A sub-pixel resolution method for non-unipolar region of pixel type cadmium zinc telluride detector

Abstract

The application discloses a sub-pixel resolution method for non-unipolar area of pixel type cadmium zinc telluride detector, which comprises the following steps: 1) using the pixel type cadmium zinc telluride detector to detect nuclear radiation, and reading out the signals generated by the detected ray energy deposition events on the cathode and anode; 2) judging whether the ray energy deposition event exists in the unipolar area according to the time and energy of the signal; 3) if the ray energy deposition event i exists in the unipolar area, the three-dimensional position information of the event i in the detector is obtained by sub-pixel resolution according to the transient signal response of the pixels around the charge collection anode corresponding to the event i; 4) if the ray energy deposition event j exists in the non-unipolar area, the energy deposition depth position of the event j in the detector is judged according to the cathode signal amplitude of the event j; then the sub-pixel position resolution of the event j is carried out by using the anode induced charge, and the three-dimensional position information of the event j in the detector is obtained.

Description

Technical Field

[0001] This invention belongs to the field of nuclear radiation detection and nuclear technology applications, and in particular relates to a sub-pixel resolution method for the non-unipolar region of a pixel-type cadmium zinc telluride detector. Background Technology

[0002] In the field of nuclear radiation detection, cadmium zinc telluride (CdZnTe, CZT) is an emerging semiconductor detector material. Compared to traditional materials such as silicon (Si) and high-purity germanium (HPGe), it has a higher atomic number, greater density, and can be used at room temperature. In recent years, it has been widely used in gamma-ray detection. The detection process is as follows: gamma rays deposit energy within the CZT detector, generating electron-hole pairs proportional to the energy deposited. These electrons and holes (also known as charge carriers) drift towards the anode and cathode under the influence of the electric field created by the potential difference between the detector and the anode. The resulting change in induced charge is used as the actual received electrical signal and transmitted to the back-end electronics for processing.

[0003] Calculating the induced charge generated on any electrode of a cadmium zinc telluride detector requires the use of the Shockley-Ramo theory. This theory introduces the concept of weighted potential and proves that the signal generated by charge carriers on the detector is proportional to the difference in weighted potential between the initial generation location of the charge carriers and the location of the detector electrode. The charge collection efficiency is defined as the amount of induced charge generated on the electrode during the process from the generation of a unit charge at a certain point to its collection by the electrode.

[0004] During carrier transport within the CZT detector, carriers may recombine or be trapped, returning from the conduction band to the valence band, leading to the disappearance of electron-hole pairs. The presence of recombination and trapping causes the number of carriers to gradually decrease with drift time, exhibiting an exponential decay. The average distance that carriers can drift within the CZT detector is called the mean free path λ. Under the same applied electric field E, this value is proportional to the carrier mobility μ, the mean carrier lifetime τ, and the electric field E within the detector. Therefore:

[0005] λ=μτE

[0006] For cadmium zinc telluride (CdZN) materials, the average lifetimes of holes and electrons are similar, but the carrier mobility of holes is one to two orders of magnitude lower than that of electrons. This makes holes much easier to trap than electrons during transport, resulting in energy information loss due to radiation deposition and increased broadening of the corresponding electrical signal. This severely affects key performance indicators of the detector, such as energy resolution.

[0007] To overcome this deficiency, cadmium zinc telluride detectors generally adopt electrode design structures guided by unipolar theory, such as hemispherical, Frisch-type, pixel-type, and coplanar-type detectors. These detectors use electrode design to specifically collect electrons with high carrier mobility, reducing the impact of holes on the detector.

[0008] For pixel-type cadmium zinc telluride detectors, the cathode is a single flat electrode, while the anode consists of many small square pixel electrodes arranged in an array, with a certain spacing between each pixel. According to the Shockley-Ramo theory, setting the weight potential of the pixel electrodes to 1 and the weight potential of the other electrodes to 0 ensures that the weight potential of most areas inside the detector farthest from the pixels is low. Figure 1 As shown, the weight potential rises sharply only in a small region near the pixel electrode. Therefore, the unipolar weight potential distribution of the pixel-type cadmium zinc telluride detector is also known as the "small pixel effect". The region where the anode weight potential remains stable is called the unipolar region, and the region where the weight potential rises significantly is called the non-unipolar region.

[0009] Under this weighted potential distribution, if electrons migrate from regions with low weighted potential, the induced charge on the pixel anode will be very low. Only when the electrons drift very close to the pixel anode does the induced charge on the electrode begin to rise rapidly, until the electrons are completely collected by the pixel anode. Under these conditions, for most areas within the detector, incident photons of the same energy can generate a consistent electrical signal response on the anode. In contrast to the distribution caused by the small pixel effect at the anode, the cathode weighted potential in the detector space increases linearly from the anode plane to the cathode plane, and the cathode-induced signal is proportional to the depth of the initial position of the charge carrier from the cathode. The electrode structure design of the pixel-type cadmium zinc telluride detector aims not only to achieve a good energy response but also to utilize the characteristics of its pixel-type anode signal readout for three-dimensional spatial position resolution. Since cadmium zinc telluride detectors can achieve both energy and position resolution, improving the position resolution of pixel-type cadmium zinc telluride detectors can enrich the detector's application scenarios and further enhance its application potential.

[0010] There are currently two main approaches to improving the position resolution of cadmium zinc telluride detectors: one is to reduce the size of the anode pixel electrode; the other is to use the response of different anodes to the same charge carrier to achieve sub-pixel resolution.

[0011] Reducing the pixel anode size can directly improve the detector's spatial resolution, but it imposes a huge burden on the readout electronics. Furthermore, since the charge carriers generated by radiation deposition energy do not drift at a single point within the detector, an excessively small anode will enhance charge sharing between electrodes, thus reducing the detector's energy resolution.

[0012] Without considering changes to the anode pixel size, current mainstream subpixel resolution technology achieves this by collecting the response signals generated on the anodes of several pixels surrounding the carrier collection pixel. For example... Figure 2 As shown, during electron drift, adjacent pixels also exhibit weighted potential changes along their drift trajectories. Therefore, before the electron is collected by the pixel anode, it generates an induced signal on adjacent pixels. The signal response amplitudes generated on adjacent pixels differ depending on the electron's position within the pixel, thus enabling sub-pixel resolution.

[0013] Traditional subpixel resolution technology currently only utilizes unipolar regions. As the requirements for detection efficiency increase, instances in the weaker unipolar regions inside the cadmium zinc telluride detector will also be included in the detection targets after correction. In this region, the change in the weight potential level of adjacent anodes decreases significantly, and there are certain limitations to using adjacent pixel signals for energy resolution. Summary of the Invention

[0014] To address the problems of existing subpixel resolution technologies, the present invention aims to provide a subpixel resolution method for non-unipolar regions of pixel-type cadmium zinc telluride (CZN) detectors. Based on the weighted potential distribution and corresponding charge collection efficiency distribution generated by the anode of a CZN detector within the detector space, the present invention designs a subpixel resolution method for non-unipolar regions of pixel-type detectors, improving detector detection efficiency while maintaining the three-dimensional spatial position resolution of the detector.

[0015] The technical solution of this invention is as follows:

[0016] A sub-pixel resolution method based on the charge collection efficiency distribution within a pixel-type detector, comprising the following steps:

[0017] (1) Nuclear radiation detection was performed using a pixel-type zinc cadmium telluride detector, and the anode and cathode signals of the detected radiation energy deposition events were read out one by one.

[0018] (2) Whether an energy deposition event exists in a unipolar region is determined by using time and energy information obtained after reading out the anode and cathode signals. For time information selection, since the time difference between the anode and cathode readouts is approximately linear, depth calibration can be used to determine whether an energy deposition event is located in a unipolar region. For energy information selection, the ratio of the cathode signal amplitude to the anode signal amplitude is linear in the unipolar region but deviates from this linear relationship in non-unipolar regions, which can be used to determine whether an event exists in a unipolar region.

[0019] (3) If the X-ray energy deposition event i exists in the unipolar region, then sub-pixel resolution is performed to obtain the detector's three-dimensional position information based on the transient signal response of the pixels around the charge collection anode corresponding to the X-ray energy deposition event i.

[0020] (4) If the X-ray energy deposition event j exists in a non-unipolar region, since the cathode signal amplitude and incident depth are approximately linearly related, the energy deposition depth position of the X-ray energy deposition event j in the detector can be determined based on the cathode signal amplitude of the X-ray energy deposition event j.

[0021] (5) When the detector depth information is determined, the equipotential lines in the detector horizontal plane where the X-ray energy deposition case j is located are determined based on the horizontal weight potential of the anode collecting the charge in the non-unipolar region space or the change in charge collection efficiency.

[0022] (6) For non-unipolar regions, the specific location of the ray energy deposited on the equipotential line in the horizontal plane is determined based on the signal response of adjacent pixels, and the three-dimensional position information of case j is obtained by combining step (4).

[0023] If the X-ray energy deposition event j exists in a non-unipolar region, the energy deposition depth position of the X-ray energy deposition event j in the pixel-type cadmium zinc telluride detector is determined based on the cathode signal amplitude of the X-ray energy deposition event j; then, the sub-pixel position resolution of the X-ray energy deposition event j is performed using the anodic induced charge to obtain the three-dimensional position information of the X-ray energy deposition event j in the pixel-type cadmium zinc telluride detector.

[0024] Furthermore, the method for sub-pixel position resolution of the X-ray energy deposition event j using anodic induced charge is as follows: based on the change in horizontal weight potential or charge collection efficiency of the anode in the non-unipolar region, the equipotential line in the horizontal plane of the pixel-type cadmium zinc telluride detector where the X-ray energy deposition event j is located is located; then, based on the signal response of the adjacent pixels of the X-ray energy deposition event j, the range in the plane directly above the pixel is determined, and the three-dimensional position information of the X-ray energy deposition event j in the pixel-type cadmium zinc telluride detector is obtained based on this range.

[0025] Furthermore, the pixel-type cadmium zinc telluride detector is based on a 22*22*5mm... 3 The tellurium zinc cadmium detector has a cathode that is a complete planar electrode and an anode that is an 11*11 pixel electrode array.

[0026] Furthermore, a negative high voltage of 600V is applied to the 5mm crystal in the pixel-type cadmium zinc telluride detector, the pixel anode is grounded, and a voltage of -600V is applied to the cathode plane.

[0027] Furthermore, to determine whether an event exists in a unipolar region, the anode-cathode signal time difference method is used. For the drifting charge carriers inside the detector, the anode-cathode signal response time is the time it takes for the induced signal on the detector to reach the set value.

[0028] Furthermore, the time difference method of anode and cathode signals is used to determine whether the X-ray energy deposition event exists in a unipolar region based on the time and energy information of the read anode and cathode signals.

[0029] The advantages of this invention are as follows:

[0030] This invention innovatively proposes a sub-pixel resolution method for non-unipolar events in pixel-type cadmium zinc telluride (CZT) detectors, filling a gap in this field. Under conditions requiring high positional resolution, this invention directly increases the usable portion within the detector, enhances detection efficiency, and reduces the cost required to achieve the same detection results. It also raises the upper limit of the detection capability of CZT detectors, given the current limitations on the fabrication volume of CZT crystal materials. Furthermore, it enhances the detector's three-dimensional positional resolution capability. Attached Figure Description

[0031] Figure 1 This is a distribution diagram of the weight potential along the central cross-section of a pixel-type cadmium zinc telluride detector.

[0032] Figure 2 It is a graph showing the change in the weighted potential of the charge-collecting pixel and its adjacent pixels during the electron drift process.

[0033] Figure 3 This is a model diagram of a pixel-type detector.

[0034] Figure 4 This is a graph showing the changes in response signals on adjacent pixels when the ray deposition is at different distances from the pixel edge.

[0035] Figure 5 It refers to the horizontal charge collection efficiency distribution of the detector at a distance of 0.9mm-1.8mm from the anode surface;

[0036] (a) Pixel anode charge collection efficiency in a horizontal plane 0.9 mm from the cathode plane.

[0037] (b) Pixel anode charge collection efficiency in a horizontal plane 1.2 mm from the cathode plane.

[0038] (c) Pixel anode charge collection efficiency in a horizontal plane 1.5 mm from the cathode plane.

[0039] (d) Pixel anode charge collection efficiency in a horizontal plane 1.8 mm from the cathode plane.

[0040] Figure 6 It is a contour curve of charge collection efficiency;

[0041] (a) Charge collection efficiency of adjacent anodes in a horizontal plane 0.9 mm from the cathode plane.

[0042] (b) Charge collection efficiency of adjacent anodes in a horizontal plane 1.2 mm from the cathode plane.

[0043] (c) Charge collection efficiency of adjacent anodes in a horizontal plane 1.5 mm from the cathode plane.

[0044] (d) Charge collection efficiency of adjacent anodes in a horizontal plane 1.8 mm from the cathode plane.

[0045] Figure 7 This is a graph showing the difference in charge collection efficiency in the horizontal direction at a certain depth from the cathode. Detailed Implementation

[0046] The present invention will now be described in further detail with reference to the accompanying drawings. The examples given are only for explaining the present invention and are not intended to limit the scope of the present invention.

[0047] The steps of this invention include:

[0048] (1) As Figure 3 Based on 22*22*5mm 3 This is a small-sized cadmium zinc telluride detector. The detector cathode is a complete planar electrode, and the anode is an 11*11 pixel-type electrode array. A negative high voltage of 600V is applied to the 5mm crystal, the pixel anode is grounded, and a voltage of -600V is applied to the cathode plane. The detector signal is read out, amplified, and acquired through a multi-channel chip.

[0049] (2) To determine whether an event exists in a unipolar region, the anode-cathode signal time difference method is used. For drifting charge carriers within the detector, the anode-cathode signal response time is the time it takes for the induced signal on the detector to reach a set value. For unipolar detectors, the difference in anode-cathode weight potential leads to differences in the efficiency of induced charge generation on the anode and cathode in different regions within the detector. The rise time difference of the anode-cathode signal for charge carriers generated at different locations within the detector has an approximately linear relationship with the depth of action. By calibrating this relationship and constructing a depth-time curve, the depth of action can be determined. Using this method for depth discrimination is a common application technique and will not be elaborated upon here.

[0050] (3) If energy deposition event i is a unipolar region event, the detector depth information has already been obtained in the above steps. Simultaneously, for the signal readout of the interior directly above the anode corresponding to the energy deposition event, the spatial resolution of event i at the sub-pixel level is performed based on the transient signal amplitude generated on the adjacent pixel anodes of that anode, thus obtaining the precise location of the energy deposition event. Figure 4 The transient signals generated on adjacent pixels vary with the charge collection efficiency at different horizontal distances from the carrier generation location to the pixel boundary. This demonstrates that the specific location within the anode directly opposite the energy deposition event affects the transient signals of adjacent pixels.

[0051] (4) If the time difference between the anode and cathode signals of the X-ray energy deposition case i is used to determine that it is a non-unipolar region case, for the characteristic X-ray energy, since the detector cathode signal is approximately proportional to the initial position distance of the electron deposition energy from the cathode, the initial carrier drift depth of case i can be determined, that is, the distance of the energy deposition position from the anode plane.

[0052] (5) For the depth range corresponding to the non-unipolar region, the charge collection efficiency of the cadmium zinc telluride detector varies significantly at different horizontal positions. Therefore, the sub-pixel position resolution of the detector can be achieved by using the anodic induced charge. Figure 5 The example shows the horizontal charge collection efficiency distribution of the detector at a distance of 0.9mm-1.8mm from the anode surface. The specific depth range that can be used for sub-pixel resolution varies with the detector thickness, pixel size, and applied high voltage value.

[0053] (6) Figure 5 As shown, step (5) can only locate the carrier generation position on a ring-shaped contour line within the detector plane. Therefore, further positioning is required: In the non-unipolar region, although the signal generated by the change in charge collection efficiency of adjacent pixels is insufficient for precise sub-pixel position resolution, it can be used for approximate position determination, thereby determining the initial carrier generation position on the contour line. At this point, sub-pixel resolution in the non-unipolar region is completed. Figure 6 The curves are contour lines showing the charge collection efficiency of adjacent pixels at the same depth as the cross section in (5) within the plane.

[0054] (7) Figure 7 The paper demonstrates the differences in charge collection efficiency at nine points at different horizontal positions (mm) with detector depths of 1.2 mm, 1.4 mm, and 1.6 m from the anode. The significant differences in charge collection efficiency indicate that the detector's energy response in the non-unipolar region can be used for horizontal position resolution.

[0055] Although specific embodiments of the invention have been disclosed for illustrative purposes to aid in understanding and implementing the invention, those skilled in the art will understand that various substitutions, variations, and modifications are possible without departing from the spirit and scope of the invention and the appended claims. Therefore, the invention should not be limited to the content disclosed in the preferred embodiments, and the scope of protection claimed by the invention is defined by the claims.

Claims

1. A sub-pixel resolution method for the non-unipolar region of a pixel-type cadmium zinc telluride detector, comprising the following steps: 1) Use a pixel-type zinc cadmium telluride detector to detect nuclear radiation, and read out the signals generated by the detected radiation energy deposition events at the cathode and anode one by one; 2) Determine whether the ray energy deposition event exists in the unipolar region based on the time and energy information of the read anode and cathode signals; 3) If the X-ray energy deposition event i exists in the unipolar region, then according to the transient signal response of the pixels around the charge collection anode corresponding to the X-ray energy deposition event i, sub-pixel resolution is performed to obtain the three-dimensional position information of the X-ray energy deposition event i in the pixel-type cadmium zinc telluride detector. 4) If the X-ray energy deposition event j exists in a non-unipolar region, the energy deposition depth position of the X-ray energy deposition event j in the pixel-type cadmium zinc telluride detector is determined based on the cathode signal amplitude of the X-ray energy deposition event j; then, sub-pixel position resolution of the X-ray energy deposition event j is performed using the anode induced charge to obtain the three-dimensional position information of the X-ray energy deposition event j in the pixel-type cadmium zinc telluride detector; wherein, the method for sub-pixel position resolution of the X-ray energy deposition event j using the anode induced charge is as follows: based on the horizontal weight potential or charge collection efficiency change of the anode in the non-unipolar region, the equipotential line in the horizontal plane of the pixel-type cadmium zinc telluride detector where the X-ray energy deposition event j is located is located; then, based on the signal response of the adjacent pixels of the X-ray energy deposition event j, the range in which the horizontal position of the X-ray energy deposition event j is located within the plane directly above the pixel is determined, and the three-dimensional position information of the X-ray energy deposition event j in the pixel-type cadmium zinc telluride detector is obtained based on this range.

2. The method according to claim 1, characterized in that, The pixel-type cadmium zinc telluride detector is based on 22 twenty two 5mm 3 The cadmium zinc telluride detector has a cathode that is a complete planar electrode and an anode that is 11... 11-pixel electrode array.

3. The method according to claim 1 or 2, characterized in that, A negative high voltage of 600V is applied to the 5mm crystal in the pixel-type cadmium zinc telluride detector, the pixel anode is grounded, and a voltage of -600V is applied to the cathode plane.

4. The method according to claim 1 or 2, characterized in that, To determine whether an event exists in a unipolar region, the anode-cathode signal time difference method is used. For drifting carriers inside the detector, the anode-cathode signal response time is the time it takes for the induced signal on the detector to reach a set value.

5. The method according to claim 1 or 2, characterized in that, The time difference method of anode and cathode signals is used to determine whether the radiation energy deposition event exists in a unipolar region based on the time and energy information of the read anode and cathode signals.

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