Energy Calibration Composite Target and Calibration Method for Photon Counting Detector

Abstract

The present invention discloses an energy calibration composite target and a calibration method for a photon counting detector. The energy calibration composite target includes a base layer and a target material layer. A circular groove is provided at the center of the upper surface of the base layer, and the target material layer is disposed in the groove. The target material layer is formed by splicing M sector-shaped target materials sharing a common vertex into a circle, and the outer wall of the circle fits the inner wall of the groove. M≥4. Each sector-shaped target material has a single material, and different sector-shaped target materials are made of different materials, which are materials that can be excited by an X-ray light source to generate X-ray fluorescence. The area ratio of each sector-shaped target material satisfies the same fluorescence intensity detected per unit time. Based on the method of the present invention, the energy resolution of the photon counting detector for particles can be achieved, and the detector with existing energy resolution function but energy drift can be recalibrated with high precision. The laboratory light source cooperates with the electric displacement stage system to achieve remote precise control, improving safety. The composite target helps save calibration time and improve calibration efficiency.

Description

Technical Field

[0001] The present invention relates to an energy calibration composite target and a calibration method, and particularly to an energy calibration composite target and a calibration method for a photon counting detector. Background Art

[0002] Photon counting, as the most advanced detection technology, can not only count single photons, but also record the energy deposited by photons and the detection material, and even the time information of each interaction. It is widely used in various fields such as elemental identification imaging, particle tracking, and radiation detection.

[0003] For applications that require obtaining particle energy information, it is first necessary to calibrate the energy of the photon counting detector. Because the deposited particle energy is converted into electron-hole pairs in the detection material and forms a clock digital signal after being collected by the electrode. Energy calibration is to further convert the digital signal into the energy value it represents to achieve particle energy identification.

[0004] Suppliers of detectors with energy resolution functions will perform calibration at the factory, but many of them are only global calibrations. For applications with high energy resolution requirements, single-pixel energy calibration is required. Moreover, as the usage time of the photon counting detector increases, both the detection material and the electronic components may change, resulting in inaccurate energy detection and the need for re-calibration.

[0005] The existing calibration methods mainly use a photon counting detector to detect several rays with known energies (wavelengths) and establish a functional relationship between energy and the detection value. The ray sources that provide specific energy rays mainly include synchrotron radiation light sources, radioactive nuclides, and fluorescence generated by X-ray sources exciting target materials. The synchrotron radiation machine time is limited and it is difficult to make an appointment, making it difficult to benefit a large number of scientific researchers. Due to time efficiency issues, radioactive nuclides that can be used for energy calibration are often non-exempt sources with high radioactivity, posing safety problems for operators.

[0006] In addition, some people have proposed using different materials as filters and scanning the threshold of the photon counting detector to find the absorption edge of the material for calibration. This method has problems such as large uncertainty in the positioning of the absorption edge in the experimental spectrum, resulting in a large standard deviation of the results, and has not been widely applied and promoted.

[0007] For laboratory conditions, the method of using an X-ray source to excite a target to generate fluorescence may be the most realistic and safe method. In the implementation of the specific method, the X-ray source is used to excite different targets in batches. Due to factors such as different excitation efficiencies of the spectrum for different materials, different fluorescence yields, and different response efficiencies of the detector to the fluorescence of different targets, the geometric position needs to be readjusted every time the target is changed to obtain a uniformly irradiated detection surface and a situation where as many photons as possible hit the target surface under single-photon conditions. If various targets can be excited simultaneously, the above problems can be solved. It is easy to think of stacking the targets together, but due to different fluorescence yields of different materials and the problem that the fluorescence of the lower-layer material is absorbed and attenuated by the upper layer due to stacking, there will also be differences in fluorescence yields. To obtain a single-photon spectrum simultaneously, similar to the cask effect, the fluorescence barrier spectrum time of the target with the fewest photons hitting the detector per unit time should be used as the standard, making the already time-consuming experiment even less efficient. Summary of the Invention

[0008] The object of the present invention is to provide an energy calibration composite target for a photon counting detector that can solve the above problems, save calibration time, improve calibration efficiency, and has accurate calibration, as well as an energy calibration composite target and a calibration method.

[0009] To achieve the above object, the technical solution adopted by the present invention is as follows: An energy calibration composite target for a photon counting detector includes a base layer and a target layer. A circular groove is provided at the center of the upper surface of the base layer, and the target layer is arranged in the groove. The target layer is formed by splicing M fan-shaped targets with a common vertex into a circle, and the outer wall of the circle fits with the inner wall of the groove. M≥4, the material of each fan-shaped target is single, and the materials of different fan-shaped targets are different, and they are materials that can be excited by an X-ray light source to generate X-ray fluorescence.

[0010] The fan-shaped targets are respectively marked as the first to the Mth, and the area ratio of each fan-shaped target satisfies the same fluorescence intensity detected per unit time.

[0011] Preferably: The base is made of PMMA material into a square with a side length of 40mm - 50mm and a thickness of 4 - 6mm; the depth of the circular groove is 1mm, the diameter is 20mm - 30mm, and the thickness of the target layer is 0.3mm - 1mm.

[0012] Preferably: The area ratio of each fan-shaped target satisfies the same fluorescence intensity detected per unit time. Specifically, the area of the ith fan-shaped target is S i , then the areas of all fan-shaped targets satisfy the following formula:

[0013]

[0014] In the formula, i = 1~M, and i is a positive integer.

[0015] Q i is the detection efficiency of the photon counting detector for the Kα fluorescence of the i-th sector target,

[0016] I(E) is the light intensity of the light source emitting light with energy E per unit time;

[0017] δ i (E) represents the Kα line fluorescence cross-section of the i-th sector target at the excitation energy E.

[0018] A calibration method for the energy calibration composite target of a photon counting detector, comprising the following steps;

[0019] (1) Construct an energy calibration composite target of a photon counting detector, prepare an X-ray light source and a photon counting detector, and the total number of pixels on the detection surface is N;

[0020] (2) Optical axis positioning:

[0021] The polychromatic X-ray beam forms a main beam after collimation. A two-dimensional motorized displacement stage is set on the vertical plane of its optical axis, and the two-dimensional motorized displacement stage can drive the object to move on the vertical plane of the optical axis;

[0022] Install the photon counting detector on the two-dimensional motorized displacement stage, with the detection surface perpendicular to the optical axis. Adjust the two-dimensional motorized displacement stage until the main beam forms a complete light spot on the detection surface, monitor the shape and flux of the light spot, and record the pixel coordinates of the center position of the light spot;

[0023] (3) Optical alignment:

[0024] Remove the photon counting detector, install the energy calibration composite target on the two-dimensional motorized displacement stage, adjust the two-dimensional motorized displacement stage to align the center of the energy calibration composite target with the center of the light spot, and the energy calibration composite target is excited by the main beam to generate X-ray fluorescence;

[0025] (4) Adjust the position of the photon counting detector;

[0026] Place the photon counting detector in the area at an angle of 120° - 170° with the main beam exit vector direction, adjust the position of the photon counting detector so that it does not block the main beam, and the fluorescence evenly hits its detection surface, and for each type of fluorescence, within the single-frame exposure time t, the number of photons n hitting the detection surface satisfies n ≤ 0.1N;

[0027] (5) During the acquisition duration T, use the photon counting detector to measure. For each sector target, take the detector count value corresponding to the fluorescence peak energy, where,

[0028] (6) Fit all the fluorescence peak energies and detector count values through the following formula to obtain the values of four constants a, b, c, t, and the functional relationship between f(x) and x;

[0029]

[0030] Wherein, x is the fluorescence peak energy of the sector target, and f(x) is the count value of the corresponding photon counting detector.

[0031] Preferably: The polychromatic X-ray beam is emitted by an X-ray light source and collimated by a collimator.

[0032] Preferably: When adjusting the position of the photon counting detector in step (4), the photon counting detector is placed on an electric three-dimensional displacement stage and adjusted through the electric three-dimensional displacement stage.

[0033] Regarding the fluorescent material of the sector target: It refers to a material that can be excited by a light source to generate X-ray fluorescence. For general laboratory light sources, Fe, Cu, Zr, Mo, Cd, In, Pb, etc. can be selected. Just select several kinds within the effective range of the photon counting detector and with a relatively uniform span of fluorescence peak positions.

[0034] Regarding the setting of the position of the photon counting detector in step (4), the fluorescence detected by the photon counting detector has passed through the absorption and attenuation of the sector target and has a relatively wide energy distribution. Therefore, generally, the photon counting detector is placed at an angle in front of the target for collection, generally in the area at an angle of 120° - 170° with respect to the main beam exit vector direction; at the same time, the fluorescence is very weak, so the distance is as close as possible to the sector target. Thus, we can determine the approximate position of the photon counting detector. In addition, the single-photon condition requires that the number of single photons detected by the detection surface is less than 10% of the pixel number. In addition, it is required that the detector does not interfere with the main beam and the photons on the detection surface are relatively uniform. Therefore, the photon counting detector is finely adjusted based on the above requirements.

[0035] Regarding the application of the present invention: After obtaining the values of the four constants a, b, c, and t by fitting through the formula in step (6), the functional relationship between f(x) and x can be determined; then the relationship between the detector count value f(x) and the fluorescence peak energy x of the target is determined. During subsequent measurements, the energy of the incident photons can be known through the detector count value, realizing various applications related to photon energy such as element identification.

[0036] Compared with the prior art, the advantages of the present invention are as follows:

[0037] (1) Using the composite target with the structure of the present invention for calibration saves calibration time and improves calibration efficiency.

[0038] (2) A new calibration method is proposed. By using the method of the present invention for calibration, for a photon counting detector without energy calibration, the energy can be accurately calibrated, that is, the corresponding conversion between the detector count value and the energy value is realized, so that the photon counting detector can not only record the incident light intensity and the position information of photons, but also perform energy resolution of single photons. For a detector with existing energy resolution, during use, due to changes in the detection material and electronics, the energy resolution will deteriorate. Using this set of methods can improve the energy resolution of the photon counting detector.

[0039] (3) In the present invention, the light source, the photon counting detector, and the energy calibration composite target can all be remotely controlled by an electric displacement stage, rather than using a radiation source at close range, which protects the calibration workers and improves safety. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 is a schematic structural diagram of the present invention;

[0041] Figure 2 is a schematic optical path diagram built for the calibration of the present invention;

[0042] Figure 3 is a flowchart of the calibration method of the present invention.

[0043] In the figure: 1. X-ray light source; 2. Collimator; 3. Two-dimensional electric displacement stage; 4. Three-dimensional electric displacement stage; 5. Main beam; 6. Photon counting detector; 7. Energy calibration composite target of the photon counting detector; 8. Base layer; 9. Target material layer. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0044] The present invention will be further described below with reference to the accompanying drawings.

[0045] Example 1: Refer to Figures 1 to 3 , an energy calibration composite target 7 of a photon counting detector, including a base layer 8 and a target material layer 9. A circular groove is provided at the center of the upper surface of the base layer 8, and the target material layer 9 is arranged in the groove. The target material layer 9 is formed by splicing M fan-shaped target materials with a common vertex into a circle, and the outer wall of the circle fits the inner wall of the groove. M≥4. The material of each fan-shaped target material is single, and the materials of different fan-shaped target materials are different, which are materials that can be excited by the X-ray light source 1 to generate X-ray fluorescence; the fan-shaped target materials are respectively marked as the first to the Mth, and the area ratio of each fan-shaped target material satisfies that the fluorescence intensity detected per unit time is the same.

[0046] The base is made of PMMA material into a square with a side length of 40 mm - 50 mm and a thickness of 4 - 6 mm; the depth of the circular groove is 1 mm, the diameter is 20 mm - 30 mm, and the thickness of the target material layer 9 is 0.3 mm - 1 mm.

[0047] The area ratio of each sector target satisfies that the fluorescence intensity detected per unit time is the same. Specifically, the area of the \(i\)-th sector target is \(S\). i Then, the areas of all sector targets satisfy the following formula:

[0048]

[0049] In the formula, \(i = 1\sim M\), and \(i\) is a positive integer.

[0050] Q i is the detection efficiency of the photon counting detector 6 for the Kα fluorescence of the \(i\)-th sector target.

[0051] \(I(E)\) is the light intensity of the light source emitting light with energy \(E\) per unit time.

[0052] δ i (E) represents the Kα line fluorescence cross-section of the \(i\)-th sector target at the excitation energy \(E\).

[0053] A calibration method for the energy calibration composite target 7 of a photon counting detector includes the following steps:

[0054] (1) Construct an energy calibration composite target 7 of a photon counting detector, and prepare an X-ray light source 1 and a photon counting detector 6, and the total number of pixels on the detection surface is \(N\).

[0055] (2) Optical axis positioning:

[0056] The polychromatic X-ray beam forms a main beam 5 after collimation. A two-dimensional electric displacement stage 3 is arranged on the vertical plane of its optical axis. The two-dimensional electric displacement stage 3 can drive the object to move on the vertical plane of the optical axis.

[0057] Install the photon counting detector 6 on the two-dimensional electric displacement stage 3, with the detection surface perpendicular to the optical axis. Adjust the two-dimensional electric displacement stage 3 until the main beam 5 forms a complete light spot on the detection surface, monitor the shape and flux of the light spot, and record the pixel coordinates of the center position of the light spot.

[0058] (3) Optical alignment:

[0059] Remove the photon counting detector 6, install the energy calibration composite target 7 on the two-dimensional electric displacement stage 3, adjust the two-dimensional electric displacement stage 3 to align the center of the energy calibration composite target 7 with the center of the light spot. The energy calibration composite target 7 is excited by the main beam 5 to generate X-ray fluorescence.

[0060] (4) Adjust the position of the photon counting detector 6;

[0061] Place the photon counting detector 6 in the area that forms an angle of 120°-170° with the outgoing vector direction of the main beam 5. Adjust the position of the photon counting detector 6 so that it does not block the main beam 5, and the fluorescence evenly hits its detection surface. Moreover, within the single-frame exposure time t of each type of fluorescence, the number of photons n hitting the detection surface satisfies n≤0.1N;

[0062] (5) During the acquisition duration T, use the photon counting detector 6 to measure. For each sector target, take the detector count value corresponding to the fluorescence peak energy thereof, where

[0063] (6) Fit all the fluorescence peak energies and detector count values through the following formula to obtain the values of four constants a, b, c, t, and the functional relationship between f(x) and x;

[0064]

[0065] In the formula, x is the fluorescence peak energy of the sector target, and f(x) is the corresponding detector count value of the photon counting detector 6.

[0066] The polychromatic X-ray beam is emitted by the X-ray source 1 and collimated by the collimator 2.

[0067] When adjusting the position of the photon counting detector 6 in step (4), place the photon counting detector 6 on the electric three-dimensional displacement stage 4 and adjust it through the electric three-dimensional displacement stage 4.

[0068] Regarding the area:

[0069] Take four sector targets with the fluorescent materials being Fe, Cu, Zr, and In as an example.

[0070] The spot of the laboratory light source is mainly circular. The center of the spot hits the center of the composite target, and the light flux irradiated on each target is proportional to the area of the target. The area distribution of the target is determined by several factors.

[0071] First, there is the following relationship between the light intensity generated by the light source per unit time and the light intensity hitting a certain target:

[0072]

[0073] Among them, S 总 is the area of the target layer 9, that is, the area of the circle formed by splicing M sector targets. The sector targets are the 1st to the Mth, and the area of the i-th sector target is S i , i = 1~M;

[0074] Then, photons are absorbed by the target, and the excited target undergoes de-excitation through electron transition to form fluorescence. From this process, it can be known that the fluorescence intensity of the Kα line generated on a certain target per unit time is

[0075] I kα (E) = I i (E)·δ kα (E) (2)

[0076] Among them, δ kα (E) is the fluorescence cross section of the target i for generating the Kα line excited by the ray with energy E, because

[0077] δ kα (E) = τ k (E)·ω k ·F kα (3)

[0078] τ k (E) is the photoionization cross section of the target i at the excitation energy E, ω k is the K-line fluorescence yield of the target i, F kα is the Kα transition probability. These can all find the corresponding values by looking up the appropriate tables as basic parameters.

[0079] In addition, the quantum efficiency Q of the photon counting detector 6 for detecting different X-rays i is also different. The final fluorescence intensity detected should be

[0080]

[0081] Among them is the solid angle subtended by the detection surface relative to the fluorescence emission point.

[0082] Combining the above equations, because the X-ray energy of the laboratory light source has a certain distribution, to make the fluorescence intensity I' kα (E) of different targets detected per unit time the same, then it is necessary

[0083]

[0084] Among them, because the fluorescence of the Kα line of the calibrated materials is involved here, the annotation of the fluorescence cross section omits Kα, δ Fe (E) represents the fluorescence cross section of Fe at the excitation energy E, Q Fe represents the detection efficiency of the detector for the Kα line of Fe; the annotation methods for other materials are the same.

[0085] In addition, for different types and thicknesses of targets, the detection efficiency for a certain fluorescence can be looked up. Therefore, Q Fe 、Q cu etc. are all known. I(E) is the light intensity of the light source emitting light with energy E per unit time, and its distribution with energy is determined by the characteristics of the X-ray light source 1 itself and the working voltage and working current. Therefore, for a set of systems, through calculation, the area ratio of the four sector targets can be known.

[0086] Regarding time saving:

[0087] Because the areas of the sector target materials in the composite target of the present invention are limited. Compared with the simple stacked composite target without calculation and area distribution, the target material with the least number of photons hitting the detector per unit exposure time determines the final time. For example, if the least number of photons hitting is half of the average number of photons hitting each target material, then it takes twice the time required for the composite target of this method to obtain the fluorescence spectra of all materials. For the case of sequentially collecting the spectra of single target materials, the time saved is the time increased by disassembling and assembling the target materials, adjusting the fluorescence geometric positions of the detector and the target materials due to target material replacement. Therefore, compared with the prior art, the composite target of the present invention can effectively save the detection time.

[0088] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. An energy calibration composite target for a photon counting detector, comprising a base layer and a target material layer. A circular groove is provided at the center of the upper surface of the base layer, and the target material layer is disposed in the groove. It is characterized in that: The target layer is formed by splicing M fan-shaped targets with a common vertex into a circle, and the outer wall of the circle fits the inner wall of the groove. M≥4. Each fan-shaped target has a single material, and different fan-shaped targets have different materials, which are materials that can be excited by an X-ray light source to generate X-ray fluorescence. The fan-shaped targets are respectively marked as the first to the Mth, and the area ratio of each fan-shaped target satisfies that the fluorescence intensity detected per unit time is the same. The area ratio of each sector target satisfies that the fluorescence intensity detected per unit time is the same. Specifically, the area of the \(i\)-th sector target is \(S\). i , then the areas of all sector targets satisfy the following formula: S1: …S i …: S M := : … …: , In the formula, i = 1~M, and i is a positive integer. Q i is the detection efficiency of the photon counting detector for the Kα fluorescence of the i-th sector target. I(E) is the light intensity of the light source emitting light with energy E per unit time. δ i (E) represents the Kα fluorescence cross-section of the i-th sector target at an excitation energy of E.

2. The energy calibration composite target of the photon counting detector according to claim 1, characterized in that: The base layer is made of PMMA material into a square with a side length of 40mm - 50mm and a thickness of 4 - 6mm; the depth of the circular groove is 1mm, the diameter is 20mm - 30mm, and the thickness of the target layer is 0.3mm - 1mm.

3. The calibration method of the energy calibration composite target of the photon counting detector according to claim 1, characterized in that: It includes the following steps; (1) Construct an energy calibration composite target for a photon counting detector, prepare an X-ray light source and a photon counting detector, and the total number of pixels on the detection surface of the photon counting detector is N. (2) Optical axis positioning: The polychromatic X-ray beam emitted by the X-ray light source forms a main beam after collimation. A two-dimensional electric displacement stage is set on the vertical plane of its optical axis, and the two-dimensional electric displacement stage can drive the object to move on the vertical plane of the optical axis. Install the photon counting detector on the two-dimensional electric displacement stage, with the detection surface perpendicular to the optical axis. Adjust the two-dimensional electric displacement stage until the main beam forms a complete light spot on the detection surface, monitor the shape and flux of the light spot, and record the pixel coordinates of the center position of the light spot. (3) Optical alignment: Remove the photon counting detector, install the energy calibration composite target on the two-dimensional electric displacement stage, adjust the two-dimensional electric displacement stage to align the center of the energy calibration composite target with the center of the light spot, and the energy calibration composite target is excited by the main beam to generate X-ray fluorescence. (4) Adjust the position of the photon counting detector; Place the photon counting detector in the area where the angle with the main beam exit vector direction is 120° - 170°. Adjust the position of the photon counting detector so that it does not block the main beam, the fluorescence evenly hits its detection surface, and for each type of fluorescence, within the single-frame exposure time t, the number of photons n hitting the detection surface ≤ 0.1N. (5) During the acquisition duration T, measurement is carried out using a photon counting detector. For each sector target, the detector count value corresponding to the energy of its fluorescence peak is taken, where T ≥ ; (6) Fit all the fluorescence peak energies and the detector count values through the following formula to obtain the values of four constants a, b, c, t, and the functional relationship between f(x) and x; ， In the formula, x is the fluorescence peak energy of the fan-shaped target, and f(x) is the corresponding count value of the photon counting detector.

4. The calibration method of the energy calibration composite target of the photon counting detector according to claim 3, characterized in that: The polychromatic X-ray beam is emitted by the X-ray light source and collimated by a collimator.

5. The calibration method of the energy calibration composite target of the photon counting detector according to claim 3, characterized in that: When adjusting the position of the photon counting detector in step (4), place the photon counting detector on an electric three-dimensional electric displacement stage and adjust it through the electric three-dimensional electric displacement stage.

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