CN118011453A - CZT detector efficiency calculating method and device based on crystal effective height - Google Patents

Abstract

The invention relates to a CZT detector efficiency calculating method and device based on the effective height of crystals, comprising S1, preparing needed equipment; preparing ²⁴¹Am、¹⁵²Eu、¹³⁷ Cs and ⁶⁰ Co point standard radioactive sources, namely point sources for short and CZT detectors; s2, adjusting working parameters of the CZT detector to enable the CZT detector to be in an optimal working state; s3, measuring ²⁴¹Am、¹⁵²Eu、¹³⁷ Cs and ⁶⁰ Co point source gamma energy spectrums; s4, establishing an efficiency calculation model based on the step S3; s5, taking the nominal parameters of the CZT detector structure as input parameters of an efficiency calculation model, and calculating the detection efficiency; s6, adjusting CZT body crystal and detector mechanism parameters, and calculating detection efficiency; and S7, finally, verifying a calculation model, and calculating the detection efficiency of the CZT detector after the model is determined. According to the invention, the effective height of the CZT crystal is calculated by adjusting the parameter in the calculation model and repeatedly comparing the parameter with experimental data, and the effective height is used as the input parameter of the calculation model, so that the measurement and calculation process is simpler, and an accurate calculation result can be obtained without a complex simulation process.

Description

CZT detector efficiency calculation method and equipment based on crystal effective height

Technical Field

The invention relates to the technical field of radionuclide activity measurement, and in particular to a CZT detector efficiency calculation method and device based on crystal effective height.

Background technique

Cadmium zinc telluride (CZT) detector is a new type of semiconductor gamma-ray spectrometer detector with the advantages of good energy resolution, operation at room temperature, light weight and low price. It is gradually used in medicine, astrophysics, homeland security, environmental protection and other fields. CZT detector is mainly used to measure the energy spectrum of X-rays or gamma-rays.

Detection efficiency is a key parameter for measuring the activity of nuclides using the gamma spectroscopy method. Before measuring the activity of radioactive nuclides, the efficiency of the detector needs to be calibrated. Passive efficiency calibration is an efficiency calculation method based on the Monte Carlo method to establish a model. The efficiency of the detector can be obtained without using a standard source. When it is difficult to use a standard source to calibrate the efficiency of the detection system, the passive efficiency calibration method has significant advantages. In order to obtain accurate detection efficiency calculation results, it is necessary to establish a calculation model based on the structural information of the detector. For traditional gamma spectrometer detectors such as HPGe, NaI (Tl), and LaBr ₃ (Ce), the closer the established calculation model is to the actual structure of the detector, the closer the calculation result is to the measurement result. The gamma-ray full energy peak in the energy spectrum measured by the CZT gamma spectrum detector has a low-energy tail, and the intrinsic detection efficiency of low-energy gamma rays and high-energy gamma rays is very different. This is a significant feature of CZT that distinguishes it from traditional gamma spectrum detectors. Many documents have confirmed that it is difficult to obtain the accurate efficiency of CZT detectors only by establishing an accurate detector efficiency calculation model. The efficiency results calculated using an ideal model (considering only the interaction between γ photons and CZT crystals and detector structural materials) are significantly higher than the measured values. This is mainly due to the inherent properties of CZT crystals, which lead to efficiency losses during the measurement process. If you want to get accurate CZT detector efficiency calculation results, you must not only simulate the interaction between photons and CZT crystals and detector structural materials, but also consider carrier drift, electron cloud diffusion, electric field action, charge induction and collection, electronic noise, signal processing and other processes to obtain efficiency results that are consistent with the measured values. The simulation process is very complicated and difficult to implement in the application of CZT.

Summary of the invention

The present invention proposes a method for calculating the efficiency of a CZT detector based on the effective height of the crystal, which can solve at least one of the above-mentioned technical problems.

To achieve the above object, the present invention adopts the following technical solutions:

A method for calculating the efficiency of a CZT detector based on the effective height of the crystal comprises the following steps:

S1. Prepare the necessary equipment, including ²⁴¹ Am, ¹⁵² Eu, ¹³⁷ Cs and ⁶⁰ Co point-shaped standard radiation sources (referred to as point sources) and CZT detectors;

S2. Adjust the working parameters of the CZT detector to make it in the best working state;

S3, measure the point source γ-ray spectra ^{of 241} Am, ¹⁵² Eu, ¹³⁷ Cs and ⁶⁰ Co;

S4, establishing an efficiency calculation model based on step S3;

S5, using the nominal parameters of the CZT detector structure as input parameters of the efficiency calculation model to calculate the detection efficiency;

S6. Adjust the CZT crystal and detector mechanism parameters, and then calculate the detection efficiency;

S7. Finally, the calculation model is verified. After the model is determined, it is used to calculate the detection efficiency of the CZT detector.

Furthermore, the step S1 of preparing the required equipment specifically includes:

Prepare a bottle of ¹⁵² Eu standard solution with activity A ₀ and volume m _0. The uncertainty of activity should not be greater than 3.0%;

Prepare 1 polyvinyl chloride sample box, 1 volumetric flask, several micro-injector liquid sampling devices, and 2 sets of sample holders for setting the distance between the sample and the detector;

Prepare a quasi-hemispherical CZT gamma spectrum detector and its supporting electronic signal processing system, and indicate the parameters such as the size of the CZT crystal, the size, material and thickness of the detector housing, and the distance between the CZT crystal surface and the inner surface of the detector housing;

Prepare data acquisition and spectrum analysis software specifically for CZT γ spectrum measurement;

Prepare a Monte Carlo method particle transport simulation platform.

Furthermore, the S2, adjusting the working parameters of the CZT detector to make it in the best working state, specifically includes:

A ¹³⁷ Cs point source is placed on the surface of the CZT detector to measure the γ-ray energy spectrum emitted by ¹³⁷ Cs;

First, a set smaller signal shaping time is fixed, and the detector working voltage is gradually adjusted from small to large, while observing the measured gamma-ray energy spectrum and calculating the half-height width of the full-energy peak at 662keV;

When the half-width reaches the minimum, record the working voltage; then adjust the signal shaping time under this working voltage, observe the changes in the system dead time and the half-width of the full-energy peak, and finally determine an optimal working parameter.

Further, the S3, measuring the point source γ-ray spectra of ²⁴¹ Am, ¹⁵² Eu, ¹³⁷ Cs and ⁶⁰ Co, specifically comprises:

The distance from the point source to the surface of the CZT detector, i.e., distance from Source to Detector (hereinafter referred to as "SD"), was adjusted by the sample holder. Under the conditions of SD = 1.5 mm, SD = 46.5 mm, and SD = 96.5 mm, the CZT detector was used to measure the γ energy spectra emitted by ^{the 241} Am, ¹⁵² Eu, ¹³⁷ Cs, and ⁶⁰ Co point sources, and the spectrum analysis software was used to calculate the full energy peak count rates of 59.5 keV emitted by ²⁴¹ Am, 121.78 keV, 244.70 keV, 344.28 keV emitted by ¹⁵² Eu, 661.77 keV emitted by ¹³⁷ Cs, and 1173.24 keV, 1332.50 keV γ rays emitted by ⁶⁰ Co, respectively.

During measurement, ensure that the count of the total energy peak is greater than 10,000; calculate the detection efficiency of CZT for 59.5 keV, 121.78 keV, 244.70 keV, 344.28 keV, 661.77 keV, 1173.24 keV and 1332.50 keV gamma rays based on the measurement results.

Further, based on step S3, an efficiency calculation model is established, including:

When the sample is placed on the central axis of the detector, the detection efficiency of CZT for point sources is calculated under the ideal model;

That is, let the width of the rectangular CZT crystal be x and z, the height be y, the distance between the upper surface of the crystal and the inner surface of the shell be h, the thickness of the shell be a, and the distance between the source and the detector surface be SD.

Furthermore, the S5, using the nominal parameters of the CZT detector structure as input parameters of the efficiency calculation model to calculate the detection efficiency, includes:

The nominal parameters of the detector structure are used as the input parameters of the efficiency calculation model. Under the conditions of SD = 1.5 mm, SD = 46.5 mm and SD = 96.5 mm, the detection efficiency of the CZT detector for γ-rays of different energies is calculated. The calculated results are compared with the detection efficiency measured in step three to guide the next parameter adjustment.

Furthermore, the S6, adjusting the CZT crystal and the detector mechanism parameters, and then calculating the detection efficiency, specifically includes:

Based on the nominal parameters of the detector structure, according to the efficiency comparison result in step S5, the values of a, h, x, z and y are continuously adjusted at intervals of 0.1 mm as input parameters of the efficiency calculation model;

Under the conditions of SD=1.5 mm, SD=46.5 mm and SD=96.5 mm, the detection efficiency of the CZT detector for gamma rays of different energies is calculated, and the calculation result is compared with the detection efficiency measured in step S3;

If the deviation between the calculated efficiency result and the measured value is greater than the set value, the values of a, h, x, z and y are readjusted as input parameters of the efficiency calculation model;

Repeat this process until the deviation between the calculated efficiency result and the measured value is no greater than the set value, and record the values of a, h, x, z and y; at this time, y= _y0 , and _y0 will be much smaller than the nominal CZT crystal height. _y0 is the effective height for calculating the CZT crystal detection efficiency.

Furthermore, the S7, finally verifies the calculation model, and after the model is determined, it is used to calculate the detection efficiency of the CZT detector; the steps of verifying the model are as follows:

Take ₁ mL of m and ₁ Bq of the ¹⁵² Eu standard solution and drop them into a 50 mL volumetric flask. Add deionized water to prepare a ¹⁵² Eu reference solution. Take 10 mL of the reference solution and put it into a Φ30 mm × 15 mm polyvinyl chloride sample box. Seal it and prepare a reference solution sample.

The reference solution sample is placed on the surface of the CZT detector to measure the gamma energy spectrum, and the detection efficiency of the CZT detector for gamma rays of different energies emitted by the ¹⁵² Eu reference sample is calculated based on the measurement results;

The values of a, h, x, z and y obtained in step S6 are used as input parameters of the detector, and the sample parameters are input according to the actual situation of the sample to establish a reference sample efficiency calculation model;

The efficiency curve calculated by the model is compared with the measured efficiency value. If the relative deviation is not greater than 5%, the efficiency calculation result is considered reliable. If the relative deviation is greater than 5%, step S6 needs to be repeated.

On the other hand, the present invention further discloses a computer-readable storage medium storing a computer program, wherein when the computer program is executed by a processor, the processor executes the steps of the above method.

On the other hand, the present invention further discloses a computer device, including a memory and a processor, wherein the memory stores a computer program, and when the computer program is executed by the processor, the processor executes the steps of the above method.

It can be seen from the above technical scheme that the CZT detector efficiency calculation method based on the effective height of the crystal of the present invention solves the problem that it is difficult to accurately calculate the CZT detection efficiency using an ideal model; a CZT detector efficiency calculation model is established based on the ideal model, and three point sources are used to calculate the effective height of the CZT crystal by adjusting the parameters in the calculation model and repeatedly comparing them with the experimental data, and use it as the input parameter of the calculation model. The measurement and calculation process is relatively simple, and accurate calculation results can be obtained without a complex simulation process. Through experimental verification, the method is used to measure the activity of the ¹⁵² Eu aqueous solution source, and the deviation of the result from the standard value is no more than 5%. The method is simple and reliable.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a CZT detector efficiency calculation operation flow of the present invention;

FIG2 is a schematic diagram of a CZT efficiency calculation model;

FIG3 is a schematic diagram of the measurement results of intrinsic detection efficiency of different detectors;

FIG4 is a schematic diagram of the half-height width at different voltages;

FIG5 is a schematic diagram showing the relationship between the forming time and the dead time of the signal processing of the electronics system;

FIG6 is a schematic diagram showing the relationship between forming time and FWHM;

FIG7 is a schematic diagram of the ¹³⁷ Cs γ-ray energy spectrum measured by CZT;

FIG8 is a schematic diagram of the γ energy spectrum of a ²⁴¹ Am point source;

FIG9 is a schematic diagram of the γ energy spectrum of a ¹³⁷ Cs point source;

FIG10 is a schematic diagram of the γ energy spectrum of a ⁶⁰ Co point source;

FIG11 is a schematic diagram of the γ energy spectrum of a ¹⁵² Eu point source;

Figure 12 is a comparison of the efficiency curve calculated with nominal parameters and the measured values when SD-1.5mm;

Figure 13 is a comparison of the efficiency curve calculated with nominal parameters and the measured values at SD-46.5mm;

Figure 14 is a comparison of the efficiency curve calculated with nominal parameters and the measured values at SD-96.5mm;

FIG15 is a comparison diagram of the efficiency result calculated using equivalent height when SD=1.5 mm and the measured value;

FIG16 is a comparison diagram of the efficiency result calculated using the equivalent height when SD=46.5 mm and the measured value;

FIG. 17 is a comparison diagram of the efficiency result calculated using the equivalent height when SD=96.5 mm and the measured value;

FIG18 is a calculation model of body source efficiency based on equivalent height;

FIG19 is a schematic diagram of the calculation results of the volume source efficiency based on the equivalent height.

Detailed ways

In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments.

There are many factors that affect the efficiency loss of CZT detectors, such as charge loss during carrier transport, "dead zone" generated by crystal fixing equipment, "dead zone" generated near the anode, imperfect electrode deposition process and uneven distribution of electric field caused by crystal defects, which will cause incomplete collection of charges generated by gamma rays, resulting in loss of full-energy peak efficiency. Compared with low-energy gamma rays, high-energy gamma rays have a deeper average action position in the CZT crystal, a larger electron cloud width, more times of interaction between the rays and the crystal, and a greater probability of incomplete charge collection, so the full-energy peak efficiency loss is greater. For quasi-hemispherical CZT detectors, the efficiency loss of gamma rays of different energies is related to the height of the crystal. After analysis, the efficiency loss caused by the above reasons can be attributed to the loss of CZT crystal height. By calculating the effective height of the crystal, the detection efficiency can be calculated using an ideal model, which makes the simulation calculation of CZT detector efficiency simple.

As shown in FIG. 1 , the CZT detector efficiency calculation method based on the effective height of the crystal described in this embodiment has an operation flow as shown in FIG. 1 ;

Step 1: Prepare the necessary equipment.

Specifically, ^241Am , ^152Eu , ^137Cs and ^60Co point standard radioactive sources (hereinafter referred to as "point sources") are prepared. The sealing structure of the point source needs to be flat and smooth, which is convenient for controlling and measuring the distance. The radioactivity of the nuclides leaving the factory is clearly given, and the uncertainty of the activity should not exceed 3.0%. The detection efficiency of CZT detectors for high-energy gamma rays is very low, so in the experiment, only the gamma-ray full energy peaks of 121.78keV, 244.70keV and 344.28keV emitted by ^152Eu are selected as the analysis objects. The energy information of gamma rays emitted by each standard source is shown in Table 1.

Table 1 Information on the energy of gamma rays emitted by standard sources

Standard Source Main gamma ray energy (keV) ²⁴¹ Am 59.54 ¹⁵² Eu 121.78, 244.70, 344.28, etc. ¹³⁷ Cs 661.77 ⁶⁰ Co 1173.24, 1332.50

Prepare a bottle of ¹⁵² Eu standard solution with activity _A0 and volume _m0 . The uncertainty of activity should be no more than 3.0%. Prepare a PVC sample box with a size of Φ30mm×15mm and a wall thickness of 3mm, a volumetric flask with a volume of 50mL, several liquid sampling devices such as micro-injectors, and 2 sets of sample holders that can set the distance between the sample and the detector.

Prepare a 10mm×10mm×5mm quasi-hemispherical CZTγ spectrum detector and its supporting electronic signal processing system. It is necessary to master the internal structural parameters of the detector in detail, and indicate the CZT crystal size, the size, material and thickness of the detector shell, the distance between the CZT crystal surface and the inner surface of the detector shell, etc. Prepare data acquisition and spectrum analysis software specifically for CZTγ spectrum measurement. 1 set of Monte Carlo method particle transport simulation platform MCNP5.

Step 2: Adjust the working parameters of the CZT detector to make it in the best working state.

Specifically, a ¹³⁷ Cs point source is placed on the surface of the CZT detector, and the gamma-ray energy spectrum emitted by ¹³⁷ Cs is measured. First, a smaller signal shaping time is fixed, and the detector operating voltage is gradually adjusted from a small increase. At the same time, the measured gamma-ray energy spectrum is observed, and the half-width of the full-energy peak at 662keV is calculated. When the half-width reaches the minimum, the operating voltage is recorded. After that, at this operating voltage, the signal shaping time is adjusted, the changes in the system dead time and the half-width of the full-energy peak are observed, and finally an optimal working parameter is determined. In general, the longer the signal shaping time, the greater the system dead time, which is more unfavorable for measuring high-activity samples, but within a certain range, a larger shaping time can improve the energy resolution capability of the detector. Therefore, it is necessary to select a suitable shaping time and ensure that the subsequent energy spectrum measurement is run under this working parameter.

Step three, measure the point source γ-ray spectra of ²⁴¹ Am, ¹⁵² Eu, ¹³⁷ Cs and ⁶⁰ Co, and calculate the detection efficiency.

Specifically, the distance from the point source to the surface of the CZT detector (distance from Source to Detector, hereinafter referred to as "SD") was adjusted by the sample holder, and the gamma energy spectrum emitted by the ²⁴¹ Am, ¹⁵² Eu, ¹³⁷ Cs and ⁶⁰ Co point sources was measured using the CZT detector under the conditions of SD = 1.5 mm, SD = 46.5 mm and SD = 96.5 mm. The energy spectrum analysis software was used to calculate the full energy peak count rate of the 59.5 keV emitted by ²⁴¹ Am, 121.78 keV, 244.70 keV, 344.28 keV emitted by ¹⁵² Eu, 661.77 keV emitted by ¹³⁷ Cs, and 1173.24 keV, 1332.50 keV gamma rays emitted by ⁶⁰ Co. During the measurement, in order to reduce the statistical error of the count, it should be ensured that the count of the full energy peak is greater than 10,000. Based on the measured results, the detection efficiency of CZT for 59.5keV, 121.78keV, 244.70keV, 344.28keV, 661.77keV, 1173.24keV and 1332.50keV γ rays was calculated.

Step 4: Establish an efficiency calculation model.

Specifically, when the sample is placed on the central axis of the detector, the detection efficiency of CZT for a point source is calculated under an ideal model. Assume that the width of the rectangular CZT crystal is x and z, the height is y, the distance between the upper surface of the crystal and the inner surface of the shell is h, the thickness of the shell (aluminum shell) is a, and the distance between the source and the detector surface is SD, as shown in Figure 2. Since the cross section of CZT is a square, that is, x=z. Under different SD conditions, the sensitivity of the efficiency calculation value to the changes in the above parameters is different.

Step 5: Use the nominal parameters of the detector structure as input parameters of the efficiency calculation model to calculate the detection efficiency.

Specifically, the nominal parameters of the detector structure are used as input parameters of the efficiency calculation model. Under the conditions of SD=1.5mm, SD=46.5mm and SD=96.5mm, the detection efficiency of the CZT detector for γ-rays of different energies is calculated. The calculated results are compared with the detection efficiency measured in step three to guide the parameter adjustment in step six.

Step six, adjust the CZT crystal and detector structure parameters, and calculate the detection efficiency.

Specifically, based on the nominal parameters of the detector structure, according to the efficiency comparison results in step five, the values of a, h, x, z and y are continuously adjusted at intervals of 0.1 mm as input parameters of the efficiency calculation model. Under the conditions of SD = 1.5 mm, SD = 46.5 mm and SD = 96.5 mm, the detection efficiency of the CZT detector for gamma rays of different energies is calculated, and the calculated results are compared with the detection efficiency measured in step three. If the deviation between the calculated efficiency result and the measured value is greater than the set value (for example: relative deviation 6%), the values of a, h, x, z and y are readjusted as input parameters of the efficiency calculation model. Repeat this process until the deviation between the calculated efficiency result and the measured value is not greater than the set value, and record the values of a, h, x, z and y. At this time, y = y ₀ , y ₀ will be much smaller than the nominal CZT crystal height, and y ₀ is the effective height of the CZT crystal detection efficiency calculation.

Step 7: Verify the calculation model.

Specifically, take m ₁ mL and A ₁ Bq standard solution from the ¹⁵² Eu standard solution and drop them into a 50 mL volumetric flask, prepare a ¹⁵² Eu reference solution by adding deionized water, take 10 mL of the reference solution and put it into a Φ30 mm × 15 mm polyvinyl chloride sample box, and seal it to make a reference solution sample. Place the reference solution sample on the surface of the CZT detector to measure the γ energy spectrum. Calculate the detection efficiency of the CZT detector for γ rays of different energies emitted by the ¹⁵² Eu reference sample based on the measurement results. Use the values of a, h, x, z and y obtained in step six as the input parameters of the detector, and then input the sample parameters according to the actual situation of the sample to establish a reference sample efficiency calculation model. Compare the efficiency curve calculated by the model with the measured efficiency value. If the relative deviation is not greater than 5%, it is considered that the efficiency calculation result is reliable. If the relative deviation is greater than 5%, step six needs to be repeated.

The following examples illustrate:

1. Differences in efficiency of different detectors

Due to some special properties of CZT crystal, the detection efficiency of CZT detector is quite different from that of common γ-spectrum detectors such as HPGe, NaI(Tl), LaBr ₃ (Ce), and CeBr ₃ , and the efficiency simulation calculation methods are also very different. In order to verify the difference between the detection efficiency of CZT detector and common γ-spectrum detectors such as HPGe, NaI(Tl), LaBr ₃ (Ce), and CeBr ₃ , five γ-spectrum detectors, CZT, HPGe, NaI(Tl), LaBr ₃ (Ce), and CeBr ₃ , were used to measure ²⁴¹ Am, ¹⁵² Eu, ¹³⁷ Cs, and ⁶⁰ Co point source standard sources, and the intrinsic detection efficiency of the detector was calculated. The size of HPGe crystal is Φ81mm×31mm, the size of CZT crystal is 10×10×5mm ³ , and the size of NaI(Tl), LaBr ₃ (Ce), and CeBr ₃ crystals are all Φ76mm×76mm. During the measurement, a stainless steel collimator with a height of 5 cm and a hole diameter of 5 mm is placed between the point source and the detector. Figure 3 shows the measurement results of the detection efficiency of various detectors for point sources.

As can be seen from Figure 3, the full energy peak intrinsic efficiency curve of the CZT detector is significantly different from that of the HPGe, NaI(Tl), LaBr ₃ (Ce) and CeBr ₃ detectors. The intrinsic detection efficiency of CZT at the low energy end is comparable to that of HPGe, but with the increase of the γ-ray energy, the detection efficiency decreases rapidly. The detection efficiency of HPGe, NaI(Tl), LaBr ₃ (Ce) and CeBr ₃ does not change much with energy relative to CZT, mainly because the volume of the CZT crystal is small, and the greater the γ-ray energy, the less likely it is that all the energy is deposited in the crystal.

2. Equipment and Standard Sources Used

The experiment uses the DT-01C11005 quasi-hemispherical CZT detector produced by Ditech. The nominal size of the CZT crystal is 10×10×5mm ³ , and the cylindrical aluminum shell is sealed. The shell size is Φ30mm×70mm, the wall thickness is 1mm and 0.5mm, and the distance between the crystal surface and the inner surface of the detector shell is 3mm. After testing, the energy resolution of the 662keV γ-ray emitted by ¹³⁷ Cs is 1.5%. The electronic signal processing system uses the Lynx multi-channel analyzer of Canberra, which can provide high voltage for the detector, amplify the pulse signal and record the pulse amplitude spectrum. The data acquisition software uses the Genie2000 γ spectrum data acquisition software of Canberra, in which parameters such as high voltage value, forming time, and signal amplification factor can be set. The peak area is calculated using the self-compiled software, which is specially designed for the characteristics of the γ spectrum measured by CZT, and uses the composite function of Gaussian function, exponential tailing function, step function and background function as the fitting function of the peak shape.

In order to cover gamma rays of different energies as much as possible, four point sources, ²⁴¹ Am, ¹⁵² Eu, ¹³⁷ Cs and ⁶⁰ Co, were used in the experiment. In order to verify the characterization effect of the detector, a Φ30mm×15mm ¹⁵² Eu aqueous solution source was prepared. The standard source information used in the experiment is shown in Table 2. Since the detection efficiency of the CZT detector for high-energy gamma rays is very low, only the gamma ray full energy peaks of 121.78keV, 244.70keV and 344.28keV emitted by ¹⁵² Eu were selected as the analysis objects in the experiment.

Table 2 Standard source information used in the experiment

Standard Source Standard source size Main gamma ray energy (keV) Activity (Bq) Activity uncertainty ²⁴¹ Am Point Source 59.54 9000 2.5% ¹⁵² Eu Point Source 121.78, 244.70, 344.28, etc. 27905 2.5% ¹³⁷ Cs Point Source 661.77 7928 2.5% ⁶⁰ Co Point Source 1173.24, 1332.50 17795 2.5% ¹⁵² Eu Φ30mm×15mm body source 121.78, 244.70, 344.28, etc. 68500 3.0%

3. Adjust the CZT detector working parameters

The energy resolution of CZT detector is the main performance index of γ spectrum detector, which is generally expressed by the full width at half maximum (FWHM) of the full energy peak of γ rays emitted by ¹³⁷ Cs at 661.77keV. In the energy spectrum measurement, it is hoped that the FWHM is as small as possible. In order to find the best working parameters of the detector and minimize the FWHM, the DT-01C11005 10×10× ^5mm3 quasi-hemispherical CZT detector produced by Ditech was used to measure the ¹³⁷ Cs point source at different working voltages. During the measurement, the point source was placed 1.5mm away from the detector surface. Figure 4 shows the FWHM value of the full energy peak of γ rays at 661.77keV measured at different working voltages.

As can be seen from Figure 4, for the DT-01C11005 CZT detector, when the operating voltage is less than 700 V, the energy resolution gradually improves with the increase of voltage, but when the voltage exceeds 700 V, the half-width (FWHM) of the 661.77 keV gamma-ray full-energy peak remains basically unchanged when the voltage is further increased.

The pulse signal shaping time of the CZT detector has a direct impact on the energy resolution capability and the dead time of the signal processing of the electronic system. In order to find the optimal pulse signal shaping time, the energy spectrum of the ¹³⁷ Cs point source was measured using the detector under different pulse shaping time parameters. Figure 5 shows the relationship between the shaping time and the dead time of the signal processing of the electronic system, and Figure 6 shows the relationship between the shaping time and the FWHM of the 661.77keV gamma-ray full energy peak.

As can be seen from Figure 5, the longer the pulse shaping time, the greater the dead time of the detection system. A large dead time is not conducive to measurement, but the shaping time is related to the energy resolution of the detection system, so it is necessary to select a suitable shaping time. As can be seen from Figure 6, when the pulse shaping time of the CZT detector is 2.4μs, the FWHM of the 661.77keV γ-ray full energy peak is the smallest. According to the results in Figure 3, the larger the shaping time, the larger the dead time of the electronic system, and a larger dead time is not conducive to measurement. Based on the above test data, it is more appropriate to set the shaping time at 2.4μs. Figure 7 shows the measured ¹³⁷ Cs γ-ray energy spectrum under the optimal working state of CZT (working voltage 900V, pulse shaping time 2.4μs).

4. Measure ²⁴¹ Am, ¹⁵² Eu, ¹³⁷ Cs and ⁶⁰ Co point source γ-ray spectra and calculate detection efficiency

The γ-ray spectra of ²⁴¹ Am, ¹⁵² Eu, ¹³⁷ Cs and ⁶⁰ Co point sources measured using CZT are shown in FIGS. 8 to 11 ; the efficiency measurement results of the point sources are shown in the experimental measurement values in FIGS. 12 to 17 .

5. Establish efficiency calculation model

According to FIG. 2 , an efficiency calculation model was established using the MCNP5 program, wherein a=0.5 mm, h=3 mm, x=10 mm, y=5 mm, z=10 mm, SD=1.5 mm, SD=46.5 mm, and SD=96.5 mm.

6. Calculate detection efficiency using nominal parameters

The efficiency curves simulated and calculated using the nominal parameters of the CZT detector under the conditions of SD of 1.5mm, 46.5mm, and 96.5mm and the comparison with the measured results are shown in Figure 12.

As can be seen from Figures 12-14, there is a large deviation between the efficiency value calculated using the nominal parameters of the CZT detector and the experimental measurement value. The deviation between the two is relatively small in the low energy range. With the increase of the γ-ray energy, the deviation increases. For the 1173.24keV γ-ray, the maximum deviation reaches 58.76%. When SD = 1.5mm, the deviation between the calculated efficiency value and the measured value of 59.54keV γ-ray is 11.54%. When SD = 46.5mm and SD = 96.5mm, the deviation is significantly reduced, which shows that the parameters affecting the solid angle of the detector such as h, x and z are inaccurate. The smaller the SD, the more significant the impact on the result. However, the corresponding deviation of higher energy γ-rays does not decrease with the increase of SD. After analysis, the deviation between the efficiency calculation result and the measurement result is not only caused by the inaccuracy of h, x and z. The effective thickness y of the CZT crystal may be the main reason for the larger deviation with the larger γ-ray energy. For low-energy γ-rays under small SD conditions, parameters such as h, x and z that affect the detector solid angle are the main factors affecting the efficiency calculation results. For high-energy γ-rays under large SD conditions, the effective thickness y of the CZT crystal is the main factor affecting the efficiency calculation results.

7. Adjust the CZT crystal and detector structure parameters to calculate the detection efficiency

Based on the nominal parameters of the detector structure, according to the comparison results in Figures 12-14, the values of a, h, x, z and y are continuously adjusted at intervals of 0.1 mm as the input parameters of the efficiency calculation model. After repeated calculations and comparisons, the efficiency calculation results under the conditions of a＝0.5mm, h＝3.5mm, y＝3.5mm, x＝10.1mm, and z＝10.1mm are finally obtained, which are in good agreement with the experimental values. At this time, y＝3.5mm is the equivalent height of the 10×10×5mm3 ^quasi -hemispherical CZT detector efficiency calculation. The efficiency results calculated using the equivalent height of the CZT crystal and the experimental comparison are shown in Figures 15-17. Table 3 lists the deviations between the efficiency calculation values and the experimental values.

Table 3 Deviation between the efficiency results calculated using equivalent height and the experimental values

From the data in Table 3, it can be seen that the calculated efficiency of 121.78keV and 244.70keV gamma rays of the characterized CZT detector under the condition of SD = 1.5mm has a large deviation from the experimental measured efficiency, which is 24.95% and 21.46% respectively. This is mainly due to the cascade coincidence of 121.78keV (emissivity 35.58%) and 244.70keV gamma rays (emissivity 7.58%) emitted by ¹⁵² Eu with 40.12keV X-rays (emissivity 37.7%), which reduces the counting rate of the full energy peak, resulting in a large deviation of the calculated efficiency relative to the experimental measured efficiency. From the energy spectrum of Figure 11, it can be clearly seen that (121+40)keV and (244+40)keV coincident peaks. The calculated efficiency of gamma rays of other energies is in good agreement with the experimental measured values, with deviations of no more than 6%. Therefore, it is feasible to calculate the efficiency of the CZT detector using the ideal model by inputting the effective height and the adjusted parameters.

Therefore, when using the passive efficiency scale to calculate the detection efficiency of the CZT used in the experiment, the input parameters are a＝0.5mm, h＝3.5mm, y＝3.5mm, x＝10.1mm, z＝10.1mm, so that the efficiency curve obtained is more reliable. In this set of parameters, h, x and z are still different from the nominal parameters, among which the parameter y is only 70% of the nominal parameter. At this time, y ₀ ＝3.5mm, which is the effective height of the 10×10×5mm ³ quasi-hemispherical CZT detector.

8. Validation of the computational model

In order to verify the feasibility of the method of using an ideal model to calculate the efficiency of CZT detectors for body sources, a Φ30×15mm ¹⁵² Eu reference solution sample was prepared with a total activity of 68500Bq (uncertainty of 3%), and the container was a cylindrical organic glass cup with a wall thickness of 3mm. First, the efficiency curve of the CZT detector was calculated using the ideal model, and then the sample activity was analyzed using this efficiency curve. Finally, the analysis results were compared with the standard source activity to verify the accuracy of the calculated efficiency. In order to eliminate the influence of the cascade coincidence effect on the results, the energy spectrum measurement and efficiency calculation were carried out under the condition of SD＝96.5mm. Figure 18 is an efficiency calculation model with a＝0.5mm, h＝3.5mm, y＝3.5mm, x＝10.1mm, z＝10.1mm as input parameters, and the detection efficiency of the CZT detector for the Φ30×15mm aqueous solution sample was calculated. Figure 19 is the calculated efficiency curve. Table 4 lists the activity calculation results of the ¹⁵² Eu aqueous solution standard source sample under the measurement condition of SD＝96.5mm.

It can be seen from the data in Table 4 that when SD = 95 mm, the influence of the cascade coincidence effect can be ignored, and the deviations of the activities of ¹⁵² Eu aqueous solution samples obtained using gamma rays of different energies relative to the standard value are all less than 5%, which fully demonstrates that the efficiency calculation method of the CZTγ spectrum detector based on the effective height of the crystal is reliable.

In yet another aspect, the present invention further discloses a computer-readable storage medium storing a computer program, wherein when the computer program is executed by a processor, the processor executes the steps of the above method.

On the other hand, the present invention further discloses a computer device, including a memory and a processor, wherein the memory stores a computer program, and when the computer program is executed by the processor, the processor executes the steps of the above method.

In another embodiment provided in the present application, a computer program product comprising instructions is also provided, which, when executed on a computer, enables the computer to execute any of the mobile source emission prediction methods based on time series feature migration in the above-mentioned embodiments.

It is understandable that the system provided by the embodiment of the present invention corresponds to the method provided by the embodiment of the present invention, and the explanation, examples and beneficial effects of the relevant contents can refer to the corresponding parts in the above method.

The embodiment of the present application also provides an electronic device, including a processor, a communication interface, a memory and a communication bus, wherein the processor, the communication interface and the memory communicate with each other through the communication bus.

Memory, used to store computer programs;

The processor is used to implement the above-mentioned mobile source emission prediction method based on time series feature migration when executing the program stored in the memory.

The communication bus mentioned in the above electronic device can be a Peripheral Component Interconnect (PCI) bus or an Extended Industry Standard Architecture (EISA) bus, etc. The communication bus can be divided into an address bus, a data bus, a control bus, etc.

The communication interface is used for communication between the above electronic device and other devices.

The memory may include a random access memory (RAM) or a non-volatile memory (NVM), such as at least one disk memory. Optionally, the memory may also be at least one storage device located away from the aforementioned processor.

The above-mentioned processor can be a general-purpose processor, including a central processing unit (CPU), a network processor (NP), etc.; it can also be a digital signal processor (DSP), an application-specific integrated circuit (ASIC), a field-programmable gate array (FPGA), or other programmable logic devices, discrete gates or transistor logic devices, and discrete hardware components.

In the above embodiments, it can be implemented in whole or in part by software, hardware, firmware or any combination thereof. When implemented using software, it can be implemented in whole or in part in the form of a computer program product. The computer program product includes one or more computer instructions. When the computer program instructions are loaded and executed on a computer, the process or function described in the embodiment of the present application is generated in whole or in part. The computer may be a general-purpose computer, a special-purpose computer, a computer network, or other programmable device. The computer instructions may be stored in a computer-readable storage medium, or transmitted from one computer-readable storage medium to another computer-readable storage medium, for example, the computer instructions may be transmitted from a website site, computer, server or data center by wired (e.g., coaxial cable, optical fiber, digital subscriber line (DSL)) or wireless (e.g., infrared, wireless, microwave, etc.) mode to another website site, computer, server or data center. The computer-readable storage medium may be any available medium that a computer can access or a data storage device such as a server or data center that includes one or more available media integrated. The available medium may be a magnetic medium, (e.g., a floppy disk, a hard disk, a tape), an optical medium (e.g., a DVD), or a semiconductor medium (e.g., a solid-state drive Solid State Disk (SSD)), etc.

It should be noted that, in this article, relational terms such as first and second, etc. are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device. In the absence of further restrictions, the elements defined by the sentence "comprise a ..." do not exclude the presence of other identical elements in the process, method, article or device including the elements.

Each embodiment in this specification is described in a related manner, and the same or similar parts between the embodiments can be referred to each other, and each embodiment focuses on the differences from other embodiments. In particular, for the system embodiment, since it is basically similar to the method embodiment, the description is relatively simple, and the relevant parts can be referred to the partial description of the method embodiment.

The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that the technical solutions described in the aforementioned embodiments may still be modified, or some of the technical features may be replaced by equivalents. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims (9)

1. A method for calculating the efficiency of a CZT detector based on the effective height of the crystal, characterized in that it comprises the following steps: S1. Prepare the necessary equipment, including ²⁴¹ Am, ¹⁵² Eu, ¹³⁷ Cs and ⁶⁰ Co point-shaped standard radiation sources (referred to as point sources) and CZT detectors; S2. Adjust the working parameters of the CZT detector to make it in the best working state; S3, measure the point source γ-ray spectra ^{of 241} Am, ¹⁵² Eu, ¹³⁷ Cs and ⁶⁰ Co; S4, establishing an efficiency calculation model based on step S3; S5, using the nominal parameters of the CZT detector structure as input parameters of the efficiency calculation model to calculate the detection efficiency; S6. Adjust the CZT crystal and detector mechanism parameters, and then calculate the detection efficiency; S7. Finally, the calculation model is verified. After the model is determined, it is used to calculate the detection efficiency of the CZT detector. 2. The CZT detector efficiency calculation method based on the effective height of the crystal according to claim 1 is characterized in that: the step S1 of preparing the required equipment specifically further comprises: Prepare a bottle of ¹⁵² Eu standard solution with activity A ₀ and volume m _0. The uncertainty of activity should not be greater than 3.0%; Prepare 1 polyvinyl chloride sample box, 1 volumetric flask, several micro-injector liquid sampling devices, and 2 sets of sample holders for setting the distance between the sample and the detector; Prepare a quasi-hemispherical CZT gamma spectrum detector and its supporting electronic signal processing system, and indicate the parameters such as the size of the CZT crystal, the size, material and thickness of the detector housing, and the distance between the CZT crystal surface and the inner surface of the detector housing; Prepare data acquisition and spectrum analysis software specifically for CZTγ spectrum measurement; Prepare a Monte Carlo method particle transport simulation platform. 3. The CZT detector efficiency calculation method based on the effective height of the crystal according to claim 2 is characterized in that: said S2, adjusting the working parameters of the CZT detector to make it in the best working state, specifically includes: A ¹³⁷ Cs point source is placed on the surface of the CZT detector to measure the γ-ray energy spectrum emitted by ¹³⁷ Cs; First, a set smaller signal shaping time is fixed, and the detector working voltage is gradually adjusted from small to large, while observing the measured gamma-ray energy spectrum and calculating the half-height width of the full-energy peak at 662keV; When the half-width reaches the minimum, record the working voltage; then adjust the signal shaping time under this working voltage, observe the changes in the system dead time and the half-width of the full-energy peak, and finally determine an optimal working parameter. 4. The CZT detector efficiency calculation method based on crystal effective height according to claim 3, characterized in that: said S3, measuring ²⁴¹ Am, ¹⁵² Eu, ¹³⁷ Cs and ⁶⁰ Co point source gamma spectra, specifically comprises: The distance from the point source to the surface of the CZT detector, i.e., distance from Source to Detector (hereinafter referred to as "SD"), was adjusted by the sample holder. Under the conditions of SD = 1.5 mm, SD = 46.5 mm, and SD = 96.5 mm, the CZT detector was used to measure the γ energy spectra emitted by ^{the 241} Am, ¹⁵² Eu, ¹³⁷ Cs, and ⁶⁰ Co point sources, and the spectrum analysis software was used to calculate the full energy peak count rates of 59.5 keV emitted by ²⁴¹ Am, 121.78 keV, 244.70 keV, 344.28 keV emitted by ¹⁵² Eu, 661.77 keV emitted by ¹³⁷ Cs, and 1173.24 keV, 1332.50 keV γ rays emitted by ⁶⁰ Co, respectively. During measurement, ensure that the count of the total energy peak is greater than 10,000; calculate the detection efficiency of CZT for 59.5 keV, 121.78 keV, 244.70 keV, 344.28 keV, 661.77 keV, 1173.24 keV and 1332.50 keV gamma rays based on the measurement results. 5. The CZT detector efficiency calculation method based on the effective height of the crystal according to claim 4 is characterized in that: based on step S3, an efficiency calculation model is established, comprising: When the sample is placed on the central axis of the detector, the detection efficiency of CZT for point sources is calculated under the ideal model; That is, let the width of the rectangular CZT crystal be x and z, the height be y, the distance between the upper surface of the crystal and the inner surface of the shell be h, the thickness of the shell be a, and the distance between the source and the detector surface be SD. 6. The CZT detector efficiency calculation method based on the effective height of the crystal according to claim 5 is characterized in that: the S5, using the nominal parameters of the CZT detector structure as input parameters of the efficiency calculation model to calculate the detection efficiency, comprises: The nominal parameters of the detector structure are used as the input parameters of the efficiency calculation model. Under the conditions of SD = 1.5 mm, SD = 46.5 mm and SD = 96.5 mm, the detection efficiency of the CZT detector for γ-rays of different energies is calculated. The calculated results are compared with the detection efficiency measured in step three to guide the next parameter adjustment. 7. The CZT detector efficiency calculation method based on the effective height of the crystal according to claim 6 is characterized in that: the step S6, adjusting the CZT body crystal and the detector mechanism parameters, and then calculating the detection efficiency, specifically includes: Based on the nominal parameters of the detector structure, according to the efficiency comparison result in step S5, the values of a, h, x, z and y are continuously adjusted at intervals of 0.1 mm as input parameters of the efficiency calculation model; Under the conditions of SD=1.5 mm, SD=46.5 mm and SD=96.5 mm, the detection efficiency of the CZT detector for gamma rays of different energies is calculated, and the calculation result is compared with the detection efficiency measured in step S3; If the deviation between the calculated efficiency result and the measured value is greater than the set value, the values of a, h, x, z and y are readjusted as input parameters of the efficiency calculation model; Repeat this process until the deviation between the calculated efficiency result and the measured value is no greater than the set value, and record the values of a, h, x, z and y; at this time, y= _y0 , and _y0 will be much smaller than the nominal CZT crystal height. _y0 is the effective height for calculating the CZT crystal detection efficiency. 8. The method for calculating the efficiency of a CZT detector based on the effective height of the crystal according to claim 7, characterized in that: said S7, finally verifies the calculation model, and after the model is determined, it is used to calculate the detection efficiency of the CZT detector; the steps of verifying the model are as follows: Take ₁ mL of m and ₁ Bq of the ¹⁵² Eu standard solution and drop them into a 50 mL volumetric flask. Add deionized water to prepare a ¹⁵² Eu reference solution. Take 10 mL of the reference solution and put it into a Φ30 mm × 15 mm polyvinyl chloride sample box. Seal it and prepare a reference solution sample. The reference solution sample is placed on the surface of the CZT detector to measure the gamma energy spectrum, and the detection efficiency of the CZT detector for gamma rays of different energies emitted by the ¹⁵² Eu reference sample is calculated based on the measurement results; The values of a, h, x, z and y obtained in step S6 are used as input parameters of the detector, and the sample parameters are input according to the actual situation of the sample to establish a reference sample efficiency calculation model; The efficiency curve calculated by the model is compared with the measured efficiency value. If the relative deviation is not greater than 5%, the efficiency calculation result is considered reliable. If the relative deviation is greater than 5%, step S6 needs to be repeated. 9. A computer device comprising a memory and a processor, wherein the memory stores a computer program, and when the computer program is executed by the processor, the processor executes the steps of the method according to any one of claims 1 to 8.