CN115390127B - A fast neutron flux high signal-to-noise ratio monitoring method and system - Google Patents

Abstract

The present invention relates to a neutron monitoring method and system, and specifically to a fast neutron flux high signal-to-noise ratio monitoring method and system, which solves the technical problems that fast neutron monitoring signals usually have energy spectrum dependence, background radiation interference and fast neutron direct noise interference, and that the fast neutron flux monitoring technology in the prior art is difficult to achieve flat energy response to neutrons of different energies, effectively remove the background interference influence and achieve high signal-to-noise ratio monitoring of fast neutron flux. The fast neutron flux high signal-to-noise ratio monitoring method can achieve a high signal-to-noise ratio for fast neutron monitoring. The fast neutron flux high signal-to-noise ratio monitoring system of the present invention includes fission material, a fast response silicon carbide detector and a waveform identification device; the fission material is attached to the fast response silicon carbide detector, which is used to undergo nuclear fission reaction with fast neutrons to generate fission fragments; the fast response silicon carbide detector is connected to the waveform identification device, which is used to collect electrical signals generated by fission fragments and input them into the waveform identification device.

Description

A fast neutron flux high signal-to-noise ratio monitoring method and system

Technical Field

The present invention relates to a neutron monitoring method and system, and in particular to a fast neutron flux high signal-to-noise ratio monitoring method and system.

Background Art

The energy range of fast neutrons is 0.1 to 20 MeV. Real-time and accurate monitoring of fast neutron flux is very important for studying nuclear reaction processes.

In the diagnosis of mixed pulse radiation fields, there are two difficulties in the existing high signal-to-noise ratio monitoring of fast neutron flux: first, the fast neutron energy distribution is wide, and fast neutrons of different energies usually generate signals of different amplitudes in the monitoring system, but the fast neutron flux only needs to monitor the number of fast neutrons per unit space, so the fast neutron flux monitoring method must solve the problem of the monitoring method's dependence on the fast neutron energy spectrum; second, the fast neutron radiation environment is often accompanied by background interference such as gamma rays, resulting in low efficiency of direct fast neutron monitoring, which needs to be converted to charged material monitoring to achieve. Since the fast neutron monitoring signal is usually affected by energy spectrum dependence, background radiation interference, and fast neutron direct illumination noise interference, high signal-to-noise ratio monitoring of fast neutron flux is not easy to achieve.

Summary of the invention

The purpose of the present invention is to solve the technical problems that fast neutron monitoring signals usually have energy spectrum dependence, background radiation interference and fast neutron direct noise interference, and that the fast neutron flux monitoring technology in the prior art is difficult to achieve flat energy response to neutrons of different energies, effectively remove the background interference influence and achieve high signal-to-noise ratio monitoring of fast neutron flux, and to provide a fast neutron flux high signal-to-noise ratio monitoring method and system to achieve a high signal-to-noise ratio of fast neutron conditioning monitoring.

In order to solve the above technical problems, the technical solution adopted by the present invention is:

A fast neutron flux high signal-to-noise ratio monitoring method is characterized in that it comprises the following steps:

1) Using fast neutrons to react with fissionable materials to release fission fragments;

2) The fission fragments enter the fast response silicon carbide detector and generate analog electrical signals; the response time half width of the fast response silicon carbide detector is 0.1 to 20ns;

3) inputting the analog electrical signal obtained in step 2) into a waveform identification device for waveform identification;

3.1) Convert the analog electrical signal output by the fast response silicon carbide detector into a digital electrical signal using a waveform identification device;

3.2) Interpolating the waveform of the digital electrical signal and extracting the background noise of the interpolated digital electrical signal;

3.3) using a waveform identification device to subtract the background noise of the digital electrical signal after interpolation in step 3.2) to obtain background noise-removed waveform data;

3.4) performing data processing on the denoised waveform data obtained in step 3.3) to obtain at least two waveform discrimination parameters in the time domain and/or frequency domain;

3.5) Use at least two waveform discrimination parameters in the time domain and/or frequency domain to perform waveform statistics and discriminate ray information to achieve fast neutron flux monitoring.

Furthermore, the interpolation in step 3.2) is specifically:

By using the sinc interpolation or linear interpolation method, N-1 digital interpolation points are inserted, where N is an integer greater than or equal to 1.

Further, step 3.4) is specifically as follows:

3.4.1) Peak search is performed on the de-noised waveform data to obtain two time domain waveform identification parameters; the time domain waveform identification parameters include the waveform half-height width and the waveform 2/3 height width;

3.4.2) Performing Fourier transform or wavelet transform on the denoised waveform data to obtain two frequency domain waveform discrimination parameters; the frequency domain waveform discrimination parameters include frequency gradient and frequency component power;

Step 3.4.1) and step 3.4.2) may be performed in any order or simultaneously.

Furthermore, the at least two waveform discrimination parameters used in step 3.5) are specifically:

The time domain waveform discrimination parameter or the frequency domain waveform discrimination parameter or the time domain waveform discrimination parameter and the frequency domain waveform discrimination parameter are used in combination.

Further, in step 1), the fissile material is specifically:

When the fission material is ²³⁸ U, the fission threshold energy of fast neutrons is greater than or equal to 1.5 MeV;

When the fission material is ²³⁷ Np, the fission threshold energy of fast neutrons is greater than or equal to 0.4 MeV;

The thickness of fissile material is less than or equal to 3 mg·cm ^-2 ;

The fission fragments are alpha rays and gamma rays.

At the same time, the present invention also provides a fast neutron flux high signal-to-noise ratio monitoring system for implementing the above-mentioned fast neutron flux monitoring method, which is special in that it includes fission material, a fast response silicon carbide detector and a waveform identification device;

The fission material is attached to the fast response silicon carbide detector and is used to undergo nuclear fission reaction with fast neutrons to generate fission fragments; the response time half width of the fast response silicon carbide detector is 0 to 20 ns;

The fast response silicon carbide detector is connected to a waveform identification device for collecting fission fragments and generating electrical signals. The waveform identification device is used to perform waveform identification according to the input analog electrical signal.

Further, the waveform identification device includes an adjustable gain amplifier, a step-adjustable attenuator, a high-speed analog-to-digital converter and a field programmable gate array connected in sequence;

The input end of the adjustable gain amplifier is connected to a fast-response silicon carbide detector;

The field programmable gate array is used for waveform recognition and to interface with external storage devices.

Furthermore, it also includes a host computer connected to the field programmable gate array.

Furthermore, the fissile material is ²³⁸ U or ²³⁷ Np, and the thickness of the fissile material is less than or equal to 3 mg·cm ^-2 ;

The fast-response silicon carbide detector is a junction semiconductor detector with a dead layer thickness of 0.05 to 2 μm;

The sampling rate of the waveform identification equipment is greater than or equal to 2GS/s, the vertical resolution is greater than or equal to 12bit, the analog bandwidth is greater than or equal to 500MHz, the record length is greater than or equal to 100k points, and the recording time is greater than or equal to 50μs.

Furthermore, the fast response silicon carbide detector is a Schottky type or PIN type detector.

Compared with the prior art, the technical solution of the present invention has the following beneficial effects:

(1) The method of the present invention can achieve high signal-to-noise ratio monitoring of fast neutron flux. The average kinetic energy of the fission fragments is about 60 MeV and 90 MeV, which is much higher than the secondary α rays (several MeV) and accompanying γ rays (less than 1 MeV) of spontaneous fission of nuclear materials, which is conducive to the high signal-to-noise ratio of the method of the present invention; the waveform discrimination device of the present invention is based on the waveform discrimination (Pulse Shape Discrimination, PSD) technology, and uses the time characteristics of fission fragments, which are different from the time characteristics of α rays and γ rays produced by nuclear fission reactions (the response time of the fast response silicon carbide detector to fission fragments is slower than the response time to the reaction products of Si nuclei and C nuclei (n, p), (n, α), (n, 3α), etc. The latter is the noise that affects the fast neutron flux monitoring results. The selection of PSD technology can eliminate the interference of the latter), so as to further eliminate the latter related noise and obtain an extremely high fast neutron flux monitoring signal-to-noise ratio.

(2) In the method of the present invention, the fission fragments and gamma rays produce different densities of ionization in the fast response silicon carbide detector, which leads to different time response characteristics of the fast response silicon carbide detector to fission fragments and gamma rays, the former has a slow time response and the latter has a fast time response. This is based on the principle of fast neutron and gamma signal discrimination based on PSD technology. The use of PSD technology can effectively eliminate the interference of gamma rays. At present, in the n (fast neutron)/gamma mixed field, PSD technology is developing in the direction of digitalization, that is, using high-speed waveform discrimination equipment to record the current pulse (analog electrical signal) output by the fast response silicon carbide detector, and using PSD technology to perform n/gamma resolution measurement, which can obtain a higher count rate and better discrimination effect.

(3) The method of the present invention uses time domain waveform discrimination parameters or frequency domain waveform discrimination parameters or a combination of time domain waveform discrimination parameters and frequency domain waveform discrimination parameters. Peak search is performed on the denoised waveform data, that is, the time domain method, and different application scenarios can be targeted according to different waveform discrimination parameters, such as n/γ, n/α, etc. The time domain waveform discrimination parameters distinguish the waveform according to the time domain characteristics such as the leading edge and trailing edge of the waveform, that is, the rise time and the fall time. It is intuitive, developed earlier, and has a mature theory and hardware system. Commonly used methods include the rise time method, the zero crossing time method, the charge comparison method, the pulse gradient analysis method, etc. The denoised waveform data is subjected to Fourier transform or wavelet transform, that is, the frequency domain method, and the frequency domain waveform discrimination parameters are used to maximize the difference of the denoised waveform data after the transformation. Different waveform discrimination methods are based on waveform discrimination parameters, and the waveform discrimination parameters corresponding to each denoised waveform data are calculated. Based on this, the ray information is discriminated, and the ray event rate can be generally around 10 ⁶ /s.

(4) The method of the present invention can realize the threshold control of the fast neutron monitoring range. By selecting fissionable materials with low fast neutron fission threshold energy such as ²³⁸ U and ²³⁷ Np, when measuring the fast neutron flux, the fast neutrons with energy below the fission threshold energy are almost not responded to, and the fast neutron flux or number with energy above the fission threshold energy can be monitored only.

(5) The method of the present invention adjusts the neutron response sensitivity by adjusting the thickness of the fissile material. Within a certain thickness range, appropriately increasing the thickness of the fissile material will increase the fast neutron response sensitivity.

(6) The system of the present invention can achieve long-term reliable operation. The use of fast-response silicon carbide detectors can achieve effective conversion of fission fragment energy into analog electrical signals. It has fast-neutron irradiation resistance and fission fragment irradiation resistance that are more than 4 orders of magnitude higher than traditional silicon detectors. When the radiation dose is high, the fast-response silicon carbide detector is less likely to experience performance degradation or failure, and can work stably for a long time.

(7) In the system of the present invention, a fast-response silicon carbide detector and a waveform discrimination device are selected, the PSD technology is used to suppress noise, and a 4H-SiC homoepitaxial material and a PIN-type detector are selected to obtain a fast time response with a response time half-width of less than 20ns and a high charge collection efficiency of nearly 100%, which can effectively convert the energy of the fission fragments induced by the incident neutrons into electrical signals, thereby realizing effective monitoring of the fast neutron flux.

(8) Since the fission cross section of the fission material ²³⁸ U has small fluctuations in the fast neutron energy region, and the average kinetic energy of the fission products has little to do with the fast neutrons, the present invention selects the fission method (nuclear fission reaction) and obtains fast neutron flux information by monitoring (secondary) fission fragments to achieve a flat energy response to fast neutrons, which can effectively get rid of the problem of fast neutron monitoring's dependence on the energy spectrum.

(9) The system of the present invention is a fully solid-state structure. The fast neutron flux monitoring results are less affected by the ambient temperature and air pressure. The system operating bias voltage is low (0-600V). Benefiting from the good radiation resistance of the fast-response silicon carbide detector, long-term stable operation can be achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a flow chart of a fast neutron flux high signal-to-noise ratio monitoring method according to the present invention.

FIG. 2 is a flow chart of waveform identification in an embodiment of a fast neutron flux high signal-to-noise ratio monitoring method of the present invention.

FIG3 is a schematic diagram of the response characteristics of the fast-response silicon carbide detector to A and B in an embodiment of the present invention, wherein A represents a characteristic waveform diagram of the fission fragment response, and B represents a characteristic waveform diagram of the gamma-ray response.

FIG4 is a fast neutron response fission spectrum obtained after respectively identifying the C curve and the D curve in an embodiment of the present invention, wherein the C curve represents the fast neutron response fission spectrum obtained without selecting the PSD technology, and the D curve represents the fast neutron response fission spectrum obtained by selecting the PSD technology.

FIG5 is a schematic diagram of inserting digital interpolation points into actual sampling points according to an embodiment of the present invention, wherein the horizontal axis represents time and the vertical axis represents amplitude.

FIG6 is a schematic diagram of the principle structure of an embodiment of a fast neutron flux high signal-to-noise ratio monitoring system of the present invention.

7 is a schematic diagram of the time response of the fast response silicon carbide detector in an embodiment of the present invention, wherein t _r represents the response rise time of 2.41 ns, t _d represents the response fall time of 7.02 ns, and t _FWHM represents the response half-maximum width of 6.93 ns.

FIG8 is a schematic diagram of the test results of the charge collection efficiency CCE of the fast-response silicon carbide detector with a thick sensitive area in an embodiment of the present invention.

FIG. 9 is a schematic diagram showing a comparison of the detection of fission fragments by a fast-response silicon carbide detector and a PIN-type detector in an embodiment of the present invention.

DETAILED DESCRIPTION

The technical solution of the present invention will be described clearly and completely below in conjunction with the accompanying drawings. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the technical solution in the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

As shown in FIG1 , a fast neutron flux high signal-to-noise ratio monitoring method of the present invention comprises the following steps:

1) Using fast neutrons to react with fissionable materials to release fission fragments;

2) The fission fragments enter the fast response silicon carbide detector and generate analog electrical signals; the response time half width of the fast response silicon carbide detector is 0.1 to 20ns;

3) inputting the analog electrical signal obtained in step 2) into a waveform identification device;

3.1) As shown in FIG2 , the analog electrical signal output by the fast response silicon carbide detector is converted into a digital electrical signal using a waveform identification device;

3.2) Interpolating the waveform of the digital electrical signal and extracting the background noise of the interpolated digital electrical signal;

By using the sinc interpolation or linear interpolation method, N-1 digital interpolation points are inserted, where N is an integer greater than or equal to 1.

3.3) using a waveform identification device to subtract the background noise of the digital electrical signal after interpolation in step 3.2) to obtain background noise-removed waveform data;

3.4) performing data processing on the denoised waveform data obtained in step 3.3) to obtain at least two waveform discrimination parameters in the time domain and/or frequency domain;

3.4.1) Peak search is performed on the denoised waveform data to obtain two time domain waveform discrimination parameters; the time domain waveform discrimination parameters include waveform half-height width (Full Width at Half Maxima, FWHM) and waveform 2/3 height width (Full Width at 2/3Maxima, FW2/3M);

3.4.2) Perform Fourier transform or wavelet transform on the denoised waveform data to obtain two frequency domain waveform identification parameters; the frequency domain waveform identification parameters include frequency gradient and frequency component power; step 3.4.1) and step 3.4.2) are performed in any order or simultaneously;

3.5) Use time domain waveform identification parameters or frequency domain waveform identification parameters or a combination of time domain waveform identification parameters and frequency domain waveform identification parameters to perform waveform identification, statistics and output ray information to achieve fast neutron flux monitoring.

Since fast neutrons are not charged, direct detection efficiency is very low, and effective fast neutron detection is usually achieved by converting them into secondary charged materials. Selecting the number of fission fragments after detecting fast neutrons to induce nuclear fission reactions to obtain fast neutron flux can effectively get rid of the dependence on fast neutron energy spectrum and obtain fast neutron flux information more accurately. The fission materials in the method of the present invention are ²³⁸ U and ²³⁷ Np, and the fission threshold energy of ²³⁸ U is greater than or equal to 1.5MeV, and the fission threshold energy of ²³⁷ Np is greater than or equal to 0.4MeV. The ray information includes the type, quantity and energy. Fission fragments are high-energy charged materials, and the energy sum of the two fragments can reach more than 150MeV; fission fragments are usually accompanied by α rays and γ rays (i.e., emitting α particles and γ rays).

As shown in Figures 3 and 4, in this embodiment, the waveform discrimination device is based on PSD technology. The principle of PSD technology is to use time domain waveform discrimination parameters or waveform discrimination parameters or a combination of time domain waveform discrimination parameters and frequency domain waveform discrimination parameters to eliminate α rays and γ rays in fast neutron measurement; wherein the time domain waveform discrimination parameters, or frequency domain waveform discrimination parameters, or a combination of time domain waveform discrimination parameters and frequency domain waveform discrimination parameters can eliminate α rays and γ rays in fast neutron measurement in real time online through FPGA; α rays and γ rays in fast neutron measurement can also be eliminated offline through the host computer. PSD technology can effectively eliminate the interference of γ rays. At present, in the n/γ mixed field, PSD technology is developing in the direction of digitalization, that is, using high-speed waveform discrimination equipment to record the current pulse waveform output by the fast response silicon carbide detector, and using PSD technology to perform n/γ resolution measurement, in order to obtain a higher count rate and better discrimination effect. As can be seen from Figure 4, the C curve is the neutron flux monitoring result without selecting PSD, and the D curve is the fast neutron flux monitoring result of selecting the PSD method of the present invention. The response of the low-energy part in the C curve is strong, and the noise is difficult to effectively distinguish from the low-energy fission fragment measurement results, which will affect the accurate acquisition of the number of fission fragments, and then affect the effective extraction of fast neutron flux information. In the D curve, using the PSD technology described in the present invention, the saddle-shaped peak of the fission fragments caused by fast neutrons is very clear, and the low-energy noise is almost invisible, which is conducive to extracting accurate fast neutron flux information therefrom.

According to the typical domain waveform identification parameters and frequency domain waveform identification parameters, the identification methods of waveform identification equipment can be divided into time domain methods and frequency domain methods. Different identification methods are targeted at different application scenarios, such as n/γ, n/α and other scenarios. The time domain method distinguishes the waveform according to the time domain characteristics such as the leading edge (rise time, fall time) and trailing edge (rise time, fall time) of the waveform. This type of method is intuitive, developed earlier, and has a mature theory and hardware system. Commonly used methods include the rise time method, zero crossing time method, charge comparison method, pulse gradient analysis method, etc. The frequency domain method first performs Fourier transform (or wavelet transform) on the waveform, and uses the frequency domain waveform identification parameters to maximize the difference in the waveform after the transformation. Different waveform identification methods are based on the waveform identification parameters, and then calculate the waveform identification parameters corresponding to each denoised waveform data, and then identify the type of ray based on this. The ray event rate is generally around 10 ⁶ /s.

Preferably, the denoised waveform data is peak-searched to obtain two time-domain waveform discrimination parameters; the time-domain waveform discrimination parameters include the waveform half-width and the waveform 2/3 height width; the denoised waveform data is Fourier transformed (or wavelet transformed) to obtain two frequency-domain waveform discrimination parameters; the frequency-domain waveform discrimination parameters include frequency gradient and frequency component power; the waveform half-width and waveform 2/3 height width (i.e., the two obtained time-domain waveform discrimination parameters) are combined in an "AND" manner to eliminate alpha rays and gamma rays in fast neutron measurement. The frequency gradient and frequency component power (i.e., the two obtained frequency-domain waveform discrimination parameters) are combined in an "AND" manner to eliminate alpha rays and gamma rays in fast neutron measurement. Of course, it is not limited to the waveform discrimination parameters of the two frequency domains and the waveform discrimination parameters of the two time domains, and those skilled in the art can flexibly set them as needed.

As shown in FIG5 , according to the ray information and through prior knowledge, 7 digital interpolation points are inserted into the denoised waveform data using sinc interpolation (linear interpolation can also be used), thereby increasing the sampling rate and vertical resolution by 8 times and reducing the sampling time interval by 8 times.

The development trends of PSD technology and digital processing in waveform identification equipment are as follows:

1. High-speed and high-resolution data acquisition technology. The indicators of waveform identification equipment have a direct impact on the measurement of analog electrical signals. The most important indicators include sampling rate, vertical resolution and storage length. Among them, sampling rate and vertical resolution affect the effect of analog electrical signal identification, and storage length determines the duration of the radiation field that can be measured. In the measurement of fast neutrons, in order to obtain the best measurement effect, the ideal waveform identification equipment should have the characteristics of high sampling rate and high bandwidth of digital oscilloscopes, high quantization accuracy and large storage length of digitizers.

2. Data processing technology. Mainly includes:

(1) Hardware digital signal processing based on Field Programmable Gate Array (FPGA) technology makes full use of the FPGA's hardware-based parallel computing structure to improve data processing capabilities. The advantage is that it can be embedded in waveform identification equipment and is expected to achieve real-time calculation of PSD technology, which has advantages in applications that require real-time measurement results.

(2) Highly parallel data processing technology based on graphics processing units (GPUs). GPUs are parallel computing hardware with multiple processor units that implement offline analysis on a host computer. By leveraging their advantages in parallel data processing, it is expected that data processing capabilities will be greatly improved. Its advantage is that when processing a large amount of stored raw data, it can significantly improve the calculation speed and identification time of PSD technology, thereby achieving higher efficiency.

As shown in FIG6 , the present invention further provides a fast neutron flux high signal-to-noise ratio monitoring system for implementing the above method, comprising nuclear fission material, a fast response silicon carbide detector and a waveform identification device; the fission material is attached to the fast response silicon carbide detector for undergoing nuclear fission reaction with fast neutrons to generate fission fragments; the response time half-width of the fast response silicon carbide detector is 0.1 to 20 ns; the fast response silicon carbide detector is connected to the waveform identification device for collecting fission fragments and generating electrical signals, and the waveform identification device is used to perform waveform identification according to the input analog electrical signal.

In this embodiment, the waveform identification device includes an adjustable gain amplifier, a step adjustable attenuator, a high-speed analog-to-digital converter and a field programmable gate array connected in sequence; the input end of the adjustable gain amplifier is connected to a fast-response silicon carbide detector; at this time, the field programmable gate array serves as a data processing center, and the waveform is processed on the field programmable gate array, the waveform is identified, counted and the radiation information is obtained, and then the radiation information is transmitted to an external storage device for storage (in this embodiment, the external storage device can also be a host computer), so as to realize real-time online identification of the field programmable gate array; in addition, the waveform identification device can also include a field programmable gate array, an adjustable gain amplifier, a step adjustable attenuator, a high-speed analog-to-digital converter and a host computer that communicate with the field programmable gate array; wherein, the input end of the adjustable gain amplifier is connected to a fast-response silicon carbide detector; at this time, the field programmable gate array is only used as a carrier for statistics and communication, and offline identification is performed on the host computer, which can be applicable to a variety of application scenarios.

The present invention can select fission materials with different thresholds according to monitoring requirements to realize fast neutron flux monitoring with thresholds. When ²³⁸ U and ²³⁷ Np are selected as fission materials, the fast neutron fission threshold energies are respectively greater than or equal to 1.5 MeV and greater than or equal to 0.4 MeV. When the fast neutron energy is lower than the fission threshold energy, it is almost difficult to monitor fast neutrons. ²³⁸ U, ²³⁷ Np and other fission materials with low-energy fast neutron threshold characteristics are selected. When realizing neutron flux measurement, neutrons with energy below the threshold can hardly respond, and only the flux or number of neutrons with energy above the fission threshold energy can be monitored. The present invention can also adjust the fast neutron response sensitivity by adjusting the thickness of the fission material. However, the maximum thickness of the fission material is generally not too thick, usually less than 3 mg cm ^-2 . Within a certain thickness range, appropriately increasing the fission target thickness will improve the sensitivity of the system of the present invention to fast neutron response. When the fission material is too thick, if the thickness of the fission material is increased, the sensitivity of the monitoring system will not increase. The fissionable material in this embodiment has a thickness of 2 mg·cm ^-2 and is attached to the surface of the fast-response silicon carbide detector by a spin coating (or deposition) process, so that the present invention can achieve threshold control of the fast neutron monitoring range.

Performance indicators of fast-response silicon carbide detectors: converting rays into analog electrical signals. The fast-response silicon carbide detector uses a junction semiconductor detector with a thin dead layer, and the dead layer thickness is 2μm, the thinner the better; in this embodiment, the fast-response silicon carbide detector is preferably a PIN type detector (it can also be a Schottky type detector), and its α-ray energy resolution is less than 10%. As shown in Figures 7 and 8, in this embodiment, a single fast-response silicon carbide detector is selected to realize fast neutron flux monitoring, so that the system of the present invention can work reliably for a long time. As can be seen from Figure 7, t _r represents the response rise time of 2.41ns, t _d represents the response fall time of 7.02ns, and t _FWHM represents the response half-width of 6.93ns; as can be seen from Figure 8, the fast-response silicon carbide detector with a thick sensitive area can realize the effective conversion of fission fragment energy into electrical signals, and has a fast neutron irradiation resistance and fission fragment irradiation resistance that is more than 4 orders of magnitude higher than that of traditional silicon detectors. When the radiation dose is high, the fast-response silicon carbide detector is less likely to have performance degradation or detector failure problems, and can work stably for a long time.

Performance indicators of waveform identification equipment: sampling rate greater than or equal to 2GS/s, vertical resolution greater than or equal to 12bit, analog bandwidth greater than or equal to 500MHz, record length greater than or equal to 100k points (recording time greater than or equal to 50μs). The waveform identification equipment has an adjustable gain (Variable Gain, VG) function to achieve the amplification or attenuation function of single-ray signals and adaptively adjust the input typical fast neutron signal amplitude. As shown in Figure 9, a comparison diagram of fast response silicon carbide detectors and PIN type detectors for detecting fission fragments. The waveform identification device used in the present invention converts the electrical signal into a digital signal, and the sampling rate is greater than or equal to 2GS/s (2×10 ⁹ /s), which is suitable for counting diagnostic systems with event rates lower than 10 ⁸ /s (single event counts at least 20 points); the analog bandwidth is greater than or equal to 500MHz, and the leading/trailing edge 1ns signal can be accurately recorded without distortion; the vertical resolution is greater than or equal to 12bit, and the measurement accuracy of the signal amplitude reaches one thousandth; the recording length is greater than or equal to 100k points (the recording time is greater than or equal to 50μs), and the maximum number of effective event counts at one time reaches 5k. As can be seen from Figure 9, the fast response silicon carbide detector and the silicon PIN detector (silicon PIN detector is the industry gold standard) have very consistent fission fragment measurement results. However, due to the poor radiation resistance of the silicon PIN detector (after being irradiated by fast neutrons and fission fragments, the performance degrades quickly and is easily damaged), it is not suitable for combining with the "fission material + PSD technology" to make a neutron flux monitoring system. The fast-response silicon carbide detector used in the method of the present invention achieves the same detection effect as the gold standard silicon PIN detector, and at the same time, solves the problem of long-term reliable operation.

In this embodiment, the waveform identification device uses a field programmable gate array as a core device for large-capacity data signal processing of the waveform identification device. By utilizing its rich digital logic resources, the digital signal output by the analog-to-digital converter (ADC) can be processed online in real time, and time domain/frequency domain signal processing such as interpolation, filtering, and Fourier transform can be performed on the digital signal. The waveform of the digital electrical 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. The sampling rate can be increased by N times, and the sampling time interval can be reduced by N times, effectively improving the measurement accuracy of the time interval in waveform identification.

In the present embodiment, two or more waveform discrimination parameters are used, and through the relationship of "AND", the interference of α rays and γ rays in fast neutron measurement is effectively eliminated. Preferably, the time domain waveform discrimination parameters and the frequency domain waveform discrimination parameters are used in combination. In other embodiments, the time domain waveform discrimination parameters or the frequency domain waveform discrimination parameters can also be used to eliminate α rays and γ rays in fast neutron measurement. When the two discrimination parameters of the waveform half-width and the waveform 2/3 height width are combined in the "AND" manner, the event rejection rate is increased by more than 30% compared to the use of one discrimination method, and the interference of α rays and γ rays in fast neutron measurement is effectively eliminated. The method of the present invention can be used for real-time online discrimination on FPGA, or offline discrimination on the host computer, and can be applied to a variety of application scenarios.

The average kinetic energy of the fission fragments is about 60MeV and 90MeV, which is much higher than the secondary α rays (several MeV) and accompanying γ rays (less than 1MeV) of spontaneous fission of nuclear materials, which is conducive to the method of the present invention to achieve a high signal-to-noise ratio; the waveform identification method uses the time characteristics of the fission fragments, which are different from the time characteristics of the α rays and γ rays produced by nuclear fission reactions (the response time of the fast-response silicon carbide detector to the fission fragments is slower than the response time of the reaction products such as (n, p), (n, α), (n, 3α) of Si nuclei and C nuclei, and the latter is the noise that affects the fast neutron flux monitoring results. The waveform identification technology can eliminate the interference of the latter), and can successfully achieve further elimination of the latter related noise, and obtain a very high fast neutron flux monitoring signal-to-noise ratio. The fission material, the fast-response silicon carbide detector and the waveform identification device in this embodiment are all solid, so they will not be affected by temperature and air pressure when used.

The working principle of the fast neutron flux high signal-to-noise ratio monitoring of the present invention is as follows:

In the mixed pulse radiation field, the present invention is based on the waveform discrimination device and uses a fast response silicon carbide detector to obtain fast neutron flux measurement with a high signal-to-noise ratio. Fast neutrons are used to react with fissionable materials to release fission fragments; the fission fragments enter the fast response silicon carbide detector and generate analog electrical signals; the obtained analog electrical signals are input into the waveform discrimination device to complete waveform discrimination (i.e., PSD technology) and realize fast neutron flux monitoring.

In the waveform identification device, the functions of amplification/attenuation and digital processing of the analog electrical signal output by the fast response silicon carbide detector are realized, and the adjustable gain amplifier and step adjustable attenuator in the waveform identification device are used to adjust the amplitude of the input analog electrical signal. The high-speed analog-to-digital converter in the waveform identification device is used to digitize the analog electrical signal to obtain the digital electrical signal. In order to obtain high-precision waveform identification parameters, data processing of the digital electrical signal is required.

Since the fission cross section of the fission material ²³⁸ U has small fluctuations in the fast neutron energy region, and the average kinetic energy of the fission products has little to do with fast neutrons, the fission method is selected, and the fast neutron flux information is obtained by monitoring the secondary fission fragments to achieve a flat energy response to fast neutrons, which can effectively get rid of the problem of fast neutron monitoring's dependence on the energy spectrum. Selecting a fast-response silicon carbide detector with fast time response and high charge collection efficiency, the waveform discrimination device is based on the key to the success of the noise suppression method based on PSD technology. At present, the PIN-type detector based on the thick sensitive area of 4H-SiC homoepitaxial material can obtain a fast time response with a response time half width of less than 20ns and a high charge collection efficiency of nearly 100%. The density of ionization generated by fission fragments and gamma rays in the fast-response silicon carbide detector is different, which leads to different time response characteristics of the fast-response silicon carbide detector to fission fragments and gamma rays, the former has a slow time response and the latter has a fast time response. This is the principle of fast neutron and gamma signal discrimination based on PSD technology.

First, the digital electrical signal is interpolated to improve the time resolution and vertical amplitude resolution of the digital electrical signal. Generally, according to the prior knowledge of the output waveform corresponding to the ray information, 7 digital interpolation points are inserted by sinc interpolation or linear interpolation. Secondly, the digital electrical signal is denoised, which is generally achieved by subtracting the average value of the waveform baseline from the overall digital electrical signal to obtain the denoised waveform data. Secondly, the denoised waveform data is processed in the time domain/frequency domain to obtain waveform discrimination parameters. For example, in order to obtain the time domain waveform discrimination parameters such as the half-width of the waveform and the 2/3 height width of the waveform, the denoised waveform data needs to be peak-searched, that is, the maximum amplitude in the denoised waveform data is found by an offline or online real-time method, and the FWHM and FW2/3M parameters are obtained according to the maximum amplitude of the waveform. Finally, the obtained waveform discrimination parameters are used to complete the discrimination, statistics and output of the pulse waveform, and the waveform discrimination function is completed.

Data processing of digital electrical signals can be implemented online in real time in the field programmable gate array in the waveform discrimination device, or offline in the host computer, based on the PSD technology, ultimately achieving high-gain measurement of fast neutron flux.

Claims (10)

1. The fast neutron flux high signal-to-noise ratio monitoring method is characterized by comprising the following steps of:

1) The fast neutrons are utilized to generate nuclear fission reaction with the fissile materials, and the fissile fragments are released;

2) The fission fragments enter a fast response silicon carbide detector and generate an analog electrical signal; 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 Using waveform discrimination equipment to convert the analog electrical signal output by the quick response silicon carbide detector into a digital electrical signal;

3.2 Interpolation is carried out on the waveform of the digital electric signal, and the background noise of the digital electric signal after interpolation is extracted;

3.3 Subtracting the digital electric signal background noise after interpolation in the step 3.2) by using a waveform discrimination device to obtain de-background noise waveform data;

3.4 Data processing is carried out on the de-background noise waveform data obtained in the step 3.3), and at least two waveform discrimination parameters of the time domain and/or the frequency domain are obtained;

3.5 Using at least two time domain and/or frequency domain waveform discrimination parameters to carry out waveform statistics and discriminate ray information, thereby realizing fast neutron flux monitoring.

2. The fast neutron flux high signal to noise ratio monitoring method of 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 greater 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) specifically comprises:

3.4.1 Peak searching is carried out on the de-background noise waveform data, and two time domain waveform discrimination parameters are obtained; the time domain waveform discrimination parameters comprise a waveform half-width and a waveform 2/3 height width;

3.4.2 Fourier transform or wavelet transform is carried out on the de-background noise waveform data 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 performed in any order or simultaneously.

4. The fast neutron flux high signal to noise ratio monitoring method according to claim 3, wherein the using at least two waveform discrimination parameters in step 3.5) is specifically:

The time domain waveform discrimination parameter or the frequency domain waveform discrimination parameter or the combination of the time domain waveform discrimination parameter and the frequency domain waveform discrimination parameter is adopted.

5. A 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 U, the fissile threshold energy of the fast neutrons is more than or equal to 1.5MeV;

When the fissile material is ²³⁷ Np, the fissile threshold energy of the fast neutrons is more than or equal to 0.4MeV;

The thickness of the fissile material is less than or equal to 3mg cm ^-2;

The fission fragments are alpha rays and gamma rays of the high-energy charged substance.

6. A fast neutron flux high signal to noise ratio monitoring system for implementing the fast neutron flux high signal to noise ratio monitoring method according to any one of claims 1 to 5, characterized in that: the device comprises a fissile material, a quick response silicon carbide detector and a waveform discrimination device;

The fissile material is attached to the fast response silicon carbide detector and is used for carrying out nuclear fission reaction with fast neutrons to generate fissile 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 a waveform discrimination device for collecting fission fragments and generating analog electric signals, and the waveform discrimination device is used for waveform discrimination 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;

the input end of the adjustable gain amplifier is connected with the fast response silicon carbide detector.

9. A fast neutron flux high signal to noise ratio monitoring system according to any of claims 6-7, wherein:

The fissile material is ²³⁸ U or ²³⁷ Np, and the thickness of the 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 device is more than or equal to 2GS/s, the vertical resolution is more than or equal to 12bit, 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.