Disclosed herein is an apparatus for detecting X-ray. The apparatus has an X-ray absorption layer with an electrode, one or more voltage comparators configured to compare a voltage of the electrode to one or more thresholds, a counter configured to register the number of X-ray photons absorbed by the X-ray absorption layer, and a controller.

TECHNICAL FIELD

The disclosure herein relates to X-ray detectors, particularly relates to semiconductor X-ray detectors.

BACKGROUND

X-ray detectors may be devices used to measure the flux, spatial distribution, spectrum or other properties of X-rays.

X-ray detectors may be used for many applications. One important application is imaging. X-ray imaging is a radiography technique and can be used to reveal the internal structure of a non-uniformly composed and opaque object such as the human body.

Early X-ray detectors for imaging include photographic plates and photographic films. A photographic plate may be a glass plate with a coating of light-sensitive emulsion. Although photographic plates were replaced by photographic films, they may still be used in special situations due to the superior quality they offer and their extreme stability. A photographic film may be a plastic film (e.g., a strip or sheet) with a coating of light-sensitive emulsion.

In the 1980s, photostimulable phosphor plates (PSP plates) became available. A PSP plate may contain a phosphor material with color centers in its lattice. When the PSP plate is exposed to X-ray, electrons excited by X-ray are trapped in the color centers until they are stimulated by a laser beam scanning over the plate surface. As the plate is scanned by laser, trapped excited electrons give off light, which is collected by a photomultiplier tube. The collected light is converted into a digital image. In contrast to photographic plates and photographic films, PSP plates can be reused.

Another kind of X-ray detectors are X-ray image intensifiers. Components of an X-ray image intensifier are usually sealed in a vacuum. In contrast to photographic plates, photographic films, and PSP plates, X-ray image intensifiers may produce real-time images, i.e., do not require post-exposure processing to produce images. X-ray first hits an input phosphor (e.g., cesium iodide) and is converted to visible light. The visible light then hits a photocathode (e.g., a thin metal layer containing cesium and antimony compounds) and causes emission of electrons. The number of emitted electrons is proportional to the intensity of the incident X-ray. The emitted electrons are projected, through electron optics, onto an output phosphor and cause the output phosphor to produce a visible-light image.

Scintillators operate somewhat similarly to X-ray image intensifiers in that scintillators (e.g., sodium iodide) absorb X-ray and emit visible light, which can then be detected by a suitable image sensor for visible light. In scintillators, the visible light spreads and scatters in all directions and thus reduces spatial resolution. Reducing the scintillator thickness helps to improve the spatial resolution but also reduces absorption of X-ray. A scintillator thus has to strike a compromise between absorption efficiency and resolution.

Semiconductor X-ray detectors largely overcome this problem by direct conversion of X-ray into electric signals. A semiconductor X-ray detector may include a semiconductor layer that absorbs X-ray in wavelengths of interest. When an X-ray photon is absorbed in the semiconductor layer, multiple charge carriers (e.g., electrons and holes) are generated and swept under an electric field towards electrical contacts on the semiconductor layer. Cumbersome heat management required in currently available semiconductor X-ray detectors (e.g., Medipix) can make a detector with a large area and a large number of pixels difficult or impossible to produce.

SUMMARY

Disclosed herein is an apparatus suitable for detecting x-ray, comprising: an X-ray absorption layer comprising an electrode; a first voltage comparator configured to compare a voltage of the electrode to a first threshold; a second voltage comparator configured to compare the voltage to a second threshold; a counter configured to register a number of X-ray photons absorbed by the X-ray absorption layer; a controller; wherein the controller is configured to start a time delay from a time at which the first voltage comparator determines that an absolute value of the voltage equals or exceeds an absolute value of the first threshold; wherein the controller is configured to activate the second voltage comparator during (including the beginning and the expiration) the time delay; wherein the controller is configured to cause the number registered by the counter to increase by one, if the second voltage comparator determines that an absolute value of the voltage equals or exceeds an absolute value of the second threshold. The first voltage comparator and the second voltage comparator may be the same component. When a voltage comparator determines whether an absolute value of a voltage equals or exceeds an absolute value of a threshold, the voltage comparator does not necessarily compare the absolute values. Instead, when the voltage and the threshold are both negative, the voltage comparator may compare the actual values of the voltage and the threshold; when the voltage is equally or more negative than the threshold, the absolute value of voltage equals or exceeds the absolute value of the threshold.

According to an embodiment, the apparatus further comprises a capacitor module electrically connected to the electrode, wherein the capacitor module is configured to collect charge carriers from the electrode.

According to an embodiment, the controller is configured to activate the second voltage comparator at a beginning or expiration of the time delay. According to an embodiment, the controller is configured to deactivate the first voltage comparator at the beginning of, or during the time delay. According to an embodiment, the controller is configured to deactivate the second voltage comparator at the expiration of the time delay or at the time when the second voltage comparator determines that the absolute value of the voltage equals or exceeds the absolute value of the second threshold.

According to an embodiment, the apparatus further comprises a voltmeter and the controller is configured to cause the voltmeter to measure the voltage upon expiration of the time delay.

According to an embodiment, the controller is configured to determine an X-ray photon energy based on a value of the voltage measured upon expiration of the time delay.

According to an embodiment, the controller is configured to connect the electrode to an electrical ground. The electrical ground may be a virtual ground. A virtual ground (also known as a “virtual earth”) is a node of a circuit that is maintained at a steady reference potential, without being connected directly to the reference potential.

According to an embodiment, a rate of change of the voltage is substantially zero at expiration of the time delay.

According to an embodiment, a rate of change of the voltage is substantially non-zero at expiration of the time delay.

According to an embodiment, the X-ray absorption layer comprises a diode.

According to an embodiment, the X-ray absorption layer comprises silicon, germanium, GaAs, CdTe, CdZnTe, or a combination thereof.

According to an embodiment, the apparatus does not comprise a scintillator.

According to an embodiment, the apparatus comprises an array of pixels.

Disclosed herein is a system comprising the apparatus described above and an X-ray source, wherein the system is configured to perform X-ray radiography on human chest or abdomen.

According to an embodiment, the system comprises the apparatus described 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 the apparatus described 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 the apparatus described 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 the apparatus described above and an X-ray source.

Disclosed herein is an X-ray computed tomography (X-ray CT) system comprising the apparatus described above and an X-ray source.

Disclosed herein is an electron microscope comprising the apparatus described above, an electron source and an electronic optical system.

Disclosed herein is a system comprising the apparatus described 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: starting a time delay from a time at which an absolute value of a voltage of an electrode of an X-ray absorption layer equals or exceeds an absolute value of a first threshold; activating a second circuit during (including the beginning and expiration of) the time delay; if an absolute value of the voltage equals or exceeds an absolute value of a second threshold, increasing a count of X-ray photon incident on the X-ray absorption layer by one.

According to an embodiment, the method further comprises connecting the electrode to an electrical ground.

According to an embodiment, the method further comprises measuring the voltage upon expiration of the time delay.

According to an embodiment, the method further comprises determining an X-ray photon energy based on a value of the voltage at expiration of the time delay.

According to an embodiment, a rate of change of the voltage is substantially zero at expiration of the time delay.

According to an embodiment, a rate of change of the voltage is substantially non-zero at expiration of the time delay.

According to an embodiment, activating the second circuit is at a beginning or expiration of the time delay.

According to an embodiment, the second circuit is configured to compare the absolute value of the voltage to the absolute value of the second threshold.

According to an embodiment, the method further comprises deactivating a first circuit at a beginning the time delay.

According to an embodiment, the first circuit is configured to compare the absolute value of the voltage to the absolute value of the first threshold. The first circuit and the second circuit may be the same circuit.

Disclosed herein is a system suitable for phase-contrast X-ray imaging (PCI), the system comprising: the apparatus described above, a second X-ray detector, a spacer, wherein the apparatus and the second X-ray detector are spaced apart by the spacer.

According to an embodiment, the apparatus and the second X-ray detector are configured to respectively capture an image of an object simultaneously.

According to an embodiment, the second X-ray detector is identical to the apparatus.

Disclosed herein is a system suitable for phase-contrast X-ray imaging (PCI), the system comprising: the apparatus described above, wherein the apparatus is configured to move to and capture images of an object exposed to incident X-ray at different distances from the object.

DETAILED DESCRIPTION

FIG. 1Aschematically shows a semiconductor X-ray detector100, according to an embodiment. The semiconductor X-ray detector100may include an X-ray absorption layer110and an electronics layer120(e.g., an ASIC) for processing or analyzing electrical signals incident X-ray generates in the X-ray absorption layer110. In an embodiment, the semiconductor X-ray detector100does not comprise a scintillator. The X-ray absorption layer110may 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 X-ray energy of interest. The X-ray absorption layer110may include one or more diodes (e.g., p-i-n or p-n) formed by a first doped region111, one or more discrete regions114of a second doped region113. The second doped region113may be separated from the first doped region111by an optional the intrinsic region112. The discrete portions114are separated from one another by the first doped region111or the intrinsic region112. The first doped region111and the second doped region113have opposite types of doping (e.g., region111is p-type and region113is n-type, or region111is n-type and region113is p-type). In the example inFIG. 1A, each of the discrete regions114of the second doped region113forms a diode with the first doped region111and the optional intrinsic region112. Namely, in the example inFIG. 1A, the X-ray absorption layer110has a plurality of diodes having the first doped region111as a shared electrode. The first doped region111may also have discrete portions.

FIG. 1Bshows a semiconductor X-ray detector100, according to an embodiment. The semiconductor X-ray detector100may include an X-ray absorption layer110and an electronics layer120(e.g., an ASIC) for processing or analyzing electrical signals incident X-ray generates in the X-ray absorption layer110. In an embodiment, the semiconductor X-ray detector100does not comprise a scintillator. The X-ray absorption layer110may 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 X-ray energy of interest. The X-ray absorption layer110may not include a diode but includes a resistor.

When an X-ray photon hits the X-ray absorption layer110including diodes, it may be absorbed and generate one or more charge carriers by a number of mechanisms. An X-ray photon may generate 10 to 100000 charge carriers. 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 contact119B may include discrete portions each of which is in electrical contact with the discrete regions114. In an embodiment, the charge carriers may drift in directions such that the charge carriers generated by a single X-ray photon are not substantially shared by two different discrete regions114(“not substantially shared” here means less than 5%, less than 2% or less than 1% of these charge carriers flow to a different one of the discrete regions114than the rest of the charge carriers). In an embodiment, the charge carriers generated by a single X-ray photon can be shared by two different discrete regions114.FIG. 2shows an exemplary top view of a portion of the device100with a 4-by-4 array of discrete regions114. Charge carriers generated by an X-ray photon incident around the footprint of one of these discrete regions114are not substantially shared with another of these discrete regions114. The area around a discrete region114in which substantially all (more than 95%, more than 98% or more than 99% of) charge carriers generated by an X-ray photon incident therein flow to the discrete region114is called a pixel associated with that discrete region114. Namely, less than 5%, less than 2% or less than 1% of these charge carriers flow beyond the pixel. By measuring the drift current flowing into each of the discrete regions114, or the rate of change of the voltage of each of the discrete regions114, the number of X-ray photons absorbed (which relates to the incident X-ray intensity) and/or the energies thereof in the pixels associated with the discrete regions114may be determined. Thus, the spatial distribution (e.g., an image) of incident X-ray intensity may be determined by individually measuring the drift current into each one of an array of discrete regions114or measuring the rate of change of the voltage of each one of an array of discrete regions114. The pixels may be organized in any suitable array, such as, a square array, a triangular array and a honeycomb array. The pixels may have any suitable shape, such as, circular, triangular, square, rectangular, and hexangular. The pixels may be individually addressable.

When an X-ray photon hits the X-ray absorption layer110including a resistor but not diodes, it may be absorbed and generate one or more charge carriers by a number of mechanisms. An X-ray photon may generate 10 to 100000 charge carriers. The charge carriers may drift to the electrical contacts119A and119B under an electric field. The field may be an external electric field. The electrical contact119B includes discrete portions. In an embodiment, the charge carriers may drift in directions such that the charge carriers generated by a single X-ray photon are not substantially shared by two different discrete portions of the electrical contact119B (“not substantially shared” here means less than 5%, less than 2% or less than 1% of these charge carriers flow to a different one of the discrete portions than the rest of the charge carriers). In an embodiment, the charge carriers generated by a single X-ray photon can be shared by two different discrete portions of the electrical contact119B. Charge carriers generated by an X-ray photon incident around the footprint of one of these discrete portions of the electrical contact119B are not substantially shared with another of these discrete portions of the electrical contact119B. The area around a discrete portion of the electrical contact119B in which substantially all (more than 95%, more than 98% or more than 99% of) charge carriers generated by an X-ray photon incident therein flow to the discrete portion of the electrical contact119B is called a pixel associated with the discrete portion of the electrical contact119B. Namely, less than 5%, less than 2% or less than 1% of these charge carriers flow beyond the pixel associated with the one discrete portion of the electrical contact119B. By measuring the drift current flowing into each of the discrete portion of the electrical contact119B, or the rate of change of the voltage of each of the discrete portions of the electrical contact119B, the number of X-ray photons absorbed (which relates to the incident X-ray intensity) and/or the energies thereof in the pixels associated with the discrete portions of the electrical contact119B may be determined. Thus, the spatial distribution (e.g., an image) of incident X-ray intensity may be determined by individually measuring the drift current into each one of an array of discrete portions of the electrical contact119B or measuring the rate of change of the voltage of each one of an array of discrete portions of the electrical contact119B. The pixels may be organized in any suitable array, such as, a square array, a triangular array and a honeycomb array. The pixels may have any suitable shape, such as, circular, triangular, square, rectangular, and hexangular. The pixels may be individually addressable.

The electronics layer120may include an electronic system121suitable for processing or interpreting signals generated by X-ray photons incident on the X-ray absorption layer110. The electronic system121may include an analog circuitry such as a filter network, amplifiers, integrators, and comparators, or a digital circuitry such as a microprocessors, and memory. The electronic system121may include components shared by the pixels or components dedicated to a single pixel. For example, the electronic system121may include an amplifier dedicated to each pixel and a microprocessor shared among all the pixels. The electronic system121may be electrically connected to the pixels by vias131. Space among the vias may be filled with a filler material130, which may increase the mechanical stability of the connection of the electronics layer120to the X-ray absorption layer110. Other bonding techniques are possible to connect the electronic system121to the pixels without using vias.

FIG. 3AandFIG. 3Beach show a component diagram of the electronic system121, according to an embodiment. The electronic system121may include a first voltage comparator301, a second voltage comparator302, a counter320, a switch305, a voltmeter306and a controller310.

The first voltage comparator301is configured to compare the voltage of an electrode of a diode300to a first threshold. The diode may be a diode formed by the first doped region111, one of the discrete regions114of the second doped region113, and the optional intrinsic region112. Alternatively, the first voltage comparator301is configured to compare the voltage of an electrical contact (e.g., a discrete portion of electrical contact119B) to a first threshold. The first voltage comparator301may be configured to monitor the voltage directly, or calculate the voltage by integrating an electric current flowing through the diode or electrical contact over a period of time. The first voltage comparator301may be controllably activated or deactivated by the controller310. The first voltage comparator301may be a continuous comparator. Namely, the first voltage comparator301may be configured to be activated continuously, and monitor the voltage continuously. The first voltage comparator301configured as a continuous comparator reduces the chance that the system121misses signals generated by an incident X-ray photon. The first voltage comparator301configured as a continuous comparator is especially suitable when the incident X-ray intensity is relatively high. The first voltage comparator301may be a clocked comparator, which has the benefit of lower power consumption. The first voltage comparator301configured as a clocked comparator may cause the system121to miss signals generated by some incident X-ray photons. When the incident X-ray intensity is low, the chance of missing an incident X-ray photon is low because the time interval between two successive photons is relatively long. Therefore, the first voltage comparator301configured as a clocked comparator is especially suitable when the incident X-ray intensity is relatively low. The first threshold may be 5-10%, 10%-20%, 20-30%, 30-40% or 40-50% of the maximum voltage one incident X-ray photon may generate in the diode or the resistor. The maximum voltage may depend on the energy of the incident X-ray photon (i.e., the wavelength of the incident X-ray), the material of the X-ray absorption layer110, and other factors. For example, the first threshold may be 50 mV, 100 mV, 150 mV, or 200 mV.

The second voltage comparator302is configured to compare the voltage to a second threshold. The second voltage comparator302may be configured to monitor the voltage directly, or calculate the voltage by integrating an electric current flowing through the diode or the electrical contact over a period of time. The second voltage comparator302may be a continuous comparator. The second voltage comparator302may be controllably activate or deactivated by the controller310. When the second voltage comparator302is deactivated, the power consumption of the second voltage comparator302may be less than 1%, less than 5%, less than 10% or less than 20% of the power consumption when the second voltage comparator302is activated. The absolute value of the second threshold is greater than the absolute value of the first threshold. As used herein, the term “absolute value” or “modulus” |x| of a real number x is the non-negative value of x without regard to its sign. Namely,

x={x,if⁢⁢x≥0-x,if⁢⁢x≤0.
The second threshold may be 200%-300% of the first threshold. The second threshold may be at least 50% of the maximum voltage one incident X-ray photon may generate in the diode or resistor. For example, the second threshold may be 100 mV, 150 mV, 200 mV, 250 mV or 300 mV. The second voltage comparator302and the first voltage comparator310may be the same component. Namely, the system121may have one voltage comparator that can compare a voltage with two different thresholds at different times.

The first voltage comparator301or the second voltage comparator302may include one or more op-amps or any other suitable circuitry. The first voltage comparator301or the second voltage comparator302may have a high speed to allow the system121to operate under a high flux of incident X-ray. However, having a high speed is often at the cost of power consumption.

The counter320is configured to register a number of X-ray photons reaching the diode or resistor. The counter320may 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 controller310may be a hardware component such as a microcontroller and a microprocessor. The controller310is configured to start a time delay from a time at which the first voltage comparator301determines that the absolute value of the voltage equals or exceeds the absolute value of the first threshold (e.g., the absolute value of the voltage increases from below the absolute value of the first threshold to a value equal to or above the absolute value of the first threshold). The absolute value is used here because the voltage may be negative or positive, depending on whether the voltage of the cathode or the anode of the diode or which electrical contact is used. The controller310may be configured to keep deactivated the second voltage comparator302, the counter320and any other circuits the operation of the first voltage comparator301does not require, before the time at which the first voltage comparator301determines that the absolute value of the voltage equals or exceeds the absolute value of the first threshold. The time delay may expire before or after the voltage becomes stable, i.e., the rate of change of the voltage is substantially zero. The phase “the rate of change of the voltage is substantially zero” means that temporal change of the voltage is less than 0.1%/ns. The phase “the rate of change of the voltage is substantially non-zero” means that temporal change of the voltage is at least 0.1%/ns.

The controller310may be configured to activate the second voltage comparator during (including the beginning and the expiration) the time delay. In an embodiment, the controller310is configured to activate the second voltage comparator at the beginning of the time delay. 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 controller310itself may be deactivated until the output of the first voltage comparator301activates the controller310when the absolute value of the voltage equals or exceeds the absolute value of the first threshold.

The controller310may be configured to cause the number registered by the counter320to increase by one, if, during the time delay, the second voltage comparator302determines that the absolute value of the voltage equals or exceeds the absolute value of the second threshold.

The controller310may be configured to cause the voltmeter306to measure the voltage upon expiration of the time delay. The controller310may be configured to connect the electrode to an electrical ground, so as to reset the voltage and discharge any charge carriers accumulated on the electrode. In an embodiment, the electrode is connected to an electrical ground after the expiration of the time delay. In an embodiment, the electrode is connected to an electrical ground for a finite reset time period. The controller310may connect the electrode to the electrical ground by controlling the switch305. The switch may be a transistor such as a field-effect transistor (FET).

In an embodiment, the system121has no analog filter network (e.g., a RC network). In an embodiment, the system121has no analog circuitry.

The voltmeter306may feed the voltage it measures to the controller310as an analog or digital signal.

The system121may include a capacitor module309electrically connected to the electrode of the diode300or the electrical contact, wherein the capacitor module is configured to collect charge carriers from the electrode. The capacitor module can include a capacitor in the feedback path of an amplifier. The amplifier configured as such is called a capacitive transimpedance amplifier (CTIA). CTIA has high dynamic range by keeping the amplifier from saturating and improves the signal-to-noise ratio by limiting the bandwidth in the signal path. Charge carriers from the electrode accumulate on the capacitor over a period of time (“integration period”) (e.g., as shown inFIG. 4, between t0to t1, or t1-t2). After the integration period has expired, the capacitor voltage is sampled and then reset by a reset switch. The capacitor module can include a capacitor directly connected to the electrode.

FIG. 4schematically shows a temporal change of the electric current flowing through the electrode (upper curve) caused by charge carriers generated by an X-ray photon incident on the diode or the resistor, and a corresponding temporal change of the voltage of the electrode (lower curve). The voltage may be an integral of the electric current with respect to time. At time t0, the X-ray photon hits the diode or the resistor, charge carriers start being generated in the diode or the resistor, electric current starts to flow through the electrode of the diode or the resistor, and the absolute value of the voltage of the electrode or electrical contact starts to increase. At time t1, the first voltage comparator301determines that the absolute value of the voltage equals or exceeds the absolute value of the first threshold V1, and the controller310starts the time delay TD1and the controller310may deactivate the first voltage comparator301at the beginning of TD1. If the controller310is deactivated before t1, the controller310is activated at t1. During TD1, the controller310activates the second voltage comparator302. The term “during” a time delay as used here means the beginning and the expiration (i.e., the end) and any time in between. For example, the controller310may activate the second voltage comparator302at the expiration of TD1. If during TD1, the second voltage comparator302determines that the absolute value of the voltage equals or exceeds the absolute value of the second threshold at time t2, the controller310causes the number registered by the counter320to increase by one. At time te, all charge carriers generated by the X-ray photon drift out of the X-ray absorption layer110. At time ts, the time delay TD1expires. In the example ofFIG. 4, time tsis after time te; namely TD1expires after all charge carriers generated by the X-ray photon drift out of the X-ray absorption layer110. The rate of change of the voltage is thus substantially zero at ts. The controller310may be configured to deactivate the second voltage comparator302at expiration of TD1or at t2, or any time in between.

The controller310may be configured to cause the voltmeter306to measure the voltage upon expiration of the time delay TD1. In an embodiment, the controller310causes the voltmeter306to measure the voltage after the rate of change of the voltage becomes substantially zero after the expiration of the time delay TD1. The voltage at this moment is proportional to the amount of charge carriers generated by an X-ray photon, which relates to the energy of the X-ray photon. The controller310may be configured to determine the energy of the X-ray photon based on voltage the voltmeter306measures. One way to determine the energy is by binning the voltage. The counter320may have a sub-counter for each bin. When the controller310determines that the energy of the X-ray photon falls in a bin, the controller310may cause the number registered in the sub-counter for that bin to increase by one. Therefore, the system121may be able to detect an X-ray image and may be able to resolve X-ray photon energies of each X-ray photon.

After TD1expires, the controller310connects the electrode to an electric ground for a reset period RST to allow charge carriers accumulated on the electrode to flow to the ground and reset the voltage. After RST, the system121is ready to detect another incident X-ray photon. Implicitly, the rate of incident X-ray photons the system121can handle in the example ofFIG. 4is limited by 1/(TD1+RST). If the first voltage comparator301has been deactivated, the controller310can activate it at any time before RST expires. If the controller310has been deactivated, it may be activated before RST expires.

FIG. 5schematically shows a temporal change of the electric current flowing through the electrode (upper curve) caused by noise (e.g., dark current, background radiation, scattered X-rays, fluorescent X-rays, shared charges from adjacent pixels), and a corresponding temporal change of the voltage of the electrode (lower curve), in the system121operating in the way shown inFIG. 4. At time t0, the noise begins. If the noise is not large enough to cause the absolute value of the voltage to exceed the absolute value of V1, the controller310does not activate the second voltage comparator302. If the noise is large enough to cause the absolute value of the voltage to exceed the absolute value of V1at time t1as determined by the first voltage comparator301, the controller310starts the time delay TD1and the controller310may deactivate the first voltage comparator301at the beginning of TD1. During TD1(e.g., at expiration of TD1), the controller310activates the second voltage comparator302. The noise is very unlikely large enough to cause the absolute value of the voltage to exceed the absolute value of V2during TD1. Therefore, the controller310does not cause the number registered by the counter320to increase. At time te, the noise ends. At time ts, the time delay TD1expires. The controller310may be configured to deactivate the second voltage comparator302at expiration of TD1. The controller310may be configured not to cause the voltmeter306to measure the voltage if the absolute value of the voltage does not exceed the absolute value of V2during TD1. After TD1expires, the controller310connects the electrode to an electric ground for a reset period RST to allow charge carriers accumulated on the electrode as a result of the noise to flow to the ground and reset the voltage. Therefore, the system121may be very effective in noise rejection.

FIG. 6schematically shows a temporal change of the electric current flowing through the electrode (upper curve) caused by charge carriers generated by an X-ray photon incident on the diode or the resistor, and a corresponding temporal change of the voltage of the electrode (lower curve), when the system121operates to detect incident X-ray photons at a rate higher than 1/(TD1+RST). The voltage may be an integral of the electric current with respect to time. At time t0, the X-ray photon hits the diode or the resistor, charge carriers start being generated in the diode or the resistor, electric current starts to flow through the electrode of the diode or the electrical contact of resistor, and the absolute value of the voltage of the electrode or the electrical contact starts to increase. At time t1, the first voltage comparator301determines that the absolute value of the voltage equals or exceeds the absolute value of the first threshold V1, and the controller310starts a time delay TD2shorter than TD1, and the controller310may deactivate the first voltage comparator301at the beginning of TD2. If the controller310is deactivated before t1, the controller310is activated at t1. During TD2(e.g., at expiration of TD2), the controller310activates the second voltage comparator302. If during TD2, the second voltage comparator302determines that the absolute value of the voltage equals or exceeds the absolute value of the second threshold at time t2, the controller310causes the number registered by the counter320to increase by one. At time te, all charge carriers generated by the X-ray photon drift out of the X-ray absorption layer110. At time th, the time delay TD2expires. In the example ofFIG. 6, time this before time te; namely TD2expires before all charge carriers generated by the X-ray photon drift out of the X-ray absorption layer110. The rate of change of the voltage is thus substantially non-zero at th. The controller310may be configured to deactivate the second voltage comparator302at expiration of TD2or at t2, or any time in between.

The controller310may be configured to extrapolate the voltage at tefrom the voltage as a function of time during TD2and use the extrapolated voltage to determine the energy of the X-ray photon.

After TD2expires, the controller310connects the electrode to an electric ground for a reset period RST to allow charge carriers accumulated on the electrode to flow to the ground and reset the voltage. In an embodiment, RST expires before te. The rate of change of the voltage after RST may be substantially non-zero because all charge carriers generated by the X-ray photon have not drifted out of the X-ray absorption layer110upon expiration of RST before te. The rate of change of the voltage becomes substantially zero after teand the voltage stabilized to a residue voltage VR after te. In an embodiment, RST expires at or after te, and the rate of change of the voltage after RST may be substantially zero because all charge carriers generated by the X-ray photon drift out of the X-ray absorption layer110at te. After RST, the system121is ready to detect another incident X-ray photon. If the first voltage comparator301has been deactivated, the controller310can activate it at any time before RST expires. If the controller310has been deactivated, it may be activated before RST expires.

FIG. 7schematically shows a temporal change of the electric current flowing through the electrode (upper curve) caused by noise (e.g., dark current, background radiation, scattered X-rays, fluorescent X-rays, shared charges from adjacent pixels), and a corresponding temporal change of the voltage of the electrode (lower curve), in the system121operating in the way shown inFIG. 6. At time t0, the noise begins. If the noise is not large enough to cause the absolute value of the voltage to exceed the absolute value of V1, the controller310does not activate the second voltage comparator302. If the noise is large enough to cause the absolute value of the voltage to exceed the absolute value of V1at time t1as determined by the first voltage comparator301, the controller310starts the time delay TD2and the controller310may deactivate the first voltage comparator301at the beginning of TD2. During TD2(e.g., at expiration of TD2), the controller310activates the second voltage comparator302. The noise is very unlikely large enough to cause the absolute value of the voltage to exceed the absolute value of V2during TD2. Therefore, the controller310does not cause the number registered by the counter320to increase. At time te, the noise ends. At time th, the time delay TD2expires. The controller310may be configured to deactivate the second voltage comparator302at expiration of TD2. After TD2expires, the controller310connects the electrode to an electric ground for a reset period RST to allow charge carriers accumulated on the electrode as a result of the noise to flow to the ground and reset the voltage. Therefore, the system121may be very effective in noise rejection.

FIG. 8schematically shows a temporal change of the electric current flowing through the electrode (upper curve) caused by charge carriers generated by a series of X-ray photons incident on the diode or the resistor, and a corresponding temporal change of the voltage of the electrode (lower curve), in the system121operating in the way shown inFIG. 6with RST expires before te. The voltage curve caused by charge carriers generated by each incident X-ray photon is offset by the residue voltage before that photon. The absolute value of the residue voltage successively increases with each incident photon. When the absolute value of the residue voltage exceeds V1(see the dotted rectangle inFIG. 8), the controller starts the time delay TD2and the controller310may deactivate the first voltage comparator301at the beginning of TD2. If no other X-ray photon incidence on the diode or the resistor during TD2, the controller connects the electrode to the electrical ground during the reset time period RST at the end of TD2, thereby resetting the residue voltage. The residue voltage thus does not cause an increase of the number registered by the counter320.

FIG. 9Ashows a flow chart for a method suitable for detecting X-ray using a system such as the system121operating as shown inFIG. 4. In step901, compare, e.g., using the first voltage comparator301, a voltage of an electrode of a diode or an electrical contact of a resistor exposed to X-ray, to the first threshold. In step902, determine, e.g., with the controller310, whether the absolute value of the voltage equals or exceeds the absolute value of the first threshold V1. If the absolute value of the voltage does not equal or exceed the absolute value of the first threshold, the method goes back to step901. If the absolute value of the voltage equals or exceeds the absolute value of the first threshold, continue to step903. In step903, start, e.g., using the controller310, the time delay TD1. In step904, activate, e.g., using the controller310, a circuit (e.g., the second voltage comparator302or the counter320) during the time delay TD1(e.g., at the expiration of TD1). In step905, compare, e.g., using the second voltage comparator302, the voltage to the second threshold. In step906, determine, e.g., using the controller310, whether the absolute value of the voltage equals or exceeds the absolute value of the second threshold V2. If the absolute value of the voltage does not equal or exceed the absolute value of the second threshold, the method goes to step910. If the absolute value of the voltage equals or exceeds the absolute value of the second threshold, continue to step907. In step907, cause, e.g., using the controller310, the number registered in the counter320to increase by one. In optional step908, measure, e.g., using the voltmeter306, the voltage upon expiration of the time delay TD1. In optional step909, determine, e.g., using the controller310, the X-ray photon energy based the voltage measured in step908. There may be a counter for each of the energy bins. After measuring the X-ray photon energy, the counter for the bin to which the photon energy belongs can be increased by one. The method goes to step910after step909. In step910, reset the voltage to an electrical ground, e.g., by connecting the electrode of the diode or an electrical contact of a resistor to an electrical ground. Steps908and909may be omitted, for example, when neighboring pixels share a large portion (e.g., >30%) of charge carriers generated from a single photon.

FIG. 9Bshows a flow chart for a method suitable for detecting X-ray using the system such as the system121operating as shown inFIG. 6. In step1001, compare, e.g., using the first voltage comparator301, a voltage of an electrode of a diode or an electrical contact of a resistor exposed to X-ray, to the first threshold. In step1002, determine, e.g., with the controller310, whether the absolute value of the voltage equals or exceeds the absolute value of the first threshold V1. If the absolute value of the voltage does not equal or exceed the absolute value of the first threshold, the method goes back to step1001. If the absolute value of the voltage equals or exceeds the absolute value of the first threshold, continue to step1003. In step1003, start, e.g., using the controller310, the time delay TD2. In step1004, activate, e.g., using the controller310, a circuit (e.g., the second voltage comparator302or the counter320) during the time delay TD2(e.g., at the expiration of TD2). In step1005, compare, e.g., using the second voltage comparator302, the voltage to the second threshold. In step1006, determine, e.g., using the controller310, whether the absolute value of the voltage equals or exceeds the absolute value of the second threshold V2. If the absolute value of the voltage does not equal or exceed the absolute value of the second threshold, the method goes to step1010. If the absolute value of the voltage equals or exceeds the absolute value of the second threshold, continue to step1007. In step1007, cause, e.g., using the controller310, the number registered in the counter320to increase by one. The method goes to step1010after step1007. In step1010, reset the voltage to an electrical ground, e.g., by connecting the electrode of the diode or an electrical contact of a resistor to an electrical ground.

The semiconductor X-ray detector100may be used for phase-contrast X-ray imaging (PCI) (also known as phase-sensitive X-ray imaging). PCI encompasses techniques that form an image of an object at least partially using the phase shift (including the spatial distribution of the phase shift) of an X-ray beam caused by that object. One way to obtain the phase shift is transforming the phase into variations in intensity.

PCI can be combined with tomographic techniques to obtain the 3D-distribution of the real part of the refractive index of the object. PCI is more sensitive to density variations in the object than conventional intensity-based X-ray imaging (e.g., radiography). PCI is especially useful for imaging soft tissues.

According to an embodiment,FIG. 10schematically shows a system1900suitable for PCI. The system1900may include at least two X-ray detectors1910and1920. One or both of the two X-ray detectors1910is the semiconductor X-ray detector100described herein. The X-ray detectors1910and1920may be spaced apart by a spacer1930. The spacer1930may have very little absorption of the X-ray. For example, the spacer1930may have a very small mass attenuation coefficient (e.g., <10 cm2g−1, <1 cm2g−1, <0.1 cm2g−1, or <0.01 cm2g−1). The mass attenuation coefficient of the spacer1930may be uniform (e.g., variation between every two points in the spacer1930less than 5%, less than 1% or less than 0.1%). The spacer1930may cause the same amount of changes to the phase of X-ray passing through the spacer1930. For example, the spacer1930may be a gas (e.g., air), a vacuum chamber, may comprise aluminum, beryllium, silicon, or a combination thereof.

The system1900can be used to obtain the phase shift of incident X-ray1950caused by an object1960being imaged. The X-ray detectors1910and1920can capture two images (i.e., intensity distributions) simultaneously. Because of the X-ray detectors1910and1920are separated by the spacer1930, the two images are different distances from the object1960. The phase may be determined from the two images, for example, using algorithms based on the linearization of the Fresnel diffraction integral.

According to an embodiment,FIG. 11schematically shows a system1800suitable for PCI. The system1800comprises the semiconductor X-ray detector100described herein. The semiconductor X-ray detector100is configured to move to and capture images of an object1860exposed to incident X-ray1850at different distances from the object1860. The images may not necessarily be captured simultaneously. The phase may be determined from the images, for example, using algorithms based on the linearization of the Fresnel diffraction integral.

FIG. 12schematically shows a system comprising the semiconductor X-ray detector100described 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 source1201. X-ray emitted from the X-ray source1201penetrates an object1202(e.g., a human body part such as chest, limb, abdomen), is attenuated by different degrees by the internal structures of the object1202(e.g., bones, muscle, fat and organs, etc.), and is projected to the semiconductor X-ray detector100. The semiconductor X-ray detector100forms an image by detecting the intensity distribution of the X-ray.

FIG. 13schematically shows a system comprising the semiconductor X-ray detector100described herein. The system may be used for medical imaging such as dental X-ray radiography. The system comprises an X-ray source1301. X-ray emitted from the X-ray source1301penetrates an object1302that is part of a mammal (e.g., human) mouth. The object1302may 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 object1302and is projected to the semiconductor X-ray detector100. The semiconductor X-ray detector100forms 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. 14schematically shows a cargo scanning or non-intrusive inspection (NII) system comprising the semiconductor X-ray detector100described 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 source1401. X-ray emitted from the X-ray source1401may backscatter from an object1402(e.g., shipping containers, vehicles, ships, etc.) and be projected to the semiconductor X-ray detector100. Different internal structures of the object1402may backscatter X-ray differently. The semiconductor X-ray detector100forms an image by detecting the intensity distribution of the backscattered X-ray and/or energies of the backscattered X-ray photons.

FIG. 15schematically shows another cargo scanning or non-intrusive inspection (NII) system comprising the semiconductor X-ray detector100described herein. The system may be used for luggage screening at public transportation stations and airports. The system comprises an X-ray source1501. X-ray emitted from the X-ray source1501may penetrate a piece of luggage1502, be differently attenuated by the contents of the luggage, and projected to the semiconductor X-ray detector100. The semiconductor X-ray detector100forms 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. 16schematically shows a full-body scanner system comprising the semiconductor X-ray detector100described herein. The full-body scanner system may detect objects on a person'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 source1601. X-ray emitted from the X-ray source1601may backscatter from a human1602being screened and objects thereon, and be projected to the semiconductor X-ray detector100. The objects and the human body may backscatter X-ray differently. The semiconductor X-ray detector100forms an image by detecting the intensity distribution of the backscattered X-ray. The semiconductor X-ray detector100and the X-ray source1601may be configured to scan the human in a linear or rotational direction.

FIG. 17schematically 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 semiconductor X-ray detector100described herein and an X-ray source1701. The semiconductor X-ray detector100and the X-ray source1701may be configured to rotate synchronously along one or more circular or spiral paths.

FIG. 18schematically shows an electron microscope. The electron microscope comprises an electron source1801(also called an electron gun) that is configured to emit electrons. The electron source1801may have various emission mechanisms such as thermionic, photocathode, cold emission, or plasmas source. The emitted electrons pass through an electronic optical system1803, which may be configured to shape, accelerate, or focus the electrons. The electrons then reach a sample1802and an image detector may form an image therefrom. The electron microscope may comprise the semiconductor X-ray detector100described 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 semiconductor X-ray detector100.

The semiconductor X-ray detector100described 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 semiconductor X-ray detector100in place of a photographic plate, a photographic film, a PSP plate, an X-ray image intensifier, a scintillator, or another semiconductor X-ray detector.