a plurality of subpixels, each of the subpixels configured to generate an electrical signal upon exposure to a radiation; a switch electrically connected to the plurality of subpixels; wherein the switch is configured to combine electrical signals generated by a subset of the subpixels. 
     According to an embodiment, the switch is configured to detect a magnitude of the electrical signal generated by each of the subpixels. 
     According to an embodiment, the switch is configured to disconnect any one of the subpixels when the magnitude of the electrical signal generated by that subpixel exceeds a magnitude threshold. 
     According to an embodiment, the switch comprises a plurality of sub-switches respectively connected to the subpixels. 
     According to an embodiment, each of the subpixels is configured to detect a magnitude of the electrical signal generated by the subpixel connected thereto. 
     According to an embodiment, each of the sub-switches is configured to disconnect the subpixel connected thereto when the magnitude exceeds a magnitude threshold. 
     According to an embodiment, the pixel comprises four subpixels. 
     According to an embodiment, the pixel further comprises a radiation absorption layer. 
     According to an embodiment, the radiation absorption layer comprises a semiconductor. 
     According to an embodiment, the semiconductor is selected from a group consisting of silicon, germanium, GaAs, CdTe, CdZnTe, or combinations thereof. 
     According to an embodiment, the switch further comprises an accumulator to combine the electrical signals generated by any subset of the subpixels. 
     According to an embodiment, the radiation detector further comprises a comparator configured to compare an output signal from the switch to an output threshold; a counter configured to register a number of particles of radiation absorbed by radiation detector; a controller; a meter configured to measure the output signal; wherein the controller is configured to start a time delay from a time at which the comparator determines that an absolute value of the output signal equals or exceeds an absolute value of the output threshold; wherein the controller is configured to cause the meter to measure the output signal upon expiration of the time delay; wherein the controller is configured to determine a number of particles by dividing the output signal measured by the meter by an output signal of the switch that a single particle generates; wherein the controller is configured to cause the number registered by the counter to increase by the number of particles. 
     According to an embodiment, the controller is configured to deactivate the comparator at a beginning of the time delay. 
     Disclosed herein is a system comprising any of the radiation detectors above and an X-ray source, wherein the system is configured to perform X-ray radiography on human chest or abdomen. 
     Disclosed herein is a system comprising any of the radiation detectors above and an X-ray source, wherein the system is configured to perform X-ray radiography on human mouth. 
     Disclosed herein is a cargo scanning or non-intrusive inspection (NII) system comprising any of the radiation detectors above and an X-ray source, wherein the cargo scanning or non-intrusive inspection (NII) system is configured to form an image using backscattered X-ray. 
     Disclosed herein is a cargo scanning or non-intrusive inspection (NII) system comprising any of the radiation detectors above and an X-ray source, wherein the cargo scanning or non-intrusive inspection (NII) system is configured to form an image using X-ray transmitted through an object inspected. 
     Disclosed herein is a full-body scanner system comprising any of the radiation detectors above and an X-ray source. 
     Disclosed herein is an X-ray computed tomography (X-ray CT) system comprising any of the radiation detectors and an X-ray source. 
     Disclosed herein is an electron microscope comprising any of the radiation detectors, an electron source and an electronic optical system. 
     Disclosed herein is a system comprising any of the radiation detectors above, wherein the system is an X-ray telescope, or an X-ray microscopy, or wherein the system is configured to perform mammography, industrial defect detection, microradiography, casting inspection, weld inspection, or digital subtraction angiography. 
     Disclosed herein is a method comprising: obtaining a radiation detector comprising a pixel, wherein the pixel comprises a plurality of subpixels, each of the subpixels being configured to generate an electrical signal upon exposure to a radiation; identifying a subset of the subpixels; combining the electrical signals generated by the subset of the subpixels. 
     According to an embodiment, in the above-mentioned method, the radiation detector comprises a switch electrically connected to the plurality of subpixels, and the switch comprises a plurality of sub-switches respectively connected to the subpixels. 
     According to an embodiment, the method further comprises detecting a magnitude of the electrical signal generated by each subpixel using the sub-switch connected thereto. 
     According to an embodiment, the method further comprising disconnecting the subpixel using the sub-switch connected thereto upon determination that the magnitude exceeds a magnitude threshold. 
    
    
     
       BRIEF DESCRIPTION OF FIGURES 
         FIG. 1  schematically shows a radiation detector, according to an embodiment. 
         FIG. 2  schematically shows a pixel of the radiation detector in  FIG. 1 , wherein the pixel comprises a plurality of subpixels. 
         FIG. 3  schematically shows a cross-sectional view of the radiation detector. 
         FIG. 4A  schematically shows a detailed cross-sectional view of the radiation detector. 
         FIG. 4B  schematically shows an alternative detailed cross-sectional view of the radiation detector. 
         FIG. 5  schematically shows a component diagram of a switch of the radiation detector in  FIG. 4A or 4B , according to an embodiment. 
         FIG. 6  schematically shows a component diagram of an electronic system of the radiation detector in  FIG. 4A  or  FIG. 4B , according to an embodiment. 
         FIG. 7  shows a temporal change of the output signal of the switch in  FIG. 5  or  FIG. 6 , caused by charge carriers generated by one or more particles incident on the diode or the resistor, according to an embodiment. 
         FIG. 8  schematically shows a flow chart for a method suitable for using a radiation detector according to an embodiment. 
         FIG. 9  shows a flow chart for a method suitable for detecting radiation using a system such as the system operating as shown in  FIG. 4 . 
         FIG. 10  schematically shows a system comprising the radiation detector described herein, suitable for medical imaging such as chest X-ray radiography, abdominal X-ray radiography, etc., according to an embodiment. 
         FIG. 11  schematically shows a system comprising the radiation detector described herein suitable for dental X-ray radiography, according to an embodiment. 
         FIG. 12  schematically shows a cargo scanning or non-intrusive inspection (NII) system comprising the radiation detector described herein, according to an embodiment. 
         FIG. 13  schematically shows another cargo scanning or non-intrusive inspection (NII) system comprising the radiation detector described herein, according to an embodiment. 
         FIG. 14  schematically shows a full-body scanner system comprising the radiation detector described herein, according to an embodiment. 
         FIG. 15  schematically shows an X-ray computed tomography (X-ray CT) system comprising the radiation detector described herein, according to an embodiment. 
         FIG. 16  schematically shows an electron microscope comprising the radiation detector described herein, according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  schematically shows a radiation detector  100 , as an example. The radiation detector  100  has an array of pixels  150 . The array may be a rectangular array, a honeycomb array, a hexagonal array or any other suitable array. Each pixel  150  is configured to detect radiation from a radiation source incident thereon and may be configured measure a characteristic (e.g., the energy of the particles, the wavelength, and the frequency) of the radiation. 
       FIG. 2A  schematically shows that a pixel  150  may include a plurality of subpixels  150 S. In the example shown, the pixel  150  includes four subpixels  150 S. However, the pixel  150  may include any suitable number of subpixels  150 S. The subpixels  150 S may each be configured to generate an electrical signal upon exposure to a radiation. The characteristic measured by the pixel  150  may be determined based on the electrical signals from the subpixels  150 S included in the pixel  150 . For example, the subpixels  150 S may each be configured to count a number of particles of radiation incident thereon that have energies within a particular bin, within a period of time. The number of the particles of radiation incident on the pixel  150  that have energies within that particular bin within that period of time can be determined by adding the numbers counted by the subpixels  150 S for that bin within that period of time. When the incident particles of radiation have similar energy, the subpixels  150 S may each be configured to simply count a number of particles of radiation incident thereon within a period of time, without measuring the energy of the particles of radiation. The number of the particles of radiation incident on the pixel  150  within that period of time can be determined by adding the numbers counted by the subpixels  150 S within that period of time. 
     Each of the subpixels  150 S may have its own analog-to-digital converter (ADC) configured to digitize the electrical signal it generates. The subpixels  150 S may be configured to operate in parallel, and operate independently from one another. For example, malfunction of one subpixel  150 S would not affect the normal operation of another subpixel  150 S in the same pixel  150 . For example, when one subpixel  150 S measures a particle of radiation, another subpixel  150 S may be waiting for a particle of radiation to arrive. The subpixels  150 S may or may not be individually addressable. 
       FIG. 3  schematically shows a cross-sectional view of the radiation detector  100 , according to an embodiment. The radiation detector  100  may include a radiation absorption layer  110  and an electronics layer  120  (e.g., an ASIC) for processing or analyzing electrical signals incident radiation generates in the radiation absorption layer  110 . Each of the pixels  150  may include a portion of the radiation absorption layer  110 . The radiation detector  100  may or may not include a scintillator. The radiation absorption layer  110  may include a semiconductor material such as, silicon, germanium, GaAs, CdTe, CdZnTe, or a combination thereof. The semiconductor may have a high mass attenuation coefficient for the radiation of interest. 
     As shown in a detailed cross-sectional view of the radiation detector  100  in  FIG. 4A , according to an embodiment, the radiation absorption layer  110  may include one or more diodes (e.g., p-i-n or p-n) formed by a first doped region  111 , one or more discrete regions  114  of a second doped region  113 . The second doped region  113  may be separated from the first doped region  111  by an optional the intrinsic region  112 . In an embodiment, the discrete regions  114  are separated from one another by the first doped region  111  or the intrinsic region  112 . The first doped region  111  and the second doped region  113  have opposite types of doping (e.g., region  111  is p-type and region  113  is n-type, or region  111  is n-type and region  113  is p-type). In the example in  FIG. 4A , each of the discrete regions  114  of the second doped region  113  forms a diode with the first doped region  111  and the optional intrinsic region  112 . Namely, in the example in  FIG. 4A , t he radiation absorption layer  110  has a plurality of diodes having the first doped region  111  as a shared electrode. The first doped region  111  may also have discrete portions. A subpixel  150 S may encompass one of the discrete regions  114 . A pixel  150  may encompass a plurality of adjacent subpixels  150 S. 
     When a particle of radiation from the radiation source hits the radiation absorption layer  110  including diodes, the particle of radiation may be absorbed and generate one or more charge carriers by a number of mechanisms. The charge carriers may drift to the electrodes of one of the diodes under an electric field. The field may be an external electric field. The electrical contact  119 B may include discrete portions each of which is in electrical contact with the discrete regions  114 . The term “electrical contact” may be used interchangeably with the word “electrode.” In an embodiment, the charge carriers may drift in directions such that the charge carriers generated by a single particle of the radiation are not substantially shared by two different discrete regions  114  (“not substantially shared” here means less than 2%, less than 0.5%, less than 0.1%, or less than 0.01% of these charge carriers flow to a different one of the discrete regions  114  than the rest of the charge carriers). Charge carriers generated by a particle of the radiation incident around the footprint of one of these discrete regions  114  are not substantially shared with another of these discrete regions  114 . A subpixel  150 S associated with a discrete region  114  may be an area around the discrete region  114  in which substantially all (more than 98%, more than 99.5%, more than 99.9%, or more than 99.99% of) charge carriers generated by a particle of the radiation incident therein flow to the discrete region  114 . Namely, less than 2%, less than 1%, less than 0.1%, or less than 0.01% of these charge carriers flow beyond the subpixel. 
     As further shown in  FIG. 4A , the subpixels  150 S of the pixel  150  are electrically connected to a switch  160 . The switch  160  is configured to combine the electrical signals generated by any subset of the subpixels  150 S of the pixel  150 . In the present disclosure, the subset always has fewer subpixels  150 S than the total number of subpixels  150 S in the pixel  150 . For example, if the pixel  150  has four subpixels  150 S, the subset may have three subpixels  150 S, two subpixels  150 S, one subpixel  150 S, or zero subpixel  150 S. In an embodiment, the magnitude of the electrical signal generated by every subpixel  150 S in the subset is below a magnitude threshold. In an embodiment, the magnitude of the electrical signal generated by every subpixel  150 S not in the subset is above the magnitude threshold. In an embodiment, the magnitude threshold is an upper limit of the magnitude of the electrical signal a non-defective subpixel  150 S generates when not receiving a particle of radiation. Namely, the magnitude threshold may be an upper limit of the dark current in a non-defective subpixel  150 S. In other words, the subset may consist of all the non-defective subpixels  150 S of the pixel  150 . 
     In an embodiment, the switch  160  is configured to detect the magnitude of the electrical signal generated by each of the subpixels  150 S. The switch  160  may disconnect a subpixel  150 S, when it has detected that the magnitude of the subpixel  150 S exceeds the magnitude threshold. Namely, the switch  160  may exclude any of the subpixels  150 S from the subset based on the magnitude of the electrical signal it generates. In an embodiment, the disconnected subpixel  150 S is grounded. 
     As shown in an alternative detailed cross-sectional view of the radiation detector  100  in  FIG. 4B , according to an embodiment, the radiation absorption layer  110  may include a resistor of a semiconductor material such as, silicon, germanium, GaAs, CdTe, CdZnTe, or a combination thereof, but does not include a diode. The semiconductor may have a high mass attenuation coefficient for the radiation of interest. 
     When a particle of radiation hits the radiation absorption layer  110  including a resistor but not diodes, it may be absorbed and generate one or more charge carriers by a number of mechanisms. A particle of the radiation may generate 10 to 100000 charge carriers. The charge carriers may drift to the electrical contacts  119 A and  119 B under an electric field. The field may be an external electric field. The electrical contact  119 B includes discrete portions. A subpixel  150 S may encompass one of the discrete portions. A pixel  150  may encompass a plurality of adjacent subpixels  150 S. In an embodiment, the charge carriers may drift in directions such that the charge carriers generated by a single particle of the radiation are not substantially shared by two different discrete portions of the electrical contact  119 B (“not substantially shared” here means less than 2%, less than 0.5%, less than 0.1%, or less than 0.01% of these charge carriers flow to a different one of the discrete portions than the rest of the charge carriers). Charge carriers generated by a particle of the radiation incident around the footprint of one of these discrete portions of the electrical contact  119 B are not substantially shared with another of these discrete portions of the electrical contact  119 B. A subpixel  150 S associated with a discrete portion of the electrical contact  119 B may be an area around the discrete portion in which substantially all (more than 98%, more than 99.5%, more than 99.9% or more than 99.99% of) charge carriers generated by a particle of the radiation incident therein flow to the discrete portion of the electrical contact  119 B. Namely, less than 2%, less than 0.5%, less than 0.1%, or less than 0.01% of these charge carriers flow beyond the subpixel  150 S associated with the one discrete portion of the electrical contact  119 B. 
     In the embodiment as shown in  FIG. 4B , the subpixels  150 S of the pixel  150  are electrically connected to a switch  160 . The switch  160  is configured to combine the electrical signals generated by any subset of the subpixels  150 S of the pixel  150 , in a manner as similarly mentioned above in connection with  FIG. 4A . 
     Similarly, the switch  160  is configured to detect a magnitude of the electrical signal generated by each of the subpixels  150 S. The switch  160  further disconnects a subpixel  150 S, when it has detected that the magnitude of the subpixel  150 S equals to or exceeds a magnitude threshold, in a similar manner as mentioned above in connection with  FIG. 4A . 
       FIG. 5  schematically shows a component diagram of the switch  160 , according to an embodiment. The switch  160  may comprise a plurality of sub-switches  311  respectively connected to the plurality of subpixels  150 S of a pixel  150 . In an embodiment as shown in  FIG. 5 , the sub-switches  311  are respectively connected to discrete portions of the electrical contact  119 B associated with the subpixels  150 S. Each of the sub-switches  311  is configured to detect the magnitude of the electrical signal generated by the subpixel  150 S connected thereto, and configured to disconnect the subpixel  150 S when it detects that the magnitude exceeds the magnitude threshold. Namely, the sub-switches  311  may exclude any of the subpixels  150 S from the subset based on the magnitude of the electrical signal it generates. In an embodiment, the disconnected subpixel  150 S is grounded. 
     In an embodiment, the switch  160  is configured to combine the electrical signals generated by any subset of the subpixels  150 S. The switch  160  may comprise an accumulator  309  electrically connected to the discrete portions of the electrical contact  119 B associated with the subpixels  150 S, for example, through the sub-switches  311 . The accumulator  309  is configured to combine the electrical signals generated by any subset of the subpixels  150 S. In an embodiment, the accumulator  309  is configured to collect charge carriers from the subpixels  150 S. In an embodiment, the accumulator  309  includes a capacitor  308  in the feedback path of an op-amp  312 . Charge carriers from the subpixels  150 S accumulate on the capacitor  308  over a period of time (“integration period”). After the integration period has expired, the voltage across the capacitor  308  is sampled and then reset by a reset switch  305 . When a subpixel  150 S is excluded from the subset, the charge carriers therefrom may be prevented from reaching the accumulator  309 . 
     The electronics layer  120  of the radiation detector  100  may include an electronic system  121  suitable for processing or interpreting signals generated by the pixels  150  from the radiation incident thereon. The electronic system  121  is electrically connected to the discrete portions of the electric contact  119 B of a pixel  150 , for example, via the switch  160 . The electronic system  121  may include analog circuitry such as a filter network, amplifiers, integrators, and comparators, or a digital circuitry such as a microprocessor, and memory. The electronic system  121  may include one or more ADCs. The electronic system  121  may include components shared by multiple pixels  150  or components dedicated to a single pixel  150 . The electronic system  121  may include components shared by all of the subpixels  150 S of a pixel  150  or components dedicated to a single subpixel  150 S. For example, the electronic system  121  may include an amplifier that is dedicated to a pixel  150  and shared among all the subpixels  150 S of this pixel  150 , and a microprocessor that is shared among all the pixels  150 . The electronic system  121  may be electrically connected to the pixels  150  by vias  131 . Space among the vias may be filled with a filler material  130 , which may increase the mechanical stability of the connection of the electronics layer  120  to the radiation absorption layer  110 . Other bonding techniques are possible to connect the electronic system  121  to the pixels  150  without using vias. 
       FIG. 6  shows a component diagram of the electronic system  121 , according to an embodiment. In this embodiment, the electronic system  121  includes a comparator  301 , a counter  320 , a meter  306  and a controller  310 . 
     The comparator  301  is configured to compare an output signal from the switch  160 , which represents the combined electrical signals generated by the subset of the subpixels  150 S, to an output threshold. The comparator  301  may be controllably activated or deactivated by the controller  310 . The comparator  301  may be a continuous comparator. Namely, the comparator  301  may be configured to be activated continuously and monitor the output signal continuously. The first comparator  301  may be a clocked comparator. The output threshold may be 5-10%, 10%-20%, 20-30%, 30-40% or 40-50% of the output signal a single particle of radiation may generate on the switch  160 . 
     The comparator  301  may include one or more op-amps or any other suitable circuitry. The comparator  301  may have a high speed to allow the system  121  to operate under a high flux of incident radiation. 
     The counter  320  is configured to register a number of particles of radiation reaching a pixel  150 . The counter  320  may be a software component (e.g., a number stored in a computer memory) or a hardware component (e.g., a 4017 IC and a 7490 IC). 
     The controller  310  may be a hardware component such as a microcontroller and a microprocessor. The controller  310  is configured to start a time delay from a time at which the comparator  301  determines that the absolute value of the output signal equals or exceeds the absolute value of the output threshold (e.g., the absolute value of the output signal increases from below the absolute value of the output threshold to a value equal to or above the absolute value of the output threshold). The absolute value is used here because the output signal may be negative or positive. The controller  310  may be configured to keep deactivated the counter  320  and any other circuits the operation of the comparator  301  does not require, before the time at which the comparator  301  determines that the absolute value of the output signal equals or exceeds the absolute value of the output threshold. The time delay may expire before or after the output signal becomes stable, i.e., the rate of change of the output signal is substantially zero. The phase “the rate of change of the output signal is substantially zero” means that temporal change of the output signal is less than 0.1%/ns. The phase “the rate of change of the output signal is substantially non-zero” means that temporal change of the output signal is at least 0.1%/ns. 
     The term “activate” means causing the component to enter an operational state (e.g., by sending a signal such as a voltage pulse or a logic level, by providing power, etc.). The term “deactivate” means causing the component to enter a non-operational state (e.g., by sending a signal such as a voltage pulse or a logic level, by cut off power, etc.). The operational state may have higher power consumption (e.g., 10 times higher, 100 times higher, 1000 times higher) than the non-operational state. The controller  310  itself may be deactivated until the output of the comparator  301  activates the controller  310  when the absolute value of the output signal equals or exceeds the absolute value of the output threshold. 
     The controller  310  may be configured to cause the meter  306  to measure the output signal upon expiration of the time delay. The controller  310  may be configured to connect the discrete portions of the electric contact  119 B to an electrical ground, so as to discharge any charge carriers accumulated thereon. The controller  310  may connect the discrete portions of the electric contact  119 B to the electrical ground by controlling the switch  305 . The switch may be a transistor such as a field-effect transistor (FET). 
     In an embodiment, the electronic system  121  has no analog filter network (e.g., a RC network). In an embodiment, the electronic system  121  has no analog circuitry. 
     The meter  306  may feed the output signal it measures to the controller  310  as an analog or digital signal. 
       FIG. 7  schematically shows a temporal change of the output signal, caused by charge carriers generated by one or more particles of radiation incident on a pixel  150 , according to an embodiment. When one or more particles of radiation hit the pixel  150  starting at time to, the absolute value of the output signal starts to increase. At time t 1 , the comparator  301  determines that the absolute value of the output signal equals or exceeds the absolute value of the output threshold V 1 , and the controller  310  starts the time delay TD 1  and the controller  310  may deactivate the comparator  301  at the beginning of TD 1 . If the controller  310  is deactivated before t 1 , the controller  310  is activated at t 1 . At time t s , the time delay TD 1  expires. The particles of radiation may continue hit the pixel  150  throughout the entirety of TD 1 . 
     The controller  310  may be configured to cause the meter  306  to measure the output signal upon expiration of the time delay TD 1 . The output signal Vt measured by the meter  306  is proportional to the amount of charge carriers generated by the incident particles of radiation on the pixel  150  from t 0  to t s , which relates to the total energy of the incident particles of radiation. When the incident particles of radiation have similar energy, the controller  310  may be configured to determine the number of incident particles of radiation from t 0  to t s , by dividing Vt with the output signal that a single particle of radiation would cause on the switch  160 . The controller  310  may increase the counter  320  by the number of particles of radiation. 
     After TD 1  expires, the controller  310  connects the discrete portions of the electric contact  119 B to an electric ground for a reset period RST to allow charge carriers accumulated thereon to flow to the ground. After RST, the electronic system  121  is ready to detect another incident particle of radiation. If the comparator  301  has been deactivated, the controller  310  can activate it at any time before RST expires. If the controller  310  has been deactivated, it may be activated before RST expires. 
       FIG. 8  shows a flow chart for a method suitable for detecting radiation using the radiation detector  100 . In procedure  4010 , a subset of the plurality of subpixels  150 S in a pixel  150  is identified. In optional procedure  4020 , a magnitude of the electrical signal generated by each of the subpixels  150 S in the subset is determined, for example, using the sub-switch  311  connected thereto. In optional procedure  4030 , that subpixel  150 S is disconnected, for example, by the sub-switch  311  connected thereto, i.e. disconnecting the subpixel using the sub-switch connected thereto upon determination that the magnitude of the electrical signal generated by that subpixel  150 S equals to or exceeds a magnitude threshold. In procedure  4040 , the electrical signals generated by the subset of the subpixels  150 S are combined. In an embodiment, the subset includes all of the non-defective subpixels  150 S of a pixel  150  and none of the defective subpixels  150 S in the pixel  150 . 
       FIG. 9  shows a flow chart for a method suitable for detecting radiation incident on a pixel  150  using a system such as the system  121  operating as shown in  FIG. 6 . In procedure  5010 , the output signal of the switch  160  is compared to the output threshold, e.g., using the comparator  301 . In procedure  5020 , whether the absolute value of the output signal equals or exceeds the absolute value of the output threshold V 1  is determined, e.g., with the controller  310 . If the absolute value of the output signal does not equal or exceed the absolute value of the output threshold, the method goes back to procedure  5010 . If the absolute value of the output signal equals or exceeds the absolute value of the output threshold, continue to procedure  5030 . In procedure  5030 , the time delay TD 1  is started, e.g., using the controller  310 . In optional procedure  5040 , a circuit (e.g., the counter  320 ) is activated, e.g., using the controller  310 , during the time delay TD 1  (e.g., at the expiration of TD 1 ). In procedure  5050 , the output signal is measured, e.g., using the meter  306 , upon expiration of the time delay TD 1 . In procedure  5070 , the number of particles of radiation incident on the pixel  150  from t 0  to t s  is determined by dividing the output signal measured by an output signal a single particle of radiation would cause on the switch  160 . The output signal that a single particle of radiation would cause on the switch  160  may be known or measured separately in advance. In procedure  5080 , the counter is increased by the number of particles of radiation. The method goes to procedure  5090  after procedure  5080 . In procedure  5090 , reset the output signal, e.g., by connecting the discrete portions of the electric contact  119 B in the pixel  150  to an electrical ground. 
     According to an embodiment, the detector  100  may use delta-sigma (sigma-delta, ΔΣ or ΣΔ) modulation. In a conventional ADC, an analog signal is integrated, or sampled, with a sampling frequency and subsequently quantized in a multi-level quantizer into a digital signal. This process introduces quantization error noise. The first step in a delta-sigma modulation is delta modulation. In delta modulation the change in the signal (its delta) is encoded, rather than the absolute value. The result is a stream of pulses, as opposed to a stream of numbers. The digital output (i.e., the pulses) is passed through a 1-bit DAC and the resulting analog signal (sigma) is added to the input signal of the ADC. During the integration of the analog signal, when the analog signal reaches the delta, a counter is increased by one and the delta is deducted from the analog signal. At the end of the integration, the registered value of the counter is the digital signal and the remaining analog signal smaller than the delta is the residue analog signal. 
     The electronic system  121  may further include another comparator  302  but omit the meter  306 , as shown in  FIG. 6 . During TD 1 , whenever the comparator  302  determines that the output signal reaches Vp, which is the output signal a single incident particle of radiation would have caused on the switch  160 , the controller  310  connects the discrete portions of the electric contact  119 B in the pixel  150  to an electric ground to allow charge carriers accumulated thereon to flow to the ground and increases the counter  320  by one. 
     After TD 1  expires, the controller  310  again connects the discrete portions of the electric contact  119 B in the pixel  150  to an electric ground for a reset period RST to allow charge carriers accumulated thereon to flow to the ground. The number of the counter  320  at the expiration of TD 1  represents the number of incident particles of radiation on the pixel  150  from t 0  to the expiration of TD 1 . 
       FIG. 10  schematically shows a system comprising the radiation detector  100  described herein. The system may be used for medical imaging such as chest X-ray radiography, abdominal X-ray radiography, etc. The system comprises an X-ray source  1201 . X-ray emitted from the X-ray source  1201  penetrates an object  1202  (e.g., a human body part such as chest, limb, abdomen), is attenuated by different degrees by the internal structures of the object  1202  (e.g., bones, muscle, fat and organs, etc.), and is projected to the radiation detector  100 . The radiation detector  100  forms an image by detecting the intensity distribution of the X-ray. 
       FIG. 11  schematically shows a system comprising the radiation detector  100  described herein. The system may be used for medical imaging such as dental X-ray radiography. The system comprises an X-ray source  1301 . X-ray emitted from the X-ray source  1301  penetrates an object  1302  that is part of a mammal (e.g., human) mouth. The object  1302  may include a maxilla bone, a palate bone, a tooth, the mandible, or the tongue. The X-ray is attenuated by different degrees by the different structures of the object  1302  and is projected to the radiation detector  100 . The radiation detector  100  forms an image by detecting the intensity distribution of the X-ray. Teeth absorb X-ray more than dental caries, infections, periodontal ligament. The dosage of X-ray radiation received by a dental patient is typically small (around 0.150 mSv for a full mouth series). 
       FIG. 12  schematically shows a cargo scanning or non-intrusive inspection (NII) system comprising the radiation detector  100  described herein. The system may be used for inspecting and identifying goods in transportation systems such as shipping containers, vehicles, ships, luggage, etc. The system comprises an X-ray source  1401 . X-ray emitted from the X-ray source  1401  may backscatter from an object  1402  (e.g., shipping containers, vehicles, ships, etc.) and be projected to the radiation detector  100 . Different internal structures of the object  1402  may backscatter X-ray differently. The radiation detector  100  forms an image by detecting the intensity distribution of the backscattered X-ray and/or energies of the backscattered X-ray particles of radiation. 
       FIG. 13  schematically shows another cargo scanning or non-intrusive inspection (NII) system comprising the radiation detector  100  described herein. The system may be used for luggage screening at public transportation stations and airports. The system comprises an X-ray source  1501 . X-ray emitted from the X-ray source  1501  may penetrate a piece of luggage  1502 , be differently attenuated by the contents of the luggage, and projected to the radiation detector  100 . The radiation detector  100  forms an image by detecting the intensity distribution of the transmitted X-ray. The system may reveal contents of luggage and identify items forbidden on public transportation, such as firearms, narcotics, edged weapons, flammables. 
       FIG. 14  schematically shows a full-body scanner system comprising the radiation detector  100  described herein. The full-body scanner system may detect objects on a person&#39;s body for security screening purposes, without physically removing clothes or making physical contact. The full-body scanner system may be able to detect non-metal objects. The full-body scanner system comprises an X-ray source  1601 . X-ray emitted from the X-ray source  1601  may backscatter from a human  1602  being screened and objects thereon, and be projected to the radiation detector  100 . The objects and the human body may backscatter X-ray differently. The radiation detector  100  forms an image by detecting the intensity distribution of the backscattered X-ray. The radiation detector  100  and the X-ray source  1601  may be configured to scan the human in a linear or rotational direction. 
       FIG. 15  schematically shows an X-ray computed tomography (X-ray CT) system. The X-ray CT system uses computer-processed X-rays to produce tomographic images (virtual “slices”) of specific areas of a scanned object. The tomographic images may be used for diagnostic and therapeutic purposes in various medical disciplines, or for flaw detection, failure analysis, metrology, assembly analysis and reverse engineering. The X-ray CT system comprises the radiation detector  100  described herein and an X-ray source  1701 . The radiation detector  100  and the X-ray source  1701  may be configured to rotate synchronously along one or more circular or spiral paths. 
       FIG. 16  schematically shows an electron microscope. The electron microscope comprises an electron source  1801  (also called an electron gun) that is configured to emit electrons. The electron source  1801  may have various emission mechanisms such as thermionic, photocathode, cold emission, or plasmas source. The emitted electrons pass through an electronic optical system  1803 , which may be configured to shape, accelerate, or focus the electrons. The electrons then reach a sample  1802  and an image detector may form an image therefrom. The electron microscope may comprise the radiation detector  100  described herein, for performing energy-dispersive X-ray spectroscopy (EDS). EDS is an analytical technique used for the elemental analysis or chemical characterization of a sample. When the electrons incident on a sample, they cause emission of characteristic X-rays from the sample. The incident electrons may excite an electron in an inner shell of an atom in the sample, ejecting it from the shell while creating an electron hole where the electron was. An electron from an outer, higher-energy shell then fills the hole, and the difference in energy between the higher-energy shell and the lower energy shell may be released in the form of an X-ray. The number and energy of the X-rays emitted from the sample can be measured by the radiation detector  100 . 
     The radiation detector  100  described here may have other applications such as in an X-ray telescope, X-ray mammography, industrial X-ray defect detection, X-ray microscopy or microradiography, X-ray casting inspection, X-ray non-destructive testing, X-ray weld inspection, X-ray digital subtraction angiography, etc. It may be suitable to use this radiation detector  100  in place of a photographic plate, a photographic film, a PSP plate, an X-ray image intensifier, a scintillator, or another semiconductor X-ray detector. 
     While various aspects and embodiments have been disclosed herein, other aspects and embodiments will be apparent to those skilled in the art. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope and spirit being indicated by the following claims.