Photon counting detector and photon counting method

Abstract

The present invention relates to a photon counting detector, the photon counting detector comprising: a scintillator (10, 20) configured to convert incident gamma radiation into optical photons; a pixelated photodetector (11, 22) configured to detect a flux of optical photons, where the pixelated photodetector (22) is a silicon photomultiplier, SiPM, detector, where each photodetector pixel comprises an array of silicon avalanche photodiodes, SPAD; and a circuit (80, 90) configured to heat the SPAD by: if a dark count rate of the SPAD exceeds a dark count rate threshold, applying an elevated reverse bias voltage to the SPAD to force the SPAD to break down to cause current to flow through the SPAD for the time of a heating period, thereby locally increasing the temperature of the SPAD; and controlling the length of the heating period.

Description

Technical Field

[0001] The present invention relates to a photon counting detector, a corresponding photon counting method and an imaging device comprising a photon counting detector for detecting radiation, in particular gamma radiation and X-ray radiation. Background Art

[0002] Conventionally, photon counting is performed using a photomultiplier tube (PMT) detector, which includes a photosensitive photocathode and a set of "multiplier" anode terminals. The photocathode emits at least one electron in response to the impact of a photon, and (the one or more) electrons then strike successive anode terminals, where each such event causes a cascade emission of electrons, thereby producing a multiplication effect that causes a measurable electrical pulse. The PMT is capable of high-speed photon counting. The PMT detector has many disadvantages, including that it is a relatively bulky discrete device with a large optical window that operates at high voltages and is prone to failure due to the vacuum tube design and high operating voltages. For example, this type of radiation detector is disclosed in US8723132B2.

[0003] Silicon photomultiplier (SiPM) devices have been developed to overcome some of these shortcomings, and SiPM devices provide photon counting detectors that are easily integrated with silicon-based signal / data processing circuits. In some embodiments, SiPM devices employ avalanche photodiodes as light sensors. When biased above its breakdown voltage, the avalanche diode breaks down in response to the collision of a single photon. Such devices are sometimes referred to as single-photon avalanche diode (SPAD) detectors. In a typical SPAD detector, the avalanche photodiode is reverse biased above its breakdown voltage and connected in series with a quenching resistor. The collision of a single photon causes the PN junction to break down in the multiplication (i.e., "avalanche") cascade of charge carriers flowing as a measurable current in the SPAD detector. Since this current reduces the reverse bias across the avalanche diode to a level below its breakdown voltage, the current is quenched relatively quickly due to the voltage across the resistor. Additionally or alternatively, an active quenching subcircuit including, for example, one or more diodes, resistors, and / or transistors can provide faster quenching. Some illustrative examples of SiPM detector arrays are disclosed in US9268033B2 and US10078141B2.

[0004] WO2010 / 100574A2 discloses a PET scanner comprising a ring of detector modules surrounding an imaging area. Each of these detector modules comprises one or more sensor avalanche photodiodes (APDs) biased in a breakdown region in Geiger mode. The sensor APD outputs a pulse in response to light from a scintillator corresponding to an incident photon. A reference APD, also biased in a breakdown region in Geiger mode, is optically shielded from light and outputs a voltage measured by an analog-to-digital converter. Based on the measurement result, a bias control feedback loop instructs a variable voltage generator to adjust the bias voltage applied to the APD so that the difference between the voltage of the breakdown pulse and a preselected logic voltage level is minimized.

[0005] Other prior art can be found in US2018 / 292550A1, WO2014 / 184714A1 and US9954124B1.

[0006] Digital silicon photomultipliers are used as photon counting detectors (sometimes also referred to as photon detectors) in high performance PET / CT systems. The fast trigger logic capability of digital silicon photomultipliers, coupled to a scintillator with time-of-flight capability, enables temporal resolution in the range of 100ps-200ps. The temporal resolution of these systems is particularly limited by the temporal spread of the scintillation process. New detector concepts using cue photons have been proposed, which potentially provide improvements in temporal resolution significantly below 100ps.

[0007] Strong radiation fields irradiating image sensors (e.g., CMOS image sensors) can cause damage to the device, resulting in higher leakage, dark noise, and changes in spectral sensitivity. In the case of a SPAD exposed to a radiation photon flux, the effect is a temporary (undesirable) increase in the dark count rate. Summary of the invention

[0008] It is an object of the present invention to mitigate potential negative effects of X-ray or gamma irradiation in SPAD-based photon counting detectors used, for example, in photon counting spectral CT applications.

[0009] In a first aspect of the present invention, a photon counting detector is provided, comprising:

[0010] a scintillator configured to convert incident gamma radiation into optical photons;

[0011] A pixelated photodetector configured to detect a flux of optical photons, wherein the pixelated photodetector is a silicon photomultiplier (SiPM) detector, wherein each photodetector pixel comprises an array of silicon avalanche photodiodes (SPADs); and a circuit configured to heat the SPADs by:

[0012] If the dark count rate of the SPAD exceeds a dark count rate threshold, applying an increased reverse bias voltage to the SPAD to force the SPAD to break down so that current flows through the SPAD for a heating period, thereby locally increasing the temperature of the SPAD, and

[0013] The length of the heating period is controlled.

[0014] In a further aspect of the invention, a corresponding photon counting method is proposed.

[0015] In yet a further aspect of the invention, an imaging device comprising a photon counting detector as disclosed herein is presented.

[0016] Preferred embodiments of the invention are defined in the dependent claims.

[0017] The present invention is based on the concept of locally increasing the temperature of the SPAD and its surroundings in order to accelerate the repair of damage caused by X-ray or gamma irradiation. Instead of using additional structural elements or hardware (such as additional heating elements or heating wires embedded in the detector), the SPAD itself is configured and used as a heating element. The SPAD, which is always biased above its breakdown voltage, will eventually break down and start to conduct current. This breakdown is usually stopped by a quenching resistor (in a passively quenched SPAD) or by activating an active quenching and recharging circuit (in an actively quenched SPAD). According to the present invention, the current continues for a certain time (also referred to as a "heating period" in this article) to heat the SPAD and thereby achieve damage repair and dark count rate (DCR) reduction. This solution does not require any modification to existing hardware (especially for actively quenched SPADs).

[0018] According to an embodiment, the circuit is configured to control the length of the heating period based on the amount of the dark count rate and / or the degree to which the dark count rate exceeds the dark count rate threshold. In particular, the greater the amount of the dark count rate and / or the higher the degree to which the dark count rate exceeds the dark count rate threshold, the longer the length of the heating period is controlled.

[0019] One or more predetermined lengths may be predefined (and, for example, stored in a lookup table or in a memory accessible by the circuit), for which the length is selected, for example, based on the amount of dark count rate and / or the extent to which the dark count rate exceeds a dark count rate threshold. Thus, the length of the heating period can be made programmable, thereby enabling simple and fast control of the heating period.

[0020] The detector may also include a circuit configured to measure the dark count rate of the SPAD. Measurements may be performed in advance to determine whether a reduction in the dark count rate is required or desired and the amount by which the dark count rate is reduced. The dark count rate may also be measured after damage repair to check the success of the damage repair.

[0021] In another embodiment, the circuit is configured to ramp up the reverse bias voltage until a bias current to flow due to application of the reverse bias voltage reaches a bias current limit.

[0022] The circuit may also be configured to: after the heating period, ramp the reverse bias voltage down to below the breakdown voltage and recharge the SPAD to become sensitive to optical photon detection. This brings the SPAD back to normal operating mode.

[0023] In an embodiment, the SPAD is insensitive to optical photon detection during the heating period. This avoids any additional heat during damage repair that may cause a further increase in the dark noise of the SPAD. The present application aims to disable high DCR SPADs in normal operation to reduce the total noise, and in the case of damage repair, reuse high DCR SPADs to select only SPADs that show a specific level of damage (and limit the requirements for SPAD current supply). The SPAD will be insensitive to optical photon detection during the heating period because the SPAD is forced to break down, especially DC breakdown. In this context, DC means that there is a constant avalanche current passing through the SPAD during the time of damage repair, which heats the SPAD internally due to the internal resistance of the SPAD. Therefore, due to the physical properties of the SPAD, and by forcing the SPAD to break down and avoiding the breakdown to be stopped by a quenching resistor or by activating an active quenching and recharging circuit, the SPAD can be made insensitive.

[0024] The circuit may be configured to monitor the temperature of the SPAD during the heating period by measuring the reverse bias voltage level, in particular while keeping the bias current level constant. In this way, overheating can be avoided and the intensity and speed of damage repair can be controlled. Thus, the circuit may be configured to control the reverse bias voltage level to not drop below a level required to maintain a constant current through the SPAD during the heating period.

[0025] Typically, it is desirable to heat the SPAD to a certain temperature and keep it there for a certain time. However, when the SPAD temperature changes, the ionization rate in the SPAD junction also changes and it determines the current. This current should be above a certain level (for example above 20 μA) in order to start a self-sustaining avalanche. A way to compensate for the temperature dependence of the ionization rate is to modulate the bias voltage so that the current remains constant to deposit a certain amount of thermal energy into the SPAD. The temperature dependence of the ionization coefficient manifests itself as a temperature dependence of the breakdown voltage (which increases by approximately 20 mV / K (process dependency)). Since this is generally known for the detector, the SPAD temperature can be estimated by looking at the drift of the bias voltage during heating.

[0026] The claimed imaging device comprising a photon counting detector as disclosed herein may be, for example, a PET device, a CT device, a SPECT device, a time-of-flight (TOF) PET device or a Compton camera. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter. In the accompanying drawings:

[0028] Figure 1 A schematic diagram of a conventional photon counting detector is shown.

[0029] Figure 2 A cross-sectional view showing the layout of an exemplary digital photon counting detector.

[0030] Figure 3 A general circuit diagram of one of the detector cells of a pixelated digital radiation detector is shown.

[0031] Figure 4 A graph showing the dark count behavior of an X-ray irradiated SPAD array.

[0032] Figure 5 A circuit diagram of an embodiment of a detector unit of a photon counting detector according to the present invention is shown.

[0033] Figure 6 A flow chart of an embodiment of a method according to the invention is shown.

[0034] Figure 7 A circuit diagram showing another embodiment of a detector unit of a photon counting detector according to the present invention is shown.

[0035] Figure 8 A flow chart showing another embodiment of the method according to the present invention is shown.

[0036] FIG9 shows a timing diagram of normal SPAD operation.

[0037] FIG10 shows a time diagram of the damage repair cycle. DETAILED DESCRIPTION

[0038] Figure 1 A schematic diagram of a conventional photon counting detector 1 (particularly a scintillator-based spectral photon counting detector with a fixed threshold) is shown. The detector 1 comprises a scintillator 10, which converts incident gamma radiation (e.g., X-ray radiation) into optical photons, and a pixelated photodetector 11 (e.g., a digital SiPM), which detects the flux of optical photons and samples them, thereby providing (e.g., every 5ns) the number of detected photons. The detected optical photons are further processed and evaluated by a circuit 12 (or a corresponding unit).

[0039] In the comparison unit 13, the number of detected photons X (or the sum of several samples) is compared with a fixed threshold value A. If the value X exceeds A, an event is detected and the finite state machine 14 starts the acquisition by accumulating a predefined number of samples in the energy accumulator 15. At the end of the acquisition, the accumulator value is stored in the X-ray energy register 16 and the accumulator 15 is reset to be ready for the next event. The values ​​stored in the energy register 16 are then sorted in the energy histogram 17, where the corresponding bins are added.

[0040] Figure 2 A cross-sectional view of the layout of an exemplary digital photon counting detector 2 is shown. When a gamma ray or an X-ray strikes a scintillator 20, the scintillator 20 generates a scintillation or light burst. A detector pixel array 22 monolithically disposed on a silicon substrate 24 receives the light burst. As will be described, the detector pixel 22 is a digital detector pixel that outputs a value including a digital representation of the photon count and a digital representation of a timestamp that indicates when the detector pixel 22 detected a light burst corresponding to a scintillation event. In addition, the plurality of radiation detectors 2 output index information including, for example, a detector index indicating which of the plurality of radiation detectors outputs a radiation detection event and a detector pixel index indicating which detector pixel or pixels of the radiation detector detected a light burst corresponding to a radiation detection event.

[0041] The scintillator 20 may be selected to provide high stopping power for gamma rays or X-rays with fast temporal decay of scintillation bursts. Some suitable scintillator materials are LSO, LYSO, MLS, LGSO, LaBr, LuAP / LuAG, GAGG, and mixtures thereof. It should be appreciated that other scintillator materials can also be used. Although Figure 2 The scintillator 20 is shown as a single crystal, but an array of crystals can be used instead. In addition, an optional planar light pipe 26 can be inserted between the scintillator 20 and the detector pixel 22 to improve the transmission of the photons of the scintillation light burst to the detector pixel 22. Optionally, the scintillator 20 and the optional light pipe 26 are encapsulated in a reflective coating 28, which directs the scintillation light to the pixel 22.

[0042] Figure 3 As shown, for example Figure 2 A general circuit diagram of one of the detector cells of a pixelated digital radiation detector is shown. Each pixel 22 of the radiation detector 2 comprises an array of detector cells. Figure 3 A general circuit diagram of one such detector unit 50 is shown. A photodiode 52 is biased in the breakdown region and serves as an input to a digitizing circuit 54. An output 56 of the digitizing circuit 54 has a first digital value corresponding to a quiescent state and transitions to a second digital value in response to the photodiode 52 detecting a photon. When the first photon of a scintillation burst is detected, switching the output 56 from the first digital value to the second digital value activates an open collector trigger line driver 60, which causes a trigger signal to be applied to a common trigger line or bus 62. The trigger signal in turn enables a photon counter / FIFO buffer 66, which counts the level transitions of the digitizing circuit 54 from the first digital value to the second digital value during an integration period initiated by the trigger signal. In some other embodiments, an acquisition enable line 67 enables the photon counter 66. A quenching circuit 70, which may be active or passive, limits the current through the photodiode 52 and is configured to facilitate the biasing circuit to transition from the second digital value back to the first digital value. Thus, if the detector cell 50 is quenched back to the static first digital value before the integration period expires, the detector cell 50 may count more than one photon. The final count stored in the photon counter / FIFO buffer 66 can be accessed via the digital bus 68 .

[0043] Single-photon avalanche diodes are devices capable of detecting single photons. One of the non-ideal properties of these sensitive devices is the presence of dark counts (detections unrelated to photons) that appear as random noise to the detector. This random noise adds to the detected signal and degrades its quality, thereby reducing the quality of the final result. One cause of dark count noise is the presence of defects and impurities in the sensitive volume of the SPAD, causing thermal generation of carriers through the Shockley-Reed-Hall process. This component of dark count noise is related to the manufacturing of the device (purity of materials and annealing steps in the semiconductor wire).

[0044] Another component of dark count noise can originate from the SPAD guard ring. The SPAD guard ring is the volume surrounding the high field region of the SPAD. Its purpose is to provide a smooth transition from the high field region to the low field region in the absence of field spikes, which would cause premature breakdown (edge ​​or corner breakdown) of the device. In a working SPAD, the guard ring is designed to not affect the performance of the SPAD, meaning that its width is wide enough to suppress edge breakdown, but not so wide that it adversely affects the fill factor of the SPAD because the area of ​​the guard ring is not sensitive to photons. Typically, the guard ring is implemented as a "virtual guard ring".

[0045] The sensitive area of ​​the SPAD exhibits the highest field strength of the electric field magnitude, while the field gradually decreases in the guard ring area towards the shallow trench isolation and the deep trench isolation. These isolation trenches usually consist of silicon dioxide, and the deep trenches can be additionally filled with polysilicon.

[0046] Typically, these isolation trench materials are neutral, as are the back-end-of-line (BEOL) oxide layers on top of the SPAD. Irradiation with high-energy photons (X-rays, gamma photons) can produce trapped charge accumulation inside these isolation layers. This charge can modify the field distribution inside the guard ring and can cause a significant increase in the dark count rate of the SPAD due to premature breakdown of the guard ring. Figure 4 The figure shows this situation. Figure 4 A diagram showing the dark count behavior of an array of X-ray irradiated SPADs, ie wherein the array of SPADs is irradiated by an X-ray source and the count rate of the SPADs is measured in a series of measurements before, during and after the irradiation.

[0047] from Figure 4 It can be seen that the dark count behavior of the array does not recover immediately when the X-ray source is turned off. The recovery of the dark count rate can take from a few minutes to a few hours (the last two measurement points were taken a few hours after irradiation). The speed of recovery depends on the ambient temperature of the sensor and therefore on the ambient temperature of the isolating oxide layer. In general, recovery proceeds faster at elevated temperatures.

[0048] One of the main elements of the present invention is a means of locally increasing the temperature of the SPAD and its surroundings in order to accelerate the repair of damage caused by X-ray or gamma ray irradiation. Instead of providing one or more additional hardware elements (e.g., a heating wire or another heating element), according to the present invention, the SPAD itself is configured as a heating element, thereby eliminating the need for additional hardware. As described above, a SPAD that is continuously biased to above its breakdown voltage, for example, by a low-ohmic path, will eventually break down and begin to conduct current. In contrast, a SPAD is typically biased above breakdown by a resistor (which limits the current to, for example, less than 20 μA, so it is a few hundred ohms), or is charged by a short pulse to a recharge switch, which is then disconnected, leaving the SPAD unconnected.

[0049] Figure 5 FIG. 8 is a circuit diagram of an embodiment of a detector unit 80 of a pixelated digital radiation detector according to the present invention. The general layout of the radiation detector can be similar to Figure 1 and Figure 2 The layout shown in is similar or identical to that shown in .

[0050] Apart from Figure 3 In addition to the detector unit 50 shown, additional circuitry 82 is provided (e.g. in the form of dedicated hardware or a programmed processor). Figure 5 The control circuit (referred to as the control circuit in the figure) is configured to: apply an increased reverse bias voltage to the SPAD if the dark count rate of the SPAD exceeds a dark count rate threshold to force the SPAD to break down as current flows through the SPAD for a period of time during the heating period; and control the length of the heating period. In this way, each SPAD (or group of SPADs, e.g., all SPADs of a pixel) can be independently heated for a specific time to implement damage repair. As will be referenced below Figure 8 As explained, an OR gate can provide a control signal from an external control line to the recharge switch. Assuming that the repair cycle will use this control line, all cells in the pixel are connected to this control line and can be controlled by an external controller or processor (e.g., FPGA) running a damage detection and repair algorithm. This FPGA will typically be located on the tile or module (i.e., one or more levels in the system architecture) that controls the sensor.

[0051] A measurement signal from the system (e.g., from the counter / FIFO buffer 66) indicative of the dark count rate of the SPAD may be provided to the circuit 82. Thus, the counter / FIFO buffer 66 may be configured to measure the dark count rate of the SPAD. Thus, the measurement may be done at the SPAD level, but in another embodiment, this functionality would be located at the next level (e.g., an FPGA or other processing circuit) and the detector would simply provide a means of externally controlling the state of the recharge FET, such as Figure 8As shown. This embodiment can be implemented with less effort. In addition, when the detector is not needed, lesion repair can be performed sequentially and between patient scans.

[0052] The measurement may be performed in advance to determine whether a reduction in the dark count rate or the amount of dark count rate reduction is needed or desired. The dark count rate may also be measured after damage repair to check the success of the damage repair.

[0053] exist Figure 6 A flow chart of an embodiment of a method 100 for use in a detector with an actively quenched SPAD according to the present invention is shown in , assuming full control of the quenching and recharge switch configuration. In step S1, the bias voltage is reduced to just below the breakdown voltage. In step S2, the recharge switch is closed, thereby pulling the anode of the SPAD to ground. In step S3, the bias voltage is slowly ramped up and the bias current is measured. When a specific current limit is reached, the bias voltage ramp is slowed down or stopped. The current is maintained for a certain period of time (heating period), which period of time can be predetermined or set based on, for example, the level of the measured dark count rate of the SPAD. In step S4, the bias voltage is ramped down to below the breakdown. In step S5, the recharge switch is opened and the sensor is reconfigured for normal operation.

[0054] Typically, each SPAD can be independently controlled at individual timings for damage repair. However, since SPADs are not functional during damage repair, it is preferred to control all SPADs or SPADs that require damage repair simultaneously for damage repair. Damage repair can be performed occasionally (e.g., on demand or at regular intervals) and when the imaging device (e.g., CT scanner) is not in use.

[0055] A dark count rate measurement is preferably performed before (and optionally after) the repair in order to prove that the measurement is needed and that the measurement was successful. A current of the order of a few mA to a few hundred mA can be adjusted in step S3 to locally increase the SPAD temperature. Since the SPAD is in close proximity to the damaged area, heating will help restore the operation of the guard ring.

[0056] When this operation is performed, the current will be closely monitored and the SPAD interconnects can be designed in a way to minimize potential aging issues due to electromigration.The SPAD current should be as low as possible and the time should be as short as possible.

[0057] exist Figure 7A flow chart of another embodiment of a method 200 according to the present invention is shown in . After starting the method (step S10), the dark count rate (DCR) of each SPAD is measured individually (step S11) using a prohibition memory and an internal or external dark count counter (not shown). It is then checked (step S12) whether the DCR is within the acceptance limit. If so, the method ends (step S13), otherwise it is determined (step S14) based on the DCR which SPADs have suffered excessive radiation damage (i.e., have excessive dark counts) and need to be disposed of. The locations of these SPADs are stored and high DCR SPADs caused by the manufacturing process are eliminated (i.e., subtracting the baseline DCR collected before radiation exposure). Subsequently, the prohibition memory is used to select one (or more) SPADs to be disposed of (step S15). All other SPADs are disabled during this disposal.

[0058] For the treatment of SPAD, the following can be applied: Figure 5 In another embodiment, as Figure 7 As shown, a global static recharge signal is applied (step S16). This connects the selected (one or more) SPADs to ground (GND), at which point the SPAD V 过量 Biased to breakdown voltage V 击穿 or more. The dark count will start a static avalanche breakdown, which will not be quenched. The current through the SPAD at this time should exceed about 30μA (conditions for self-sustaining avalanche), but should be limited to tens of mA to prevent lifetime effects due to electromigration. This continuous avalanche current will cause heat accumulation inside the SPAD, causing an increase in the breakdown voltage. Under ideal conditions, the current is monitored by the bias power supply and is limited to a constant level during the heating process. Depending on the amount of damage (DCR level), the duration of the annealing cycle can be constant or variable and can be measured from the beginning of the avalanche current.

[0059] The power dissipated inside the SPAD junction will cause a local increase in the SPAD temperature (step S17). Since the temperature coefficient of SPADs is generally well known (in the range of 20 mV / K), it is possible to use a bias voltage V 偏置 The junction temperature is determined by the change in resistance (assuming the bias supply is configured to provide a constant bias current) (changes in resistance due to heating of the interconnects are neglected here). Higher temperatures accelerate the reduction of radiation damage in SPADs.

[0060] Finally, a check is made to see if all SPADs have been treated (step S18). If so, a wait is made (step S19) until the sensor has cooled sufficiently, and the method then proceeds to step 11 (or waits for a new damage repair iteration to start). If not all SPADs have been treated, the method proceeds to step S14 to process one or more additional SPADs.

[0061] Figure 8 Another embodiment of a circuit diagram of a detector cell 90 of a pixelated digital radiation detector according to the present invention is shown. FIG9 shows a timing diagram of normal SPAD operation, and FIG10 shows a timing diagram of a damage repair (annealing) cycle. In this context, Fig.9A and Fig. 10A shows the logic recharge signal, Fig. 9B and Fig. 10B The SPAD voltage V across the SPAD 91 is shown spad , Fig. 9C and Fig. 10C The SPAD current I through the SPAD 91 is shown spad ,and Fig.9D and Fig. 10D The timing of the different operating modes is shown.

[0062] In normal operation ( Fig.9D is indicated as at all times T op During the period of the local recharge signal (signal which has been in the same (normal) operating mode for a period of time), as explained above, the local recharge signal is applied by the cell electronics 92 to the OR gate 93 to close the first switch 94 (recharge switch) so that the anode A of the SPAD 91 is connected to the ground GND. In addition, the enable signal from the memory 95 closes the second switch 96 (enable switch). The enable switch 96 disconnects the SPAD from the recharge switch 94, i.e., when the memory 95 is set to 0, the SPAD will not be recharged / repaired, and when the memory 95 is set to 1, the enable switch 96 will be closed and the SPAD will be connected to the recharge.

[0063] SPAD voltage V spad (Bias voltage) from breakdown voltage V 击穿 The ramp increases to a higher voltage V 击穿 +V 过量 The SPAD is now sensitive to single photons. The photon (event) that reaches the SPAD causes a current I through the SPAD. spad Short increases to the quiescent current I 静态 above and breakdown, making the SPAD voltage V spad breakdown and decreases back to (or even below) the breakdown voltage V 击穿 The SPAD is then recharged again to make it sensitive to the arrival of photons.

[0064] In the damage repair (damage annealing) operation (in Fig. 10D is indicated as the normal operating time T op Normal operation and damage repair operation time T 退火 During the period of switching between the signals (i.e., switching between the signals), a global recharge signal is provided from a system 97 (which is not part of the detector unit 90) (e.g., a central controller) to the OR gate 93. The recharge switch 94 is closed, which pulls the anode of the SPAD to ground, causing the SPAD voltage I spad The SPAD current I spad is measured. When a certain current limit is reached, the bias voltage ramp is slowed or stopped. The current is maintained for the annealing time (also known as the heating period). Thereafter, the bias voltage ramp is reduced to the breakdown voltage or below the breakdown voltage. Finally, the recharge switch 93 is opened and the sensor is reconfigured for normal operation.

[0065] In summary, according to the present invention, a current is provided through the SPAD itself (e.g. by appropriately configuring the active quenching and recharging circuits and the DC current through the device). The solution does not require (or at least does not substantially) modify existing hardware, in particular as long as the recharging switch can be controlled externally. This enables damage repair of the corresponding SPAD and thus mitigates potential negative effects of X-ray or gamma irradiation in SPAD-based photon counting detectors (e.g. higher leakage, higher dark noise and changes in spectral sensitivity).

[0066] Although the invention has been described and illustrated in detail in the drawings and the foregoing description, such description and illustration should be considered illustrative or exemplary rather than restrictive; the invention is not limited to the disclosed embodiments. Other variations of the disclosed embodiments can be understood and implemented by those skilled in the art in practicing the claimed invention based on a study of the drawings, the disclosure, and the appended claims.

[0067] In the claims, the word "comprising" does not exclude other elements or steps, and the word "a" or "an" does not exclude a plurality. A single element or other unit may perform the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

[0068] Any reference signs in the claims should not be construed as limiting the scope.

Claims

1. A photon counting detector, comprising: a scintillator (10, 20) configured to convert incident gamma radiation into optical photons; A pixelated photodetector (11, 22) configured to detect a flux of optical photons, wherein the pixelated photodetector (22) is a silicon photomultiplier SiPM detector, wherein each photodetector pixel comprises an array of silicon avalanche photodiodes SPAD; and A circuit (80, 90) configured to heat a SPAD by: If the dark count rate of the SPAD exceeds a dark count rate threshold, applying an increased reverse bias voltage to the SPAD to force the SPAD to break down so that current flows through the SPAD for a heating period, thereby locally increasing the temperature of the SPAD, and The length of the heating period is controlled.

2. The photon counting detector according to claim 1, in, The circuit (80, 90) is configured to control the length of the heating period based on an amount of the dark count rate and / or an extent to which the dark count rate exceeds the dark count rate threshold.

3. The photon counting detector according to claim 2, in, The circuit (80, 90) is configured to control the length of the heating period to be longer the greater the amount of the dark count rate and / or the higher the degree to which the dark count rate exceeds the dark count rate threshold.

4. The photon counting detector according to claim 2, in, The circuit (80, 90) is configured to control the length of the heating period according to one or more predetermined lengths.

5. A photon counting detector according to any one of the preceding claims, Also includes: Circuitry configured to measure the dark count rate of the SPAD.

6. A photon counting detector according to any one of the preceding claims, in, The circuit (80, 90) is configured to ramp up the reverse bias voltage until a bias current to flow due to application of the reverse bias voltage reaches a bias current limit.

7. A photon counting detector according to any one of the preceding claims, in, The circuit (80, 90) is configured to ramp the reverse bias voltage down to below a breakdown voltage after the heating period and recharge the SPAD to become sensitive to optical photon detection.

8. A photon counting detector according to any one of the preceding claims, in, The SPAD is insensitive to optical photon detection during the heating period.

9. A photon counting detector according to any one of the preceding claims, in, The circuit (80, 90) is configured to monitor the temperature of the SPAD during the heating period by measuring the reverse bias voltage level.

10. The photon counting detector according to claim 9, in, The circuit (80, 90) is configured to control the reverse bias voltage level to not drop below a level required to maintain a constant current through the SPAD during the heating level.

11. An imaging device comprising a photon counting detector as claimed in any one of the preceding claims.

12. A photon counting method of a photon counting detector, the photon counting detector comprising: a scintillator (10, 20) configured to convert incident gamma radiation into optical photons; A pixelated photodetector (11, 22) configured to detect a flux of optical photons, wherein the pixelated photodetector (11, 22) is a silicon photomultiplier (SiPM) detector, wherein each photodetector pixel comprises an array of silicon avalanche photodiodes (SPAD); and a circuit (80, 90) configured to heat the SPADs, wherein the photon counting method comprises: If the dark count rate of the SPAD exceeds a dark count rate threshold, applying an increased reverse bias voltage to the SPAD to force the SPAD to break down so that current flows through the SPAD for a heating period, thereby locally increasing the temperature of the SPAD, and The length of the heating period is controlled.

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