CN118011453A - CZT detector efficiency calculating method and device based on crystal effective height - Google Patents
CZT detector efficiency calculating method and device based on crystal effective height Download PDFInfo
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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
Technical Field
The invention relates to the technical field of radionuclide activity measurement, in particular to a CZT detector efficiency calculating method and device based on the effective height of crystals.
Background
The tellurium-zinc-cadmium (cadmium zinc telluride, CZT for short) detector is a novel semiconductor gamma energy spectrum detector, has the advantages of good energy resolution, capability of working at room temperature, light equipment, low price and the like, is gradually applied to the fields of medicine, astrophysics, homeland security, environmental protection and the like, and mainly uses the CZT detector to measure the energy spectrum of X-rays or gamma rays.
The detection efficiency is a key parameter for measuring the activity of nuclides by using gamma spectrometry, and the efficiency of the detector needs to be calibrated before measuring the activity of radionuclides. The passive efficiency scale is an efficiency calculation method based on a Monte Carlo method building model, and the efficiency of the detector can be obtained without using a standard source. Passive efficiency scaling has significant advantages when it is difficult to use the efficiency of a standard source scale detection system. In order to obtain an accurate detection efficiency calculation result, a calculation model needs to be established according to the structure information of the detector, and for the traditional gamma energy spectrum detectors such as HPGe, naI (Tl), laBr 3 (Ce) and the like, the established calculation model is closer to the real structure of the detector, and the calculation result is closer to the measurement result. The CZT gamma energy spectrum detector detects that the gamma-ray full-energy peak in the energy spectrum has low-energy trailing, and the intrinsic detection efficiency of the low-energy gamma-ray and the high-energy gamma-ray is poor, which is a remarkable characteristic that the CZT is different from the traditional gamma energy spectrum detector. Many documents have demonstrated that it is difficult to obtain accurate efficiencies of CZT detectors simply by building accurate detector efficiency calculation models, with efficiency results calculated using ideal models (which only consider interactions of gamma photons with CZT crystals and detector structural materials) being significantly higher than the actual values. The method mainly comprises the steps of obtaining an accurate CZT detector efficiency calculation result due to the intrinsic property of the CZT crystal, simulating the photon and CZT crystal and detector structure material action process, considering carrier drift, electron cloud diffusion, electric field action, charge induction and collection, electronic noise, signal processing and other processes, obtaining an efficiency result which accords with an actual measurement value, and having complicated simulation process and difficult implementation in the CZT application process.
Disclosure of Invention
The invention provides a CZT detector efficiency calculating method based on the effective height of crystals, which can at least solve one of the technical problems.
In order to achieve the above purpose, the present invention adopts the following technical scheme:
a CZT detector efficiency calculating method based on the effective height of a crystal comprises the following steps,
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.
Further, the step S1 prepares the required equipment, and specifically further includes:
Preparing a standard solution 1 bottle 152 Eu standard solution with activity A 0 and volume m 0, wherein uncertainty of activity is not more than 3.0%;
preparing 1 polyvinyl chloride sample box, 1 volumetric flask, a plurality of microsyringe liquid sampling devices and a sample support 2 sleeve for setting the distance between a sample and a detector;
Preparing a quasi-hemispherical CZT gamma energy spectrum detector and an electronic signal processing system matched with the detector, and marking parameters such as the size of a CZT crystal, the size, the material and the thickness of a detector shell and the distance between the surface of the CZT crystal and the inner surface of the detector shell;
Preparing data acquisition and energy spectrum analysis software special for CZT gamma energy spectrum measurement;
and preparing a Monte Carlo method particle transport simulation platform 1 set.
Further, the step S2 of adjusting the working parameters of the CZT detector to be in the optimal working state comprises the following steps of,
Placing a 137 Cs point source on the surface of a CZT detector, and measuring the gamma ray energy spectrum emitted by 137 Cs;
Firstly, fixing a set small signal forming time, gradually adjusting the working voltage of a detector by small increase, simultaneously observing the measured gamma ray energy spectrum, and calculating the half-width of the full-energy peak at 662 keV;
Recording the working voltage when the half-width reaches the minimum; and then, under the working voltage, adjusting the signal forming time, observing the change of the dead time and the full-energy peak half-width of the system, and finally determining an optimal working parameter.
Further, the S3, measured 241Am、152Eu、137 Cs and 60 Co point source γ spectra specifically include:
The distance from the point source to the surface of the CZT detector, namely distance from Source to Detector, hereinafter referred to as SD, is regulated by a sample support, under the conditions of SD=1.5 mm, SD=46.5 mm and SD=96.5 mm, gamma energy spectrums emitted by the point sources of 241Am、152Eu、137 Cs and 60 Co are measured by the CZT detector, and the total energy peak count rates of the gamma rays of 59.5keV emitted by 241 Am, 121.78keV emitted by 152 Eu, 244.70keV, 344.28keV, 661.77keV emitted by 137 Cs and 1173.24keV and 1332.50keV emitted by 60 Co are respectively calculated by using energy spectrum analysis software;
During measurement, ensuring that the count of the total energy peaks is more than 10000; the detection efficiency of CZT for 59.5keV, 121.78keV, 244.70keV, 344.28keV, 661.77keV, 1173.24keV and 1332.50keV gamma rays was calculated from the measurement results.
Further, an efficiency calculation model is built based on step S3, including,
When the sample is placed on the central axis of the detector, calculating the detection efficiency of the CZT to the point source under an ideal model;
That is, the cuboid CZT crystal has the width of x and z, the height of y, the distance between the upper surface of the crystal and the inner surface of the shell is h, the thickness of the shell is a, and the distance between the source and the detector surface is SD.
Further, S5, taking the nominal parameters of the CZT detector structure as input parameters of an efficiency calculation model, calculating the detection efficiency, comprising,
And (3) taking the nominal parameters of the detector structure as input parameters of an efficiency calculation model, calculating the detection efficiency of the CZT detector on gamma rays with different energies under the conditions of SD=1.5 mm, SD=46.5 mm and SD=96.5 mm, comparing the calculation result with the detection efficiency measured in the step (III), and guiding the parameter adjustment in the next step.
Further, the step S6 of adjusting CZT body crystal and detector mechanism parameters and then calculating the detection efficiency comprises the following steps of,
Taking the nominal parameters of the detector structure as the basis, and continuously adjusting a, h, x, z and y values as input parameters of an efficiency calculation model according to the efficiency comparison result in the step S5 at intervals of 0.1 mm;
Under the conditions of SD=1.5 mm, SD=46.5 mm and SD=96.5 mm, calculating the detection efficiency of the CZT detector on gamma rays with different energies, and comparing the calculation result with the detection efficiency measured in the step S3;
if the deviation between the calculated efficiency result and the measured value is larger than the set value, readjusting a, h, x, z and the numerical value of y to serve as input parameters of the efficiency calculation model;
Repeating the steps until the deviation between the calculated efficiency result and the measured value is not larger than a set value, and recording the numerical values of a, h, x, z and y; at this point y=y 0,y0 will be much smaller than the nominal CZT crystal height, and y 0 is the effective height for CZT crystal detection efficiency calculation.
Further, S7, finally verifying a calculation model, and calculating the detection efficiency of the CZT detector after the model is determined; the verification model comprises the following steps:
Dripping m 1mL、A1 Bq standard solution from 152 Eu standard solution into a 50mL volumetric flask, preparing 152 Eu reference solution by adding deionized water, loading 10mL reference solution into a polyvinyl chloride sample box with phi of 30mm multiplied by 15mm, and sealing to prepare a reference solution sample;
Placing a reference solution sample on the surface of a CZT detector to measure gamma energy spectrum, and calculating the detection efficiency of the CZT detector on gamma rays with different energies emitted by 152 Eu reference sample according to the measurement result;
Taking the numerical values a, h, x, z and y obtained in the step S6 as input parameters of a detector, inputting sample parameters according to the actual condition of the sample, and establishing a reference sample efficiency calculation model;
Comparing the efficiency curve calculated by the model with the measured efficiency value, and if the relative deviation is not more than 5%, considering that the calculation result of the efficiency is reliable; if the relative deviation is greater than 5%, it is also necessary to repeat step S6.
In yet another aspect, the invention also discloses a computer readable storage medium storing a computer program which, when executed by a processor, causes the processor to perform the steps of the method as described above.
In yet another aspect, the invention also discloses a computer device comprising a memory and a processor, the memory storing a computer program which, when executed by the processor, causes the processor to perform the steps of the method as above.
According to the technical scheme, the CZT detector efficiency calculating method based on the effective height of the crystal solves the problem that the CZT detection efficiency is difficult to accurately calculate by using an ideal model; the CZT detector efficiency calculation model is established based on an ideal model, 3 point sources are used, the effective height of the CZT crystal is calculated by adjusting the parameters in the calculation model and repeatedly comparing the experimental data, the effective height is used as the input parameters of the calculation model, the measurement and calculation process is simple, and an accurate calculation result can be obtained without a complex simulation process. Experiments prove that the method is simple and reliable, and the deviation between the result and the standard value is not more than 5 percent when the activity of 152 Eu aqueous solution source is measured.
Drawings
FIG. 1 is a flow chart of the CZT detector efficiency calculation operation of the present invention;
FIG. 2 is a schematic illustration of a CZT efficiency calculation model;
FIG. 3 is a schematic diagram of the intrinsic detection efficiency measurements of different detectors;
FIG. 4 is a half-width schematic diagram at different voltages;
FIG. 5 is a schematic diagram of the relationship between forming time and dead time of signal processing by the electronics system;
FIG. 6 is a graph showing the relationship between forming time and FWHM;
FIG. 7 is a schematic representation of CZT measured 137 Cs gamma-ray energy spectrum;
FIG. 8 is a schematic diagram of the gamma energy spectrum of 241 Am point sources;
FIG. 9 is a schematic diagram of the gamma energy spectrum of 137 Cs point sources;
FIG. 10 is a schematic diagram of the gamma energy spectrum of 60 Co point sources;
FIG. 11 is a schematic diagram of a gamma energy spectrum of 152 Eu point sources;
FIG. 12 is a graph showing the comparison of the calculated efficiency curve with the measured value of the SD-1.5mm time scale parameter;
FIG. 13 is a graph showing the comparison of the calculated efficiency curve with the measured value of the SD-46.5mm time scale parameter;
FIG. 14 is a graph showing the comparison of the calculated efficiency curve with the measured value of the SD-96.5mm time scale parameter;
Fig. 15 is a graph showing comparison of efficiency results and measured values obtained by calculating sd=1.5mm using equivalent height;
fig. 16 is a graph showing comparison of efficiency results and measured values obtained by calculating sd=46.5mm using equivalent height;
Fig. 17 is a graph showing comparison of efficiency results and measured values obtained by calculating sd=96.5mm using equivalent height;
FIG. 18 is a volumetric source efficiency calculation model based on equivalent height;
Fig. 19 is a schematic diagram of a result of calculating the body source efficiency based on the equivalent height.
Detailed Description
For the purpose of making the objects, technical solutions and advantages of the embodiments of the present invention more apparent, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention, and it is apparent that the described embodiments are some embodiments of the present invention, but not all embodiments of the present invention.
The factor influencing the efficiency loss of the CZT detector is more, and charge loss in the carrier transportation process, dead zone generated by crystal fixing equipment, dead zone generated near an anode, imperfect electrode deposition process, uneven electric field distribution generated by crystal defects and the like can lead to incomplete collection of charges generated by gamma rays, thereby leading to loss of full peak efficiency. Compared with low-energy gamma rays, the high-energy gamma rays have deep average action positions in CZT crystals, the generated electron cloud is large in width, the rays and the crystals act for a plurality of times, the probability of incomplete charge collection is higher, and therefore the loss of the total energy peak efficiency is more. For a quasi-hemispherical CZT detector, the amount of different energy gamma ray efficiency loss is related to the height of the crystal. Through analysis, the efficiency loss caused by the reasons can be attributed to the loss of the height of the CZT crystal, and the detection efficiency can be calculated by adopting an ideal model by calculating the effective height of the crystal, so that the simulation calculation of the efficiency of the CZT detector is simplified.
As shown in fig. 1, the operation flow of the CZT detector efficiency calculating method based on the crystal effective height according to the embodiment is shown in fig. 1;
step one, preparing required equipment.
Specifically, a 241Am、152Eu、137 Cs and 60 Co punctiform standard radioactive source (hereinafter referred to as a point source) is prepared, the sealing structure of the point source needs to be flat and smooth, the distance is convenient to control and measure, the radioactivity of nuclide delivery is clearly given, and the uncertainty of the activity is not more than 3.0%. The CZT detector has low detection efficiency on high-energy gamma rays, so only gamma ray full-energy peaks of 121.78keV, 244.70keV and 344.28keV emitted by 152 Eu are selected as analysis objects in experiments. The gamma-ray energy information emitted by each standard source is shown in table 1.
TABLE 1 Standard Source emission gamma ray energy information Table
| Standard source | Energy of principal gamma rays (keV) |
| 241Am | 59.54 |
| 152Eu | 121.78, 244.70, 344.28, Etc |
| 137Cs | 661.77 |
| 60Co | 1173.24、1332.50 |
A standard solution 1 bottle 152 Eu standard solution with activity of A 0 and volume of m 0 is prepared, and uncertainty of activity is not more than 3.0%. 1 polyvinyl chloride sample box with phi 30mm multiplied by 15mm and wall thickness of 3mm, 1 volumetric flask with volume of 50mL, a plurality of liquid sampling devices such as microsyringes and the like and a sample support 2 sleeve capable of setting the distance between a sample and a detector are prepared.
Preparing a quasi-hemispherical CZT gamma energy spectrum detector with the dimensions of 10mm multiplied by 5mm and a matched electronic signal processing system thereof, and needing to grasp the internal structural parameters of the detector in detail to mark parameters such as the size of a CZT crystal, the size, the material and the thickness of a detector shell, the distance from the surface of the CZT crystal to the inner surface of the detector shell and the like. Data acquisition and spectroscopy software specific to CZT gamma spectroscopy was prepared. Monte Carlo method particle transport simulation platform MCNP 51 sets.
And step two, adjusting working parameters of the CZT detector to enable the CZT detector to be in an optimal working state.
Specifically, a 137 Cs point source is placed on the surface of a CZT detector, and gamma ray energy spectrum emitted by 137 Cs is measured. Firstly, fixing a smaller signal forming time, gradually adjusting the working voltage of the detector by small increase, simultaneously observing the measured gamma ray energy spectrum, and calculating the half-width of the full-energy peak at 662 keV. When the half width reaches the minimum, the operating voltage is recorded. And then, under the working voltage, adjusting the signal forming time, observing the change of the dead time and the full-energy peak half-width of the system, and finally determining an optimal working parameter. In general, the longer the signal shaping time, the greater the dead time of the system, which is detrimental to measuring high activity samples, but within a certain range, the greater the shaping time can improve the energy resolving power of the detector. It is therefore necessary to select a suitable shaping time and to ensure that the latter spectral measurements are run at this operating parameter.
And thirdly, measuring 241Am、152Eu、137 Cs and 60 Co point source gamma energy spectrums, and calculating detection efficiency.
Specifically, the distance of the point source from the CZT detector surface (distance from Source to Detector, hereinafter referred to as "SD") was adjusted by the sample holder, and the gamma energy spectra emitted by the 241Am、152Eu、137 Cs and 60 Co point sources were measured using the CZT detector under conditions of sd=1.5 mm, sd=46.5 mm, and sd=96.5 mm. And using energy spectrum analysis software to calculate total peak count rates of gamma rays of 1173.24keV and 1332.50keV emitted by 661.77keV and 60 Co emitted by 241 Am emitted by 121.78keV, 244.70keV, 344.28keV and 137 Cs emitted by 59.5keV and 152 Eu respectively. In order to reduce the statistical error of the count, the count of the total energy peak should be ensured to be more than 10000. The detection efficiency of CZT for 59.5keV, 121.78keV, 244.70keV, 344.28keV, 661.77keV, 1173.24keV and 1332.50keV gamma rays was calculated from the measurement results.
And step four, establishing an efficiency calculation model.
Specifically, the detection efficiency of CZT for a point source is calculated under an ideal model when the sample is placed on the central axis of the detector. Let the width of the cuboid 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 housing be h, the thickness of the housing (aluminum shell) be a, the distance between the source and the detector surface be SD, the diagram being shown in fig. 2. Since CZT is square in cross section, i.e., x=z. The sensitivity of the efficiency calculation value to the variation of the above parameters is different for different SDs.
And fifthly, taking the nominal parameters of the detector structure as input parameters of an efficiency calculation model to calculate the detection efficiency.
Specifically, nominal parameters of the detector structure are used as input parameters of an 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 on gamma rays with different energies is calculated, and the calculated result is compared with the detection efficiency measured in the step three to guide parameter adjustment in the step six.
And step six, adjusting CZT body crystal and detector mechanism parameters, and calculating detection efficiency.
Specifically, based on the nominal parameters of the detector structure, according to the efficiency comparison result in the fifth step, the numerical values of a, h, x, z and y are continuously adjusted to be input parameters of the efficiency calculation model at intervals of 0.1 mm. 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 with different energies is calculated, and the calculation result is compared with the detection efficiency measured in the step three. If the deviation between the calculated efficiency result and the measured value is greater than the set value (e.g., 6% relative deviation), the values of a, h, x, z and y are readjusted as input parameters to the efficiency calculation model. And repeating the steps until the deviation between the calculated efficiency result and the measured value is not larger than the set value, and recording a, h, x, z and y values. At this point y=y 0,y0 will be much smaller than the nominal CZT crystal height, and y 0 is the effective height for CZT crystal detection efficiency calculation.
And step seven, verifying the calculation model.
Specifically, taking m 1mL、A1 Bq standard solution from 152 Eu standard solution, dripping the standard solution into a 50mL volumetric flask, preparing 152 Eu reference solution by adding deionized water, taking 10mL of reference solution, filling the reference solution into a polyvinyl chloride sample box with phi of 30mm multiplied by 15mm, and sealing to prepare a reference solution sample. A reference solution sample was placed on the CZT detector surface to measure gamma spectra. And calculating the detection efficiency of the CZT detector on gamma rays with different energies emitted by 152 Eu reference samples according to the measurement result. And D, taking the numerical values a, h, x, z and y obtained in the step six as input parameters of a detector, inputting sample parameters according to the actual condition of the sample, and establishing a reference sample efficiency calculation model. Comparing the efficiency curve calculated by the model with the measured efficiency value, and if the relative deviation is not more than 5%, considering that the calculation result of the efficiency is reliable. If the relative deviation is greater than 5%, the sixth step is repeated.
The following examples are given:
1. differences in efficiency of different detectors
Because of some special properties of CZT crystals, the detection efficiency of the CZT detector is greatly different from that of common gamma energy spectrum detectors such as HPGe, naI (Tl), laBr 3(Ce)、CeBr3 and the like, and the difference of methods for efficiency simulation calculation is also great. In order to verify the difference between the detection efficiency of the CZT detector and the common gamma energy spectrum detectors such as HPGe, naI (Tl), laBr 3(Ce)、CeBr3 and the like, five gamma energy spectrum detectors such as CZT, HPGe, naI (Tl), laBr 3 (Ce) and CeBr 3 are used for respectively measuring 241Am、152Eu、137 Cs and 60 Co point source standard sources, and the intrinsic detection efficiency of the detector is calculated. HPGe crystal size was Φ81 mm. Times.31 mm, CZT crystal size was 10X 5mm 3,NaI(Tl)、LaBr3 (Ce) and CeBr 3 crystal size was Φ76mm. Times.76 mm. During measurement, a stainless steel collimator with the height of 5cm is placed between the point source and the detector, and the diameter of a collimator hole is 5mm. Fig. 3 is a graph of various detector measurements of the efficiency of point source detection.
As can be seen from fig. 3, the full-energy peak intrinsic efficiency curves of the CZT detector are significantly different from those of the HPGe, naI (Tl), laBr 3 (Ce) and CeBr 3 detectors, and the intrinsic detection efficiency of CZT at the low energy end is comparable to that of HPGe, but as the gamma ray energy increases, the detection efficiency decreases rapidly, while the change in the detection efficiency with energy for HPGe, naI (Tl), laBr 3 (Ce) and CeBr 3 with respect to CZT is not large, mainly due to the smaller volume of the CZT crystal, the larger the gamma ray energy, and the smaller the possibility that all the energy is deposited in the crystal.
2. Apparatus and standard source for use
The experiment adopts a DT-01C11005 quasi-hemispherical CZT detector manufactured by Ditec company, wherein the nominal size of a CZT crystal manufacturer is 10 multiplied by 5mm 3, a cylindrical aluminum shell is sealed, the size of the shell is phi 30mm multiplied by 70mm, the wall thickness is 1mm and 0.5mm, and the distance between the crystal surface and the inner surface of the shell of the detector is 3mm. Through testing, the energy resolution of 662keV gamma rays emitted by 137 Cs is 1.5%, and an electronic signal processing system adopts a Lynx multi-channel analyzer of Canberra company, which can provide high voltage, amplify pulse signals for a detector and record pulse amplitude spectra. The data acquisition software uses Genie2000 gamma energy spectrum data acquisition software of Canberra company, and parameters such as a high voltage value, forming time, signal amplification factor and the like can be set in the data acquisition software. The peak area is calculated by using self-programming software, the software is specially designed aiming at the characteristics of CZT gamma energy spectrum measurement, and a composite function of a Gaussian function, an index tailing function, a step function and a background function is used as a fitting function of peak shape.
To cover as much of the gamma rays of different energies as possible, four point sources of 241Am、152Eu、137 Cs and 60 Co were used in the experiments. To verify the characterization effect of the detector, 152 Eu aqueous solution source of Φ30mm×15mm was formulated, and standard source information used in the experiments is shown in table 2. Since the CZT detector has low detection efficiency for high-energy gamma rays, only gamma ray full-energy peaks of 121.78keV, 244.70keV and 344.28keV emitted by 152 Eu are selected as analysis objects in experiments.
Table 2 standard source information used in experiments
| Standard source | Standard source size | Energy of principal gamma rays (keV) | Activity (Bq) | Uncertainty of activity |
| 241Am | Point source | 59.54 | 9000 | 2.5％ |
| 152Eu | Point source | 121.78, 244.70, 344.28, Etc | 27905 | 2.5％ |
| 137Cs | Point source | 661.77 | 7928 | 2.5％ |
| 60Co | Point source | 1173.24、1332.50 | 17795 | 2.5％ |
| 152Eu | Phi 30mm x 15mm bulk source | 121.78, 244.70, 344.28, Etc | 68500 | 3.0％ |
3. Adjusting CZT detector operating parameters
The energy resolution of a CZT detector is a main performance index of a gamma energy spectrum detector, and is generally expressed by using full width at half maximum (FWHM) of a gamma ray full energy peak of 661.77keV emitted by 137 Cs. In energy spectrum measurement, it is desirable that the smaller the FWHM, the better, in order to find the optimal operating parameters of the detector, the FWHM is minimized, and the 137 Cs point source is measured at different operating voltages using DT-01C11005 type 10×10×5mm 3 mm quasi-hemispherical CZT detector manufactured by ditak company, respectively, and the point source is placed at a distance of 1.5mm from the detector surface. FIG. 4 shows the FWHM of the gamma ray full energy peak at 661.77keV measured at various operating voltages.
As can be seen from FIG. 4, for the DT-01C11005 type CZT detector, the energy resolving power becomes better with increasing voltage when the operating voltage is less than 700V, but the full width at half maximum (FWHM) of the full energy peak of 661.77keV gamma rays remains substantially unchanged when the voltage is increased beyond 700V.
Pulse signal shaping time of CZT detectors has a direct influence on energy resolving power, dead time of signal processing of electronics systems. In order to find the optimal pulse signal shaping time, the energy spectrum of 137 Cs point source is measured under different pulse shaping time parameters by using a detector, fig. 5 is a graph of shaping time versus dead time of signal processing of an electronic system, and fig. 6 is a graph of shaping time versus full energy peak FWHM of 661.77keV gamma rays.
As can be seen from fig. 5, the longer the shaping time of the pulse, the greater the dead time of the detection system, which is detrimental to the measurement, but the shaping time is in turn related to the energy resolution of the detection system, so that a suitable shaping time needs to be chosen. As can be seen from FIG. 6, the full energy peak FWHM of 661.77keV gamma rays is minimal at a CZT detector pulse forming time of 2.4 μs. According to the results in fig. 3, the greater the forming time, the greater the electronic system dead time, which is detrimental to the measurement. In view of the above test data, the molding time was set at 2.4. Mu.s. FIG. 7 shows the measured 137 Cs gamma ray energy spectrum under CZT optimum operating conditions (900V operating voltage, 2.4 μs pulse forming time).
4. Gamma energy spectrum calculation detection efficiency of 241Am、152Eu、137 Cs and 60 Co point sources is measured
241Am、152Eu、137 Cs and 60 Co point source gamma spectra measured using CZT are shown in FIGS. 8 to 11; the efficiency measurements of the point sources are shown in the experimental measurements in fig. 12-17.
5. Establishing an efficiency calculation model
Efficiency calculation models were built using the MCNP5 procedure according to fig. 2, where 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. Calculating detection efficiency using nominal parameters
The comparison of the calculated efficiency curves and measurements using the nominal parameters of the CZT detector at SD of 1.5mm, 46.5mm, 96.5mm, respectively, is shown in fig. 12.
As can be seen from fig. 12-14, there is a large deviation between the efficiency value calculated using the CZT detector nominal parameter and the experimental measurement value, the deviation between the two is relatively small in the low energy section, the deviation is in an increasing trend with increasing gamma-ray energy, and the deviation reaches 58.76% at maximum for the gamma-ray of 1173.24 keV. At sd=1.5 mm, the calculated efficiency value of 59.54keV gamma rays deviates from the measured value by 11.54%, and at sd=46.5 mm and sd=96.5 mm, the deviation is significantly reduced, which means that the parameters affecting the detector solid angle such as h, x and z are inaccurate, the smaller SD has more significant effect on the result, but the corresponding deviation of higher energy gamma rays does not decrease with the increase of SD. Through analysis, the deviation of the efficiency calculation result and the measurement result is not only caused by inaccuracy of h, x and z, but the effective thickness y of the CZT crystal may be a main reason for causing larger deviation of gamma ray energy. Parameters such as h, x, z and the like which influence the solid angle of the detector are main factors which influence the efficiency calculation result under the condition of small SD for low-energy gamma rays, and the effective thickness y of the CZT crystal is the main factor which influences the efficiency calculation result under the condition of large SD for high-energy gamma rays.
7. Adjusting CZT body crystal and detector mechanism parameter to calculate detection efficiency
Based on the nominal parameters of the detector structure, the values a, h, x, z and y are continuously adjusted as input parameters of the efficiency calculation model at intervals of 0.1mm according to the comparison results of fig. 12-14. Through repeated calculation and comparison, the efficiency calculation result under the conditions of a=0.5 mm, h=3.5 mm, y=3.5 mm, x=10.1 mm and z=10.1 mm is obtained, and is better matched with the experimental value, at this time, y=3.5 mm is the equivalent height calculated by the efficiency of the 10×10×5mm 3 quasi-hemispherical CZT detector, the efficiency result calculated by the equivalent height of the CZT crystal and the experimental comparison are shown in fig. 15-17, and the deviation between the efficiency calculation value and the experimental value is listed in table 3.
TABLE 3 deviation between efficiency results and experimental values calculated using equivalent heights
As can be seen from the data in table 3, the calculated efficiencies of 121.78keV and 244.70keV gamma rays at sd=1.5 mm for the characterized CZT detectors were large with the experimental measurement efficiencies at 24.95% and 21.46%, respectively. This is mainly due to the cascade coincidence of 121.78keV (35.58% emissivity) and 244.70keV gamma rays (7.58% emissivity) emitted by 152 Eu with 40.12keV X-rays (37.7% emissivity), and the reduced count rate of the full-energy peak, resulting in a larger deviation of the calculation efficiency from the experimental measurement efficiency. The coincident peaks of (121+40) keV and (244+40) keV are evident from the energy spectrum of fig. 11. The calculation efficiency of other energy gamma rays accords with the experimental measurement value well, and the deviation is not more than 6%. Thus, it is possible to calculate the efficiency of the CZT detector using an ideal model, with the effective height and adjusted parameters entered.
Thus, when the passive efficiency scale is used to calculate the detection efficiency of the CZT used in the experiment, the input parameters are a=0.5 mm, h=3.5 mm, y=3.5 mm, x=10.1 mm, and z=10.1 mm, respectively, so that the obtained efficiency curve is relatively reliable. In this set of parameters, h, x and z are still different from the nominal parameter, where the parameter y is only 70% of the nominal parameter, where y 0 =3.5 mm, i.e. the effective height of the 10×10×5mm 3 quasi-hemispherical CZT detector.
8. Validating a computational model
To verify the feasibility of the method for calculating the CZT detector efficiency using the ideal model for bulk sources, 152 Eu reference solution samples of Φ30x15 mm were prepared, total activity 68500Bq (uncertainty 3%), and the vessel was a cylindrical organic glass cup with a wall thickness of 3 mm. Firstly, calculating an efficiency curve of the CZT detector by adopting an ideal model, analyzing the activity of a sample by utilizing the efficiency curve, and finally comparing an analysis result with a standard source activity to verify the accuracy of calculation efficiency. To eliminate the effect of cascade coincidence effect on the results, the energy spectrum measurement and efficiency calculation were performed under sd=96.5 mm. Fig. 18 is an efficiency calculation model using a=0.5 mm, h=3.5 mm, y=3.5 mm, x=10.1 mm, and z=10.1 mm as input parameters, and the detection efficiency of the CZT detector for Φ30×15mm aqueous solution samples is calculated, and fig. 19 is a calculated efficiency curve. Table 4 shows the results of activity calculations for standard source samples of 152 Eu aqueous solutions under SD = 96.5mm measurement conditions.
As can be seen from the data in table 4, when sd=95 mm, the effect of cascade coincidence effect is negligible, and the deviation of the activity of the 152 Eu aqueous solution sample obtained by using different energy γ rays from the standard value is not more than 5%, which fully indicates that the method for calculating the efficiency of the CZT γ energy spectrum detector based on the effective height of the crystal is reliable.
In yet another aspect, the invention also discloses a computer readable storage medium storing a computer program which, when executed by a processor, causes the processor to perform the steps of the method as described above.
In yet another aspect, the invention also discloses a computer device comprising a memory and a processor, the memory storing a computer program which, when executed by the processor, causes the processor to perform the steps of the method as above.
In yet another embodiment of the present application, there is also provided a computer program product containing instructions that, when run on a computer, cause the computer to perform any of the methods of mobile source emission prediction based on time series feature migration described in the previous embodiments.
It may be understood that the system provided by the embodiment of the present invention corresponds to the method provided by the embodiment of the present invention, and explanation, examples and beneficial effects of the related content may refer to corresponding parts in the above method.
The embodiment of the application also provides an electronic device, which comprises a processor, a communication interface, a memory and a communication bus, wherein the processor, the communication interface and the memory are communicated with each other through the communication bus,
A memory for storing a computer program;
And the processor is used for realizing the mobile source emission prediction method based on time sequence characteristic migration when executing the program stored in the memory.
The communication bus mentioned by the above electronic device may be a peripheral component interconnect standard (english: PERIPHERAL COMPONENT INTERCONNECT, abbreviated as PCI) bus or an extended industry standard architecture (english: extended Industry Standard Architecture, abbreviated as EISA) bus, etc. The communication bus may be classified as an address bus, a data bus, a control bus, or the like.
The communication interface is used for communication between the electronic device and other devices.
The Memory may include random access Memory (RAM, english: random Access Memory) or nonvolatile Memory (NVM, english: non-Volatile Memory), such as at least one disk Memory. Optionally, the memory may also be at least one memory device located remotely from the aforementioned processor.
The processor may be a general-purpose processor, including a central processing unit (english: central Processing Unit, abbreviated as CPU), a network processor (english: network Processor, abbreviated as NP), etc.; it may also be a digital signal processor (English: DIGITAL SIGNAL Processing: DSP), an Application specific integrated Circuit (English: application SPECIFIC INTEGRATED Circuit: ASIC), a Field Programmable gate array (English: field-Programmable GATE ARRAY; FPGA) or other Programmable logic device, discrete gate or transistor logic device, discrete hardware components.
In the above embodiments, it may be implemented in whole or in part by software, hardware, firmware, or any combination thereof. When implemented in software, may 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 loaded and executed on a computer, produces a flow or function in accordance with embodiments of the present application, in whole or in part. The computer may be a general purpose computer, a special purpose computer, a computer network, or other programmable apparatus. The computer instructions may be stored in or transmitted from one computer-readable storage medium to another, for example, by wired (e.g., coaxial cable, optical fiber, digital Subscriber Line (DSL)), or wireless (e.g., infrared, wireless, microwave, etc.). The computer readable storage medium may be any available medium that can be accessed by a computer or a data storage device such as a server, data center, etc. that contains an integration of one or more available media. The usable medium may be a magnetic medium (e.g., floppy disk, hard disk, tape), an optical medium (e.g., DVD), or a semiconductor medium (e.g., solid state disk Solid STATE DISK (SSD)), etc.
It is noted that relational terms such as first and second, and the like are used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. Moreover, the terms "comprises," "comprising," or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. Without further limitation, an element defined by the phrase "comprising one … …" does not exclude the presence of other like elements in a process, method, article, or apparatus that comprises the element.
In this specification, each embodiment is described in a related manner, and identical and similar parts of each embodiment are all referred to each other, and each embodiment mainly describes differences from other embodiments. In particular, for system embodiments, since they are substantially similar to method embodiments, the description is relatively simple, as relevant to see a section of the description of method embodiments.
The above embodiments are only for illustrating the technical solution of the present invention, and are not limiting; although the invention has been described in detail with reference to the foregoing embodiments, it will be understood by those of ordinary skill in the art that: the technical scheme described in the foregoing embodiments can be modified or some technical features thereof can be replaced by equivalents; such modifications and substitutions do not depart from the spirit and scope of the technical solutions of the embodiments of the present invention.
Claims (9)
1. A CZT detector efficiency calculating method based on the effective height of a crystal is characterized by comprising the following steps,
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.
2. The method for calculating the CZT detector efficiency based on the effective height of the crystal according to claim 1, wherein: the step S1 prepares the required equipment, and specifically further includes:
Preparing a standard solution 1 bottle 152 Eu standard solution with activity A 0 and volume m 0, wherein uncertainty of activity is not more than 3.0%;
preparing 1 polyvinyl chloride sample box, 1 volumetric flask, a plurality of microsyringe liquid sampling devices and a sample support 2 sleeve for setting the distance between a sample and a detector;
Preparing a quasi-hemispherical CZT gamma energy spectrum detector and an electronic signal processing system matched with the detector, and marking parameters such as the size of a CZT crystal, the size, the material and the thickness of a detector shell and the distance between the surface of the CZT crystal and the inner surface of the detector shell;
Preparing data acquisition and energy spectrum analysis software special for CZT gamma energy spectrum measurement;
and preparing a Monte Carlo method particle transport simulation platform 1 set.
3. The method for calculating the CZT detector efficiency based on the effective height of the crystal according to claim 2, wherein: s2, adjusting working parameters of the CZT detector to enable the CZT detector to be in an optimal working state, wherein the working parameters comprise,
Placing a 137 Cs point source on the surface of a CZT detector, and measuring the gamma ray energy spectrum emitted by 137 Cs;
Firstly, fixing a set small signal forming time, gradually adjusting the working voltage of a detector by small increase, simultaneously observing the measured gamma ray energy spectrum, and calculating the half-width of the full-energy peak at 662 keV;
Recording the working voltage when the half-width reaches the minimum; and then, under the working voltage, adjusting the signal forming time, observing the change of the dead time and the full-energy peak half-width of the system, and finally determining an optimal working parameter.
4. A method of calculating CZT detector efficiency based on crystal effective height according to claim 3, wherein: the S3, measuring 241Am、152Eu、137 Cs and 60 Co point source gamma energy spectrum specifically comprises the following steps:
The distance from the point source to the surface of the CZT detector, namely distance from Source to Detector, hereinafter referred to as SD, is regulated by a sample support, under the conditions of SD=1.5 mm, SD=46.5 mm and SD=96.5 mm, gamma energy spectrums emitted by the point sources of 241Am、152Eu、137 Cs and 60 Co are measured by the CZT detector, and the total energy peak count rates of the gamma rays of 59.5keV emitted by 241 Am, 121.78keV emitted by 152 Eu, 244.70keV, 344.28keV, 661.77keV emitted by 137 Cs and 1173.24keV and 1332.50keV emitted by 60 Co are respectively calculated by using energy spectrum analysis software;
During measurement, ensuring that the count of the total energy peaks is more than 10000; the detection efficiency of CZT for 59.5keV, 121.78keV, 244.70keV, 344.28keV, 661.77keV, 1173.24keV and 1332.50keV gamma rays was calculated from the measurement results.
5. The method for calculating the CZT detector efficiency based on the effective height of the crystal according to claim 4, wherein: an efficiency calculation model is built based on step S3, including,
When the sample is placed on the central axis of the detector, calculating the detection efficiency of the CZT to the point source under an ideal model;
That is, the cuboid CZT crystal has the width of x and z, the height of y, the distance between the upper surface of the crystal and the inner surface of the shell is h, the thickness of the shell is a, and the distance between the source and the detector surface is SD.
6. The method for calculating the CZT detector efficiency based on the effective height of the crystal according to claim 5, wherein: s5, taking the nominal parameters of the CZT detector structure as input parameters of an efficiency calculation model, calculating the detection efficiency, comprising,
And (3) taking the nominal parameters of the detector structure as input parameters of an efficiency calculation model, calculating the detection efficiency of the CZT detector on gamma rays with different energies under the conditions of SD=1.5 mm, SD=46.5 mm and SD=96.5 mm, comparing the calculation result with the detection efficiency measured in the step (III), and guiding the parameter adjustment in the next step.
7. The method for calculating the CZT detector efficiency based on the effective height of the crystal according to claim 6, wherein: s6, adjusting CZT body crystal and detector mechanism parameters, and then calculating detection efficiency, wherein the method specifically comprises the steps of,
Taking the nominal parameters of the detector structure as the basis, and continuously adjusting a, h, x, z and y values as input parameters of an efficiency calculation model according to the efficiency comparison result in the step S5 at intervals of 0.1 mm;
Under the conditions of SD=1.5 mm, SD=46.5 mm and SD=96.5 mm, calculating the detection efficiency of the CZT detector on gamma rays with different energies, and comparing the calculation result with the detection efficiency measured in the step S3;
if the deviation between the calculated efficiency result and the measured value is larger than the set value, readjusting a, h, x, z and the numerical value of y to serve as input parameters of the efficiency calculation model;
Repeating the steps until the deviation between the calculated efficiency result and the measured value is not larger than a set value, and recording the numerical values of a, h, x, z and y; at this point y=y 0,y0 will be much smaller than the nominal CZT crystal height, and y 0 is the effective height for CZT crystal detection efficiency calculation.
8. The method for calculating the CZT detector efficiency based on the effective height of the crystal according to claim 7, wherein: s7, finally verifying a calculation model, and calculating the detection efficiency of the CZT detector after the model is determined; the verification model comprises the following steps:
Dripping m 1mL、A1 Bq standard solution from 152 Eu standard solution into a 50mL volumetric flask, preparing 152 Eu reference solution by adding deionized water, loading 10mL reference solution into a polyvinyl chloride sample box with phi of 30mm multiplied by 15mm, and sealing to prepare a reference solution sample;
Placing a reference solution sample on the surface of a CZT detector to measure gamma energy spectrum, and calculating the detection efficiency of the CZT detector on gamma rays with different energies emitted by 152 Eu reference sample according to the measurement result;
Taking the numerical values a, h, x, z and y obtained in the step S6 as input parameters of a detector, inputting sample parameters according to the actual condition of the sample, and establishing a reference sample efficiency calculation model;
Comparing the efficiency curve calculated by the model with the measured efficiency value, and if the relative deviation is not more than 5%, considering that the calculation result of the efficiency is reliable; if the relative deviation is greater than 5%, it is also necessary to repeat step S6.
9. A computer device comprising a memory and a processor, the memory storing a computer program that, when executed by the processor, causes the processor to perform the steps of the method of any of claims 1 to 8.
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