CN115390127A - Fast neutron flux high signal-to-noise ratio monitoring method and system - Google Patents
Fast neutron flux high signal-to-noise ratio monitoring method and system Download PDFInfo
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Abstract
The invention relates to a neutron monitoring method and a system, in particular to a fast neutron flux high signal-to-noise ratio monitoring method and a system, which solve the technical problems that fast neutron monitoring signals usually have the influence of energy spectrum dependence, background radiation interference and fast neutron direct-irradiation noise interference, and the fast neutron flux monitoring technology in the prior art is difficult to realize flat energy response to neutrons with different energies, effectively remove background interference influence and realize fast neutron flux high signal-to-noise ratio monitoring. The fast neutron flux high signal-to-noise ratio monitoring method can achieve the high signal-to-noise ratio of fast neutron monitoring. The invention relates to a fast neutron flux high signal-to-noise ratio monitoring system, which comprises a fissile material, a fast response silicon carbide detector and waveform discrimination equipment; the fissile material is attached to the fast response silicon carbide detector and used for generating a nuclear fission reaction with fast neutrons to generate fission fragments; the fast response silicon carbide detector is connected with the waveform discrimination equipment and used for collecting electric signals generated by fission fragments and inputting the electric signals into the waveform discrimination equipment.
Description
Technical Field
The invention relates to a neutron monitoring method and system, in particular to a fast neutron flux high signal-to-noise ratio monitoring method and system.
Background
The energy range of fast neutrons is 0.1-20 MeV, and the real-time accurate monitoring of fast neutron flux is very important for researching the nuclear reaction process.
In the diagnosis of the mixed pulse radiation field, the high signal-to-noise ratio monitoring of the conventional fast neutron flux has two problems: firstly, fast neutrons have wide energy distribution, fast neutrons with different energies can generate signals with different amplitudes in a monitoring system generally, but fast neutron flux only needs to monitor the number of the fast neutrons in a unit space, so that the fast neutron flux monitoring method must solve the dependence problem of the fast neutron energy spectrum of the monitoring method; secondly, background interference such as gamma rays and the like is usually accompanied in a fast neutron radiation environment, so that the direct monitoring efficiency of fast neutrons is low, and the fast neutron radiation environment needs to be converted into charged substance monitoring to realize.
Disclosure of Invention
The invention aims to solve the technical problems that fast neutron flux monitoring technology in the prior art is difficult to realize flat energy response to neutrons with different energies, effectively removes background interference influence and realizes fast neutron flux high signal-to-noise ratio monitoring, and the fast neutron flux high signal-to-noise ratio monitoring method and system realize high signal-to-noise ratio of fast neutron conditioning monitoring.
In order to solve the technical problems, the technical scheme adopted by the invention is as follows:
a fast neutron flux high signal-to-noise ratio monitoring method is characterized by comprising the following steps:
1) The fast neutrons and fissile materials are used for generating nuclear fission reaction to release fission fragments;
2) Fission fragments enter a quick response silicon carbide detector and generate analog electric signals; the half width of the response time of the fast response silicon carbide detector is 0.1-20 ns;
3) Inputting the analog electric signal obtained in the step 2) into waveform discrimination equipment for waveform discrimination;
3.1 Utilizing a waveform discrimination device to convert an analog electrical signal output by the fast response silicon carbide detector into a digital electrical signal;
3.2 Interpolating a waveform of the digital electric signal and extracting a background noise of the interpolated digital electric signal;
3.3 Subtracting the background noise of the digital electric signal interpolated in the step 3.2) by utilizing waveform discrimination equipment to obtain background noise-removed waveform data;
3.4 Carrying out data processing on the bottom noise-removed waveform data obtained in the step 3.3) to obtain at least two waveform discrimination parameters of a time domain and/or a frequency domain;
3.5 At least two waveform discrimination parameters of time domain and/or frequency domain are used for carrying out waveform statistics and discriminating ray information, thereby realizing fast neutron flux monitoring.
Further, the interpolation in step 3.2) is specifically:
inserting N-1 digital interpolation points by a sinc interpolation or linear interpolation method, wherein N is an integer greater than or equal to 1.
Further, the step 3.4) is specifically as follows:
3.4.1 Carrying out peak searching on the waveform data without the bottom noise to obtain two time domain waveform discrimination parameters; the time domain waveform discrimination parameters comprise waveform half-height width and waveform 2/3 height width;
3.4.2 Carrying out Fourier transform or wavelet transform on the waveform data without the bottom noise to obtain two frequency domain waveform discrimination parameters; the frequency domain waveform discrimination parameters comprise frequency gradient and frequency component power;
step 3.4.1) is performed in any order or simultaneously with step 3.4.2).
Further, the using of at least two waveform discrimination parameters in step 3.5) is specifically:
and combining the time domain waveform discrimination parameters or the frequency domain waveform discrimination parameters or the time domain waveform discrimination parameters and the frequency domain waveform discrimination parameters for use.
Further, in step 1), the fissile material is specifically:
when the fissile material is 238 When U is adopted, the fission threshold energy of fast neutrons is more than or equal to 1.5MeV;
when the fissile material is 237 Np, the fission threshold energy of the fast neutron is more than or equal to 0.4MeV;
the thickness of fissile material is less than or equal to 3mg cm -2 ；
Fission fragments are alpha rays and gamma rays.
Meanwhile, the invention also provides a fast neutron flux high signal-to-noise ratio monitoring system, which is used for realizing the fast neutron flux monitoring method and is characterized in that: the device comprises a fissile substance, a fast response silicon carbide detector and waveform discrimination equipment;
the fissile material is attached to the fast response silicon carbide detector and used for generating a nuclear fission reaction with fast neutrons to generate fission fragments; the half width of the response time of the fast response silicon carbide detector is 0-20 ns;
the fast response silicon carbide detector is connected with the waveform screening device and used for collecting fission fragments and generating electric signals, and the waveform screening device is used for carrying out waveform screening according to input analog electric signals.
Furthermore, the waveform screening device comprises an adjustable gain amplifier, a stepping adjustable attenuator, a high-speed analog-digital converter and a field programmable gate array which are connected in sequence;
the input end of the adjustable gain amplifier is connected with the fast response silicon carbide detector;
the field programmable gate array is used for waveform discrimination and is connected with an external storage device.
Furthermore, the system also comprises an upper computer connected with the field programmable gate array.
Further, the fissile material is 238 U or 237 Np, thickness of fissile material is less than or equal to 3mg cm -2 ；
The fast response silicon carbide detector is a junction type semiconductor detector, and the thickness of a dead layer is 0.05-2 mu m;
the sampling rate of the waveform discrimination equipment is more than or equal to 2GS/s, the vertical resolution is more than or equal to 12 bits, the analog bandwidth is more than or equal to 500MHz, the recording length is more than or equal to 100k points, and the recording time is more than or equal to 50 mu s.
Further, the fast response silicon carbide detector is a Schottky type or PIN type detector.
Compared with the prior art, the technical scheme of the invention has the beneficial effects that:
(1) The method can realize high signal-to-noise ratio monitoring of fast neutron flux. The average kinetic energy of fission fragments is about 60MeV and 90MeV, which is much higher than that of secondary alpha rays (several MeV) and accompanying gamma rays (less than 1 MeV) of spontaneous fission of nuclear materials, so that the method is favorable for realizing high signal-to-noise ratio; the waveform Discrimination equipment is based on a waveform Discrimination (PSD) technology, utilizes the time characteristics of fission fragments and the time characteristics of alpha rays and gamma rays generated by nuclear fission reaction to be different (the response time of a fast response silicon carbide detector to the fission fragments is slower than the response time of reaction products such as (n, p), (n, alpha), (n, 3 alpha) and the like to Si nucleus and C nucleus, the latter is noise influencing the monitoring result of fast neutron flux, the interference of the latter can be eliminated by selecting the PSD technology), the further elimination of the related noise of the latter is realized, and the extremely high signal-to-noise ratio of the fast neutron flux monitoring is obtained.
(2) In the method, the fission fragments and the gamma rays have different ionization densities in the fast response silicon carbide detector, so that the fast response silicon carbide detector has different time response characteristics to the fission fragments and the gamma rays, wherein the time response characteristics of the fission fragments and the gamma rays are slow, and the time response characteristics of the gamma rays are fast. The method is based on the principle of realizing fast neutron and gamma signal discrimination by PSD technology. The PSD technology can be used for effectively eliminating the interference of gamma rays, the PSD technology is developed towards the direction of digital in an n (fast neutron)/gamma mixed field at present, namely, a high-speed waveform discrimination device is used for recording current pulses (analog electric signals) output by a fast response silicon carbide detector, and the PSD technology is used for carrying out n/gamma resolution measurement, so that higher counting rate and better discrimination effect can be obtained.
(3) The method adopts the time domain waveform discrimination parameters or the frequency domain waveform discrimination parameters or the combination of the time domain waveform discrimination parameters and the frequency domain waveform discrimination parameters. The method carries out peak searching on the waveform data without the bottom noise, and can be used for different application scenes such as n/gamma, n/alpha and the like according to different waveform discrimination parameters. The time domain waveform discrimination parameters discriminate the waveform according to the front edge and the back edge of the waveform, namely the time-domain characteristics such as the rise time, the fall time and the like, and the waveform is visual and has early development and mature theory and hardware system. Common methods include rise time method, zero-crossing time method and electricityCharge comparison method, pulse gradient analysis method, etc. And carrying out Fourier transform or wavelet transform on the bottom noise-removed waveform data, namely a frequency domain method, and utilizing the frequency domain waveform discrimination parameters to maximize the difference of the bottom noise-removed waveform data after the transform. The different waveform discrimination methods are that waveform discrimination parameters corresponding to each bottom noise-removed waveform data are calculated according to the waveform discrimination parameters, and ray discrimination information is carried out according to the waveform discrimination parameters, so that the ray event rate can be generally 10 6 Around/s.
(4) The method can realize threshold regulation of the fast neutron monitoring range. Selecting 238 U、 237 Np and other fissile materials with low-energy fast neutron fission threshold energy hardly respond to fast neutrons with energy below the fission threshold energy during fast neutron flux measurement, and the monitoring of the fast neutron flux or the number of the fast neutrons with energy above the fission threshold energy is realized.
(5) The method of the present invention adjusts neutron response sensitivity by adjusting the thickness of the fissile material. Within a certain thickness range, the thickness of the fissile material is properly increased, and the response sensitivity of fast neutrons is improved.
(6) The system can realize long-term reliable work. The fast response silicon carbide detector is selected to realize the effective conversion of fission fragment energy to analog electric signals, has the fast neutron irradiation resistance and the anti-deformation fragment irradiation resistance which are higher than the traditional silicon detector by more than 4 magnitude levels, is less prone to the problems of performance degradation or failure when the radiation dose is higher, and can stably work for a long time.
(7) In the system, a fast response silicon carbide detector and a waveform discrimination device are selected, noise is suppressed by utilizing a PSD technology, a 4H-SiC-based homoepitaxial material and a PIN type detector are selected, fast time response with the half width of response time less than 20ns and high charge collection efficiency of nearly 100 percent can be obtained, the energy of fission fragments caused by incident neutrons is effectively converted into electric signals, and the efficient monitoring of fast neutron flux is realized.
(8) Due to fissile material 238 The fission cross section of U has small fluctuation in the fast neutron energy region, and the average kinetic energy of fission products has little relation with the fast neutron, so the invention selects the fission method (nuclear fission reaction) and passes (secondary)The fast neutron flux information is obtained by monitoring the fission fragments, the flat energy response to the fast neutrons is realized, and the problem of dependence of fast neutron monitoring on an energy spectrum can be effectively solved.
(9) The system is of an all-solid-state structure, the fast neutron flux monitoring result is slightly influenced by the ambient temperature and the atmospheric pressure, the working bias of the system is low (0-600V), and the system can realize long-term stable work by benefiting from the good anti-irradiation performance of the fast response silicon carbide detector.
Drawings
FIG. 1 is a flow chart of the fast neutron flux high SNR monitoring method of the present invention.
FIG. 2 is a flow chart of waveform discrimination in an embodiment of the fast neutron flux high signal-to-noise ratio monitoring method.
FIG. 3 is a response characteristic diagram of a fast response silicon carbide detector in an embodiment of the invention for A and B, wherein A represents a fission fragment response characteristic waveform diagram, and B represents a gamma-ray response characteristic waveform diagram.
Fig. 4 is a fast neutron response fission spectrogram obtained by screening a C curve and a D curve respectively in the embodiment of the present invention, where the C curve represents the fast neutron response fission spectrogram obtained without selecting the PSD technology, and the D curve represents the fast neutron response fission spectrogram obtained with selecting the PSD technology.
Fig. 5 is a schematic diagram of inserting digital interpolation points into actual sampling points according to an embodiment of the present invention, where the horizontal axis represents time and the vertical axis represents amplitude.
Fig. 6 is a schematic structural diagram of the principle of the fast neutron flux high signal-to-noise ratio monitoring system according to the embodiment of the present invention.
FIG. 7 is a schematic time response diagram of a fast response silicon carbide detector in an embodiment of the present invention, where t r Indicating a response rise time of 2.41ns d Indicating a response fall time of 7.02ns FWHM Indicating a response full width at half maximum of 6.93ns.
Fig. 8 is a schematic diagram of a result of a CCE test of a thick-sensitive-region fast-response silicon carbide detector according to an embodiment of the present invention.
FIG. 9 is a schematic diagram of a fast response silicon carbide detector in comparison with a PIN detector for detecting fission fragments in an embodiment of the invention.
Detailed Description
The technical solutions of the present invention will be described clearly and completely with reference to the accompanying drawings, and it should be understood that the described embodiments are some, but not all embodiments of the present invention. All other embodiments obtained by those skilled in the art without creative efforts based on the technical solutions of the present invention belong to the protection scope of the present invention.
As shown in fig. 1, the method for monitoring fast neutron flux with high signal-to-noise ratio of the invention comprises the following steps:
1) The fission reaction is carried out between fast neutrons and fissile materials to release fission fragments;
2) The fission fragments enter a fast response silicon carbide detector and generate analog electric signals; the half width of the response time of the fast response silicon carbide detector is 0.1-20 ns;
3) Inputting the analog electric signal obtained in the step 2) into waveform discrimination equipment;
3.1 As shown in fig. 2), an analog electrical signal output by the fast-response silicon carbide detector is converted into a digital electrical signal by a waveform discrimination apparatus;
3.2 Interpolating a waveform of the digital electric signal and extracting a background noise of the interpolated digital electric signal;
inserting N-1 digital interpolation points by a sinc interpolation or linear interpolation method, wherein N is an integer greater than or equal to 1.
3.3 Subtracting the background noise of the digital electric signal interpolated in the step 3.2) by utilizing waveform discrimination equipment to obtain background noise-removed waveform data;
3.4 Carrying out data processing on the bottom noise-removed waveform data obtained in the step 3.3) to obtain at least two waveform discrimination parameters of a time domain and/or a frequency domain;
3.4.1 Carrying out peak searching on the waveform data without the bottom noise to obtain two time domain waveform discrimination parameters; the time domain waveform discrimination parameters comprise a Full Width at Half maximum (FWHM) and a Full Width at 2/3 maximum (FW2/3M);
3.4.2 Carrying out Fourier transform or wavelet transform on the bottom noise-removed waveform data to obtain two frequency domain waveform discrimination parameters; the frequency domain waveform discrimination parameters comprise frequency gradient and frequency component power; step 3.4.1) and step 3.4.2) are performed in any order or simultaneously;
3.5 Time domain waveform discrimination parameters or frequency domain waveform discrimination parameters or the combination of the time domain waveform discrimination parameters and the frequency domain waveform discrimination parameters are adopted for waveform discrimination, statistics and ray information output, so that fast neutron flux monitoring is realized.
Since fast neutrons are uncharged, direct detection efficiency is very low, and efficient fast neutron detection is usually achieved by conversion to secondary charged species. The number of fission fragments after the nuclear fission reaction is initiated by the detection of the fast neutrons is selected to obtain the fast neutron flux, so that the dependence on the fast neutron energy spectrum can be effectively eliminated, and the fast neutron flux information can be more accurately obtained. The fissile material in the process of the invention is 238 U and 237 np, and 238 the fission threshold energy of U is greater than or equal to 1.5MeV, 237 the fission threshold energy of Np is greater than or equal to 0.4MeV. The ray information includes the type, quantity and energy. Fission fragments are high-energy charged substances, and the energy sum of the two fragments can reach more than 150 MeV; fission fragments are usually associated with alpha rays and gamma rays (i.e., emit alpha particles and gamma rays).
As shown in fig. 3 and 4, in this embodiment, the waveform discrimination apparatus is based on a PSD technique, and the PSD technique is based on the principle that an α -ray and a γ -ray in fast neutron measurement are rejected by using a time-domain waveform discrimination parameter or a combination of the time-domain waveform discrimination parameter and a frequency-domain waveform discrimination parameter; the time domain waveform discrimination parameters, the frequency domain waveform discrimination parameters or the combination of the time domain waveform discrimination parameters and the frequency domain waveform discrimination parameters can be used for removing alpha rays and gamma rays in fast neutron measurement in real time on line through the FPGA; alpha rays and gamma rays in the fast neutron measurement can be removed off line through the upper computer. The PSD technology can be used for effectively eliminating the interference of gamma rays, the PSD technology is developed towards the direction of digital in the current n/gamma mixed field, namely, high-speed waveform discrimination equipment is used for recording the current pulse waveform output by the fast-response silicon carbide detector, and the PSD technology is used for carrying out n/gamma discrimination measurement so as to obtain higher counting rate and better discrimination effect. As can be seen from fig. 4, curve C is the neutron flux monitoring result without PSD selection, and curve D is the fast neutron flux monitoring result with PSD selection according to the present invention. The C curve has strong response of a low-energy part, and the measurement results of noise and low-energy fission fragments are difficult to distinguish effectively, so that the accurate acquisition of the number of the fission fragments is influenced, and the effective extraction of the fast neutron flux information is further influenced. In the curve D, by using the PSD technology, a saddle-shaped peak of fission fragments initiated by fast neutrons is clear, low-energy noise is hardly visible, and accurate fast neutron flux information can be extracted from the saddle-shaped peak.
According to typical domain waveform discrimination parameters and frequency domain waveform discrimination parameters, the discrimination methods of the waveform discrimination apparatus can be divided into a time domain method and a frequency domain method. Different screening methods are used for different application scenes, such as n/gamma, n/alpha and the like. The time domain method distinguishes the waveform according to the front edge (rising time, falling time), the back edge (rising time, falling time) and other time domain characteristics of the waveform, and the method is visual and has early development and mature theory and hardware system. The method is commonly used in a rise time method, a zero-crossing time method, a charge comparison method, a pulse gradient analysis method and the like. The frequency domain method performs Fourier transform (or wavelet transform) on the waveform, and uses the waveform discrimination parameters of the frequency domain to maximize the difference of the waveform after the transform. According to different waveform discrimination methods, waveform discrimination parameters corresponding to each background noise-removed waveform data are calculated according to waveform discrimination parameters, and then ray types are discriminated according to the waveform discrimination parameters, wherein the ray event rate is generally 10 6 Around/s.
Preferably, carrying out peak searching on the waveform data without the bottom noise to obtain two time domain waveform discrimination parameters; the time domain waveform discrimination parameters comprise waveform half-height width and waveform 2/3 height width; carrying out Fourier transform (or wavelet transform) on the waveform data without the bottom noise to obtain two frequency domain waveform discrimination parameters; the frequency domain waveform discrimination parameters comprise frequency gradient and frequency component power; and by combining the half-height width of the waveform and the 2/3 height width of the waveform (namely the obtained two time domain waveform discrimination parameters), alpha rays and gamma rays in fast neutron measurement are eliminated. And eliminating alpha rays and gamma rays in the fast neutron measurement by combining and using the frequency gradient and the frequency component power (namely the obtained two frequency domain waveform discrimination parameters) in an AND mode. Certainly, the method is not limited to the waveform discrimination parameters of two frequency domains and the waveform discrimination parameters of two time domains, and those skilled in the art can flexibly set the parameters according to the needs.
As shown in fig. 5, according to the ray information, 7 digital interpolation points are inserted into the bottoming noise waveform data by using sinc interpolation (or a linear interpolation method) through a priori knowledge, so that the sampling rate and the vertical resolution are both improved by 8 times, and the sampling time interval is reduced by 8 times.
The trend in PSD technology and digital processing in waveform discrimination devices is as follows:
1. high-speed high-resolution data acquisition technology. The indexes of the waveform discrimination equipment have direct influence on measuring analog electric signals, and the most important indexes comprise sampling rate, vertical resolution and storage length; the sampling rate and the vertical resolution affect the discrimination effect of the analog electric signals, and the storage length determines the duration time of the measurable radiation field. In the measurement of fast neutrons, in order to obtain an optimal measurement effect, an ideal waveform discrimination device should have the series characteristics of high sampling rate and high bandwidth of a digital oscilloscope, high quantization precision and large storage length of a digitizer, and the like.
2. Data processing techniques. The method mainly comprises the following steps:
(1) Hardware digital signal processing based on Field Programmable Gate Array (FPGA) technology, and a parallel computing structure of the FPGA based on hardware is fully utilized, so that the data processing capability is improved. The method has the advantages that the method can be embedded into waveform discrimination equipment, is expected to realize real-time calculation of a PSD technology, and has advantages in application requiring real-time acquisition of measurement results.
(2) Graphics Processing Unit (GPU) based highly parallel data Processing techniques. The GPU is parallel computing hardware with a plurality of processor units, and off-line analysis is achieved on an upper computer. It is expected to greatly improve the data processing capability by utilizing its advantages in parallel data processing. The method has the advantages that when a large amount of stored original data are processed, the calculation speed and the discrimination time effectiveness of the PSD technology can be obviously improved, and higher efficiency can be obtained.
As shown in fig. 6, the invention also provides a fast neutron flux high signal-to-noise ratio monitoring system for implementing the method, which includes a nuclear fission substance, a fast response silicon carbide detector and a waveform discrimination device; the fissile material is attached to the fast response silicon carbide detector and used for generating a nuclear fission reaction with fast neutrons to generate fission fragments; the half width of the response time of the fast response silicon carbide detector is 0.1-20 ns; the fast response silicon carbide detector is connected with the waveform screening device and used for collecting fission fragments and generating electric signals, and the waveform screening device is used for carrying out waveform screening according to input analog electric signals.
In this embodiment, the waveform discrimination apparatus includes an adjustable gain amplifier, a step adjustable attenuator, a high-speed analog-to-digital converter, and a field programmable gate array, which are connected in sequence; the input end of the adjustable gain amplifier is connected with the fast response silicon carbide detector; at the moment, the field programmable gate array is used as a data processing center, the waveform is processed on the field programmable gate array, the waveform is screened and counted to obtain ray information, and then the ray information is transmitted to an external storage device for storage (the external storage device can also adopt an upper computer in the embodiment), so that the real-time online screening of the field programmable gate array is realized; in addition, the waveform discrimination equipment can also comprise a field programmable gate array, an adjustable gain amplifier communicated with the field programmable gate array, a stepping adjustable attenuator, a high-speed analog-digital converter and an upper computer; the input end of the adjustable gain amplifier is connected with the fast response silicon carbide detector; at the moment, the field programmable gate array is only used as a carrier for statistics and communication, offline discrimination is carried out on an upper computer, and the method can be suitable for various application scenes.
The invention can select fissile materials with different thresholds according to monitoring requirements, realizes fast neutron flux monitoring with the thresholds, and selects the fissile materials as the fissile materials 238 U and 237 and Np, the fission threshold energy of the fast neutron is respectively more than or equal to 1.5MeV and more than or equal to 0.4MeV, and when the energy of the fast neutron is lower than the fission threshold energy, the fast neutron is hardly monitored. Selecting 238 U、 237 The fissile materials such as Np and the like with the low-energy fast neutron threshold characteristics can hardly respond to neutrons with energy below the threshold when neutron flux measurement is realized, and the monitoring of the neutron flux or the number of the neutrons with energy above the fission threshold is realized. The invention can also adjust the response sensitivity of fast neutrons by adjusting the thickness of fissile materials. However, the maximum thickness of fissile material generally must not be too thick, and is generally less than 3mg cm -2 . Within a certain thickness range, increasing the thickness of the fission target properly will increase the sensitivity of the system of the present invention to fast neutron response. When the fissile material is too thick, the sensitivity of the monitoring system will not increase if the fissile material thickness is increased further. The fissile material in this example was 2 mg-cm thick -2 The fast neutron monitoring device is attached to the surface of a fast response silicon carbide detector through a spin coating (or deposition) process, so that the fast neutron monitoring device can realize threshold value regulation of a fast neutron monitoring range.
The performance indexes of the fast response silicon carbide detector are as follows: the radiation is converted into an analog electrical signal. The fast response silicon carbide detector is a junction type semiconductor detector with a thin dead layer, wherein the thickness of the dead layer is 2 microns, and the thinner the thickness is, the better the thickness is; the fast response silicon carbide detector in this embodiment is preferably a PIN-type detector (which may also be a schottky-type detector) with < 10% resolution for alpha ray energy. As shown in fig. 7 and 8, in this embodiment, a single fast-response silicon carbide detector is selected to implement fast neutron flux monitoring, so that the system of the present invention works reliably for a long time. As can be seen in FIG. 7, t r Indicating a response rise time of 2.41ns d Indicating a response fall time of 7.02ns FWHM Indicating a response full width at half maximum of 6.93ns; it can be seen from fig. 8 that the fast-response silicon carbide detector in the thick sensitive region can realize the effective conversion of fission fragment energy to an electrical signal, has the fast neutron irradiation resistance and the anti-fission fragment irradiation resistance which are higher than those of the conventional silicon detector by more than 4 orders, and when the radiation dose is higher, the fast-response silicon carbide detector is less prone to performance degradation or detector failure, and can stably work for a long time.
Performance indexes of the waveform discrimination apparatus: sampling rate is more than or equal to 2GS/s, vertical resolution is more than or equal to 12 bits, analog bandwidth is more than or equal to 500MHz, and recordingThe length is 100k points or more (recording time is 50. Mu.s or more). The waveform discrimination equipment has a function of adjustable Gain (VG), realizes the amplification or attenuation function of a single-ray signal, and adaptively adjusts the amplitude of an input typical fast neutron signal. As shown in fig. 9, a comparative schematic of a fast response silicon carbide detector and a PIN detector for detecting fission fragments is shown. The waveform discrimination equipment used in the invention converts the electric signal into a digital signal, and the sampling rate is more than or equal to 2GS/s (2 multiplied by 10) 9 /s) for event rates below 10 8 A count-type diagnostic system of/s (single event minimum count 20 points); the analog bandwidth is more than or equal to 500MHz, and the signal of the leading edge/trailing edge of 1ns can be accurately recorded without distortion; the vertical resolution is more than or equal to 12 bits, and the measurement precision of the signal amplitude reaches one thousandth; the recording length is more than or equal to 100k points (the recording time is more than or equal to 50 mu s), and the number of the one-time most effective events reaches 5k. As can be seen in fig. 9, the fast response silicon carbide probe is very consistent with the fission fragment measurement of the silicon PIN probe (which is the industry gold standard). However, the silicon PIN detector has poor radiation resistance (after being irradiated by fast neutrons and fission fragments, the performance is degraded quickly and is easy to damage), so that the silicon PIN detector is not suitable for being combined with a fissile material + PSD technology to manufacture a neutron flux monitoring system. The quick response silicon carbide detector adopted in the method realizes the same detection effect as a gold standard silicon PIN detector, and simultaneously solves the problem of long-term reliable work.
In this embodiment, the waveform screening apparatus employs a field programmable gate array as a large-capacity data signal processing core device of the waveform screening apparatus. By utilizing abundant Digital logic resources, the Digital signals output by an Analog-to-Digital Converter (ADC) can be processed on line in real time, and the Digital signals can be subjected to time domain/frequency domain signal processing such as interpolation, filtering, fourier transform and the like. The waveform of the digital electric signal is interpolated, and (N-1) digital interpolation points are inserted between two actual sampling points through sinc interpolation (or linear interpolation and other methods) between the actual sampling digital interpolation points, so that the sampling rate can be improved by N times, the sampling time interval is reduced by N times, and the measurement precision of the time interval in waveform discrimination is effectively improved.
In the embodiment, two or more waveform discrimination parameters are adopted, and the interference of alpha rays and gamma rays is effectively eliminated in the fast neutron measurement through the AND relation. The time domain waveform screening parameters and the frequency domain waveform screening parameters are preferably used in a combined manner, and in other embodiments, the time domain waveform screening parameters or the frequency domain waveform screening parameters can also be adopted to reject alpha rays and gamma rays in fast neutron measurement. When two discrimination parameters of the half-height width of the waveform and the 2/3 height width of the waveform are combined and used in an AND mode, compared with a discrimination method, the event rejection rate is improved by more than 30%, and the interference of alpha rays and gamma rays in fast neutron measurement is effectively rejected. The method can be used for real-time online screening on the FPGA, can also be used for offline screening on an upper computer, and can be suitable for various application scenes.
The average kinetic energy of fission fragments is about 60MeV and 90MeV, which is much higher than that of secondary alpha rays (several MeV) and accompanying gamma rays (less than 1 MeV) of spontaneous fission of nuclear materials, so that the method of the invention is favorable for realizing high signal-to-noise ratio; the waveform discrimination method utilizes the time characteristics of fission fragments to be different from the time characteristics of alpha rays and gamma rays generated by nuclear fission reaction (the response time of a fast response silicon carbide detector to the fission fragments is slower than the response time of reaction products of (n, p), (n, alpha), (n, 3 alpha) and the like of a Si nucleus and a C nucleus, the latter is noise influencing a fast neutron flux monitoring result, the interference of the latter can be eliminated by selecting a waveform discrimination technology), the further elimination of the related noise of the latter can be successfully realized, and the extremely high fast neutron flux monitoring signal-to-noise ratio is obtained. The fissile material, the fast response silicon carbide detector and the waveform discrimination device in the embodiment are solid, so that the fissile material, the fast response silicon carbide detector and the waveform discrimination device are not influenced by temperature and air pressure when in use.
The working principle of the fast neutron flux high signal-to-noise ratio monitoring is as follows:
in a mixed pulse radiation field, the invention is based on waveform discrimination equipment, and fast neutron flux measurement with high signal-to-noise ratio can be obtained by adopting a fast response silicon carbide detector. The fast neutrons and fissile materials are used for generating nuclear fission reaction to release fission fragments; fission fragments enter a quick response silicon carbide detector and generate analog electric signals; the obtained analog electric signal is input into a waveform discrimination device to finish waveform discrimination (namely PSD technology), and fast neutron flux monitoring is realized.
In the waveform discrimination equipment, the functions of amplification/attenuation, digital processing and the like of analog electric signals output by the fast response silicon carbide detector are realized, and the amplitude of the input analog electric signals is adjusted by adopting an adjustable gain amplifier and a step adjustable attenuator in the waveform discrimination equipment. The high-speed analog-digital converter in the waveform discrimination equipment is adopted to realize the digitization of the analog electric signal, and a digital electric signal is obtained. In order to obtain high-precision waveform discrimination parameters, data processing needs to be performed on the digital electric signals.
Due to fissile material 238 The fission cross section of the U has small fluctuation in a fast neutron energy area, and the average kinetic energy of fission products has little relation with fast neutrons, so that a fission method is selected, fast neutron flux information is obtained through monitoring of secondary fission fragments, flat energy response to the fast neutrons is realized, and the problem of dependence of fast neutron monitoring on an energy spectrum can be effectively solved. A fast-response silicon carbide detector with fast time response and high charge collection efficiency is selected, and the key of the success of a noise suppression method based on the PSD technology is the waveform discrimination equipment. At present, a PIN type detector based on a thick sensitive region of a 4H-SiC homoepitaxial material can obtain a fast time response with the half width of the response time less than 20ns and high charge collection efficiency of nearly 100 percent. The fission fragments and the gamma rays have different ionization densities in the fast response silicon carbide detector, so that the fast response silicon carbide detector has different time response characteristics to the fission fragments and the gamma rays, wherein the time response of the fission fragments is slow, and the time response of the gamma rays is fast. The method is based on the principle of realizing fast neutron and gamma signal discrimination by PSD technology.
Firstly, the digital electrical signal is interpolated to improve the time resolution and the vertical amplitude resolution of the digital electrical signal, and generally, 7 digital interpolation points are inserted by a sinc interpolation or linear interpolation method according to the priori knowledge of the output waveform corresponding to the ray information. And secondly, performing background noise removal processing on the digital electric signal, generally adopting a method of subtracting the average value of the waveform baseline from the whole digital electric signal, and obtaining the waveform data with the background noise removed. Secondly, time domain/frequency domain processing is carried out on the waveform data with the background noise removed to obtain waveform discrimination parameters, for example, in order to obtain parameters such as the full width at half maximum of the waveform and the width at 2/3 height of the waveform of the time domain waveform discrimination parameters, peak searching processing needs to be carried out on the waveform data with the background noise removed, namely, the maximum amplitude in the waveform data with the background noise removed is searched through an off-line or on-line real-time method, and the FWHM and FW2/3M parameters are obtained according to the maximum amplitude of the waveform. And finally, the obtained waveform discrimination parameters are utilized to complete the processing of discrimination, statistics, output and the like of the pulse waveform, thereby completing the waveform discrimination function.
The data processing of the digital electric signals can be realized on line in real time in a field programmable gate array in the waveform discrimination equipment, can also be realized off line on an upper computer, and finally realizes the high-gain measurement of the fast neutron flux based on the PSD technology.
Claims (10)
1. A fast neutron flux high signal-to-noise ratio monitoring method is characterized by comprising the following steps:
1) The fast neutrons and fissile materials are used for generating nuclear fission reaction to release fission fragments;
2) The fission fragments enter a fast response silicon carbide detector and generate analog electric signals; the half width of the response time of the fast response silicon carbide detector is 0.1-20 ns;
3) Inputting the analog electric signal obtained in the step 2) into waveform discrimination equipment for waveform discrimination;
3.1 Utilizing a waveform discrimination device to convert an analog electric signal output by the fast response silicon carbide detector into a digital electric signal;
3.2 Interpolating a waveform of the digital electric signal and extracting a background noise of the interpolated digital electric signal;
3.3 Utilizing waveform discrimination equipment to subtract the background noise of the digital electric signal interpolated in the step 3.2) to obtain background noise-removed waveform data;
3.4 Carrying out data processing on the bottom noise-removed waveform data obtained in the step 3.3) to obtain at least two waveform discrimination parameters of a time domain and/or a frequency domain;
3.5 At least two waveform discrimination parameters of time domain and/or frequency domain are used for carrying out waveform statistics and discriminating ray information, thereby realizing fast neutron flux monitoring.
2. The fast neutron flux high signal-to-noise ratio monitoring method according to claim 1, wherein the interpolation in step 3.2) is specifically:
inserting N-1 digital interpolation points by a sinc interpolation or linear interpolation method, wherein N is an integer which is more than or equal to 1.
3. The fast neutron flux high signal-to-noise ratio monitoring method according to claim 2, wherein the step 3.4) is specifically:
3.4.1 Carrying out peak searching on the waveform data without the bottom noise to obtain two time domain waveform discrimination parameters; the time domain waveform discrimination parameters comprise waveform half-height width and waveform 2/3 height width;
3.4.2 Carrying out Fourier transform or wavelet transform on the waveform data without the bottom noise to obtain two frequency domain waveform discrimination parameters; the frequency domain waveform discrimination parameters comprise frequency gradient and frequency component power;
the step 3.4.1) and the step 3.4.2) are executed in any order or simultaneously.
4. The method for high-signal-to-noise-ratio monitoring of fast neutron flux according to claim 3, wherein the step 3.5) uses at least two waveform discrimination parameters specifically:
time domain waveform discrimination parameters or frequency domain waveform discrimination parameters or the combination of the time domain waveform discrimination parameters and the frequency domain waveform discrimination parameters are adopted for use.
5. The fast neutron flux high signal-to-noise ratio monitoring method according to claim 3, wherein in step 1), the fissile material is specifically:
when the fissile material is 238 When the number of the neutrons is U, the fission threshold energy of the fast neutrons is more than or equal to 1.5MeV;
when the fissile material is 237 Np, the fission threshold energy of the fast neutron is more than or equal to 0.4MeV;
the thickness of the fissile material is less than or equal to 3 mg-cm -2 ；
The fission fragments are high-energy charged substances alpha rays and gamma rays.
6. A fast neutron flux high signal-to-noise ratio monitoring system is used for realizing the fast neutron flux high signal-to-noise ratio monitoring method of claims 1-5, and is characterized in that: the device comprises a fissile substance, a fast response silicon carbide detector and waveform discrimination equipment;
the fissile material is attached to the fast response silicon carbide detector and used for generating a nuclear fission reaction with fast neutrons to generate fission fragments; the half width of the response time of the fast response silicon carbide detector is 0.1-20 ns;
the fast response silicon carbide detector is connected with waveform screening equipment and used for collecting fission fragments and generating analog electric signals, and the waveform screening equipment is used for carrying out waveform screening according to the input analog electric signals.
7. The fast neutron flux high signal-to-noise ratio monitoring system of claim 6, wherein: the waveform discrimination equipment comprises an adjustable gain amplifier, a stepping adjustable attenuator, a high-speed analog-digital converter and a field programmable gate array which are connected in sequence;
the input end of the adjustable gain amplifier is connected with the fast response silicon carbide detector;
the field programmable gate array is used for waveform discrimination and is connected with an external storage device.
8. The fast neutron flux high signal-to-noise ratio monitoring system of claim 6, wherein: the waveform discrimination equipment comprises a field programmable gate array, an adjustable gain amplifier communicated with the field programmable gate array, a stepping adjustable attenuator, a high-speed analog-digital converter and an upper computer;
and the input end of the adjustable gain amplifier is connected with the fast response silicon carbide detector.
9. The fast neutron flux high signal-to-noise ratio monitoring system of any one of claims 6-7, characterized in that:
the fissile material is 238 U or 237 Np, thickness of fissile material is less than or equal to 3mg cm -2 ；
The fast response silicon carbide detector is a junction type semiconductor detector, and the thickness of a dead layer is 0.05-2 mu m;
the sampling rate of the waveform discrimination equipment is more than or equal to 2GS/s, the vertical resolution is more than or equal to 12 bits, the analog bandwidth is more than or equal to 500MHz, the recording length is more than or equal to 100k points, and the recording time is more than or equal to 50 mu s.
10. The fast neutron flux high signal-to-noise ratio monitoring system of claim 9, wherein:
the fast response silicon carbide detector is a Schottky type or PIN type detector.
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