Neutron detection method based on boron-coated straw tube

Abstract

The invention discloses a neutron detection method based on a boron-coated straw tube, which relates to the technical field of radiation detection, and is technically characterized in that a B4C coating is electroplated in a substrate, so that thermal neutrons interact with the coating in a detector in different modes and are converted into charged particles, ionization of working gas is caused, and the neutron detection efficiency is improved. And finally, an electronic pulse signal is generated to realize neutron detection.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of radiation detection, in particular to a neutron detection method based on boron-coated straw tube. BACKGROUND

[0002] The nuclear radiation emitted by nuclear materials is mainly neutrons and gamma rays, wherein the detector for detecting neutrons generally adopts 3 He gas detector. In recent years, with 3 The shortage of He gas, 3 The price of He tube rises, and the contradiction between supply and demand is very prominent. The world has seen 3 A hot wave of research on alternative products for He gas detectors. Boron-coated straw tube neutron detectors have the advantages of high quality factor, low working voltage, low inflation pressure, high reliability, stable performance, high gamma ray suppression level, etc., and have attracted great attention. It has gradually become the most potential alternative 3 He gas detector.

[0003] With the continuous rapid development of neutron analysis and measurement technology, the latest research results in the international show that the boron-coated straw tube neutron detection technology has great advantages in extracting the properties and characteristic information of nuclear materials, and will be widely used in nuclear material accounting, neutron diffraction, nondestructive testing and other fields.

[0004] Although boron-coated straw tubes provide a feasible solution to the problem of 3 He gas shortage, and have achieved successful application in nuclear safety monitoring, nondestructive testing and other fields, but they still have certain technical and application limitations in detection efficiency, signal decay, environmental adaptability, long-term stability, cost and manufacturing.

[0005] Therefore, the present application aims to provide a neutron detection method based on boron-coated straw tube to solve the above problems. SUMMARY

[0006] The purpose of the present application is to solve the above problems, provide a neutron detection method based on boron-coated straw tube, by electroplating B4C coating in the substrate, so that thermal neutrons interact with the coating in the detector in different ways, convert into charged particles, cause ionization of the working gas, and finally generate electronic pulse signals to realize neutron detection.

[0007] In order to achieve the above purpose, the technical scheme of the present application is as follows:

[0008] The present application provides a neutron detection method based on boron-coated straw tube, which comprises the following steps:

[0009] S1, using metal Al as the substrate of the boron-coated straw tube, electroplating B4C coating in the substrate;

[0010] S2, when the neutrons enter the detector, the neutrons interact with the reaction 10 B generates secondary charged particles 7Li and alpha particles, and the reaction equation is: Therefore, the nuclear reaction generates two groups of charged particles with different energy combinations: alpha_1: 1775.45KeV (6.1%), 7 Li_1: 1014.5KeV (6.1%), alpha_2: 1470KeV (93.9%) and 7 Li_1: 840KeV (93.9%);

[0011] S3, when the charged particles with energy of 0KeV-20MeV pass through the target material, four types of interactions occur between the charged particles and the target material through Coulomb force, including non-elastic collision between the charged particles and the target atomic nucleus, non-elastic collision between the charged particles and the target atomic nucleus, elastic collision between the charged particles and the target atomic nucleus, and elastic collision between the charged particles and the target atomic nucleus, wherein the non-elastic collision between the charged particles and the target atomic nucleus is the main energy loss mode. Coulomb interaction occurs between the charged particles and the target atomic nucleus, energy is transferred to the electrons, the energy state of the material atoms is changed, and ionization or excitation is caused;

[0012] S4, two charged particles generated in the B4C coating each time the nuclear reaction occurs are emitted in any two opposite directions, and part of the particles emitted in the direction of the boron-coated straw tube enter the detection gas through the coating, and the charged particles deposit energy by ionizing gas molecules and atoms in the working gas; when the central anode of the boron-coated straw tube is connected to high voltage and the wall of the boron-coated straw tube is grounded, the detector works in the proportional counting area, the ionized electrons drift to the anode, and an electron avalanche occurs in a very small area near the anode wire, and then an electric signal is collected on the anode wire, completing the detection of the neutrons.

[0013] Compared with the prior art, the beneficial effects of the present scheme are:

[0014] The present application realizes the detection of neutrons by plating a B4C coating in the substrate, so that thermal neutrons interact with the coating in the detector in different ways, are converted into charged particles, cause ionization of the working gas, and finally generate an electronic pulse signal; and further simulation analysis shows that the detector has good gamma ray suppression ability, high gamma suppression ratio, and can effectively reduce the interference of gamma rays; the detector exhibits high detection efficiency and low noise level in different energy ranges, proving its excellent performance and wide application potential. BRIEF DESCRIPTION OF DRAWINGS

[0015] Figure 1 is a schematic diagram of the movement of neutrons into the detector in the embodiment of the present application;

[0016] Figure 2 is the SRIM software operation interface diagram in the embodiment of the present application;

[0017] Figure 3 is the schematic diagram of the range of 1775.45 KeV alpha particles in B4C in the embodiment of the present application;

[0018] Figure 4 is the diagram of thermal neutrons emitting in the same direction in the embodiment of the present application;

[0019] Figure 5 is the charged detection efficiency of a single straw tube at different detection distances in the embodiment of the present application;

[0020] Figure 6 is the corresponding detection efficiency of PTI-204 at different distances in the embodiment of the present application;

[0021] Figure 7 is the energy deposition output diagram of gamma rays in the working gas of a straw tube simulated by superMC in the embodiment of the present application;

[0022] Figure 8 is the diagram of gamma rays parallelly entering a straw tube simulated by Geant4 in the embodiment of the present application;

[0023] Figure 9 is the energy spectrum of 400800000 gamma rays deposited in a straw tube in the embodiment of the present application;

[0024] Figure 10 is the diagram of counting the number of particles in different energy intervals by matlab in the embodiment of the present application;

[0025] Figure 11 is the schematic diagram of the electronic readout system of a boron-coated straw tube detector in the embodiment of the present application;

[0026] Figure 12 is the front and back views of a DAQ board in the embodiment of the present application;

[0027] Figure 13 is the plan view (left) and internal photo (right) of BL20 in the embodiment of the present application;

[0028] Figure 14 is the schematic diagram of the connection of the electronic system in the embodiment of the present application (left side), and the physical diagram of the electronic system (right side); 252 schematic diagram of the neutron energy spectrum of a Cf source;

[0029] Figure 15 is the schematic diagram of the connection of the electronic system in the embodiment of the present application (left side), and the physical diagram of the electronic system (right side);

[0030] Figure 16The high-voltage and low-voltage power supplies in the embodiment of the present invention are respectively used to power the detector and electronics (left side), and the actual image of the detector system (right side);

[0031] Figure 17 Figures 1 and 2 show the noise test results for the first-layer tube (left), the second-layer tube (center), and the third-layer tube (right) in accordance with an embodiment of the present invention.

[0032] Figure 18 The noise test diagram of the fourth-layer tube (left) and the noise test diagram of the fifth-layer tube (right) in the embodiment of the present invention are shown;

[0033] Figure 19 1. The waveform diagram of the T0 signal 50Ω matching (left) and the waveform diagram of the T0 signal 1MΩ matching (right) in accordance with the present invention;

[0034] Figure 20 It is the standard in the embodiment of the present invention 3 Time-of-flight spectrum of He tube test (left) and time-of-flight spectrum measured by boron-coated straw tube detector (right);

[0035] Figure 21 The boron-coated straw tube detector and 3 Comparison of counting results of He tube detectors;

[0036] Figure 22 In the embodiment of the present invention 3 Simulation results of He tube detector detection efficiency (left), and analysis results of detection efficiency (right);

[0037] Figure 23 is the pulse amplitude spectrum in an embodiment of the present invention;

[0038] Figure 24 It is a simulated AWCC model and neutron collision diagram in an embodiment of the present invention;

[0039] Figure 25 It is an AWCC model based on straw tube simulation in an embodiment of the present invention. DETAILED DESCRIPTION

[0040] In order to enable those skilled in the art to better understand the present invention, the technical solution of the present invention will be further described in detail below in conjunction with the embodiments of the present invention and the accompanying drawings. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts should fall within the scope of protection of the present invention.

[0041] It should be noted that, in the absence of conflict, the embodiments and features of the embodiments of the present invention can be combined with each other. The present invention will be described in detail below with reference to the embodiments.

[0042] Example 1: Simulation process

[0043] Detection process:

[0044] The detection process of the detector is that thermal neutrons are first emitted in all directions. A part of the emitted neutrons enters the detector. The neutrons entering the detector will have the following three situations: directly pass through the detector; a small amount of neutrons are scattered out of the detector by Al; the other part is deposited on the inner wall coating of the straw tube. 10 B undergoes the above nuclear reaction. Figure 1 shown.

[0045] Nuclear reactions convert uncharged neutrons into charged particles, which are emitted in any opposite direction at different reaction locations, such as in case 3. Particles emitted toward the working gas enter the working gas with a certain probability, causing Coulomb interaction, resulting in ionization or excitation, and generating primary electrons. The primary electrons drift in the electric field of the proportional counter tube, and then electron avalanche occurs in the strong electric field very close to the central anode wire. A large number of electrons are finally collected at the anode wire, generating electronic pulse signals, which are recorded and analyzed using an electronic system to achieve neutron detection.

[0046] Charged particle range expectation simulation:

[0047] For charged particle range simulations, we use SRIM software. SRIM utilizes quantum mechanics to analyze particle-matter collisions and calculates the energy loss and range of particles in matter. It can quickly establish the energy loss and range per unit distance traveled by incident particles over a wide energy range. TRIM is a Monte Carlo calculation that tracks the particle's trajectory through the target material, calculating in detail the energy gained by each collision with a target atom.

[0048] Input the incident particle type, energy, target material thickness, composition, atomic density of constituent elements, and target material density into the software. The ranges of the four charged particles can be obtained as 6.39um, 2.93um, 5.17um, and 2.61um respectively. The software operation interface is as follows: Figure 2 The results of simulating the range of 1775.45KeV α particles are as follows Figure 3 shown.

[0049] Simulation study of single tube detection efficiency:

[0050] Firstly, the model of single straw tube was established by superMC to simulate the research. superMC is a set of nuclear design and radiation safety evaluation software based on Monte Carlo method developed by Institute of Nuclear and Radiation Safety, Chinese Academy of Sciences, which can be used to calculate the transport of 10-11 MeV to 20 MeV neutrons, 1 KeV to 1000 KeV photons and electrons in three-dimensional complex structure and criticality problems. superMC can calculate the surface flow, surface flux, volume flux, point or ring probe flux, average deposited energy, average fission deposited energy, probe energy spectrum and deposited charge statistics. In the simulation of this project, superMC was mainly used to track neutrons and photons. The hot neutron emission mode was the positive direction of X axis, and the center point of the straw tube was placed at a distance of 3 cm from the source, as shown in Figure 4 The calculated detection efficiency is 4.34%, and the detection efficiency obtained at this time is the absolute detection efficiency (source detection efficiency). For this method, the length, diameter and detection distance of the straw tube have little effect on the detection efficiency, and the only influencing factor is the coating thickness. The simulation results are only used to verify the absorption performance of B4C coating for neutrons. In view of this, the detection efficiency of isotropic emission of hot neutrons was studied.

[0051] When the isotropic emission is used, the model is established as above, and only the neutron source emission mode is changed to arbitrary direction emission. Due to the small size of the straw tube, the number of neutrons entering the probe becomes smaller, and at this time the absolute detection efficiency also becomes very small, so we study the relative detection efficiency. The main factor affecting the detection efficiency is the detection distance, and the detection efficiency corresponding to the detection distance of 1 cm-10 cm is simulated, as shown in Figure 5

[0052] It can be seen from Figure 5 that when the detection distance reaches 4 cm or more, the relative detection efficiency is basically stable. The distance is no longer the main factor affecting the detection efficiency.

[0053] PTI-204 straw tube array detection efficiency research:

[0054] 1) Theoretical calculation

[0055] In order to verify the accuracy of the data and the model, the same method was used to theoretically calculate and simulate the PTI-204 type straw tube of PTI company, and the results were compared. PTI-204 type straw tube is arranged on the basis of structure, and 7 tubes of 7.5 mm are used to form a counting tube, and the diameter of the counting tube is 25.4 mm. The thickness of B4C coating on each basic structure tube is 1 um. The 7 counting tubes are arranged in a row to form a probe module.

[0056] When calculating the theoretical results, the theoretical calculation method proposed by Lacy was used to calculate the hot neutrons absorbed by the counting tube, and the theoretical detection efficiency was calculated as 0.000 6%.​10 The probability of boron capture Pf and the probability of charged particles penetrating the coating into the working gas P c The product of the two is the thermal neutron detection efficiency. The calculation formula is as follows:

[0057]

[0058] Where N is the nuclear density, i.e. the number of nuclei per unit area; σ is 10 B thermal neutron capture cross section, taken as 3840b here; D1, D2 are the diameters of the counter tube and the straw tube respectively; d is the thickness of the boron carbide coating, taken as 1um; Pf i is the reaction probability of the i-th charged particle; R i is the range expectation of the i-th charged particle in the coating.

[0059] Based on the above formula, the data is substituted to calculate the theoretical thermal neutron detection efficiency of 17.58%.

[0060] 2) Model establishment and simulation study

[0061] The superMC program is used to establish a model identical to the PTI-204 detector module, and the model establishment result is basically consistent with the PTI-204 array.

[0062] After the straw tube model is established, the information of the radiation source is set. In order to allow more neutrons to enter the detector, the neutron source is placed at the origin, and the center coordinate of the detector module is on the X axis, with the detector surface being 3cm away from the source. The geometric factor is required to calculate the probability of neutrons entering the detector. Direct simulation in superMC shows that among the 1000000 neutrons emitted by the source, about 357283 can enter the sensitive volume of the detector, i.e. the corresponding solid angle when the detector surface is 3cm away from the source is about 257.243°. By recording the number and position of neutrons deposited in the coating, a total of 88666 neutrons are deposited at different positions in the coating. The number of neutrons at different positions is counted using MATLAB, and the range expectation is used for calculation. Finally, 75936 neutrons enter the working gas of the detector and are detected, and the simulated thermal neutron detection efficiency of the straw tube is 21.254%, which is 3.674% different from the theoretical calculation value. The main reason for the difference in detection efficiency is that the different measurement distances result in different coating thicknesses in the direction of neutron incidence, which has a greater impact on the detection efficiency.

[0063] To further analyze the impact of detection distance on detector efficiency, simulations were conducted for distances ranging from 1 cm to 10 cm from the detector surface to the source. Theoretical analysis shows that as distance increases, the corresponding solid angle decreases, and the average coating thickness corresponding to the neutron incident direction also decreases, leading to lower detection efficiency. Furthermore, the impact on detection efficiency decreases with increasing distance. Based on this concept, simulations of detection efficiency at different distances yield the results shown in Table 1:

[0064] Table 1 Simulated efficiency at different detection distances

[0065]

[0066] Depend on Figure 6 It can be seen that the detection efficiency of the straw tube decreases with the increase of the detection distance, which is consistent with the previous analysis results. When the detection distance reaches 8 cm, the detection efficiency basically reaches a stable level.

[0067] 3) Summary

[0068] When the neutron source emits in any direction, the relative detection efficiency of the straw tube will decrease as the detection distance increases, and this effect will gradually decrease as the distance increases. This is because as the distance increases, the coating thickness corresponding to the neutron emission direction will gradually decrease and gradually approach the thickness of the coating itself, so the detection efficiency will gradually approach a value. Studies have found that a detection distance that is too close will have a greater impact on the efficiency, and a detection distance that is too far will cause the solid angle corresponding to the detector to become smaller, and ultimately the number of neutrons entering the detector will decrease, and the absolute detection efficiency will be too low. In order to choose a suitable distance, according to Figure 6 At a detection distance of 8 cm, the detection efficiency is essentially stable. The simulated PTI-204 thermal neutron detection efficiency of 17.57% is closest to the theoretical calculation result, with an error of only 0.01%. When neutrons are directed into the straw tube, the absolute detection efficiency is 4.34%. At this point, the only factor affecting the detection efficiency is the coating thickness. Simulations of the detection efficiency of a single tube at different distances show that the efficiency quickly stabilizes. This is because the solid angle occupied by the straw tube itself is very small, and the corresponding coating thickness differences when neutrons enter the detector are not significant. Therefore, the detection efficiency of a single straw tube is approximately 8.8%. This result is quite impressive for a single tube, and the detection efficiency can be improved by establishing an array with a certain arrangement, which is of high research value.

[0069] Example 2: Gamma Inhibitory Capacity

[0070] The γ rejection capability is represented by γ rejection ratio, which is characterized by γ detection efficiency in the neutron detector. Since the neutron source usually emits γ rays of higher order of magnitude than the neutron source when emitting neutrons, the energy of these γ rays is usually several hundred KeV, and the γ rays below 1.022 MeV mainly react with the wall of the detector through photoelectric effect and Compton scattering to generate secondary electrons. The secondary electrons have a certain probability of entering the working gas, and the electrons entering the working gas will drift towards the central anode wire under the action of the electric field. When the electrons drift to a very close distance from the central anode wire, electron avalanche occurs, and finally the charge is collected to the central anode wire, causing the detector to count. In this process, the counts caused by γ rays in the neutron detector are called false counts, which are mistaken for neutron events, resulting in an increase in neutron detection efficiency. Through research, it is found that the energy deposition distribution of neutrons in the working gas is relatively wide, ranging from 0 to several MeV, while the energy deposition caused by γ rays ultimately does not exceed 100 KeV, and only a small part of the neutrons in this energy range.

[0071] Based on the above analysis, it can be known that the response of the neutron detector to the γ rays is due to the generation of secondary electrons by the reaction of the γ rays with the detector wall. However, it is mentioned in the literature that the energy deposition of γ rays in the working gas of the detector is statistically analyzed using the Monte Carlo method. In order to verify this, we use 0.662 MeV γ rays to study the straw tube. It is found that γ rays can directly react with the working gas of the straw tube detector to deposit energy, and the maximum energy deposition exceeds the previous theoretical value of 100 KeV. The energy deposition simulated by superMC is shown in Figure 7

[0072] According to the simulation results, the maximum energy deposition reaches 480 KeV, which deviates greatly from the theoretical value. At the same time, this is also the result of not simulating the reaction with the detector wall. The possible reason for this phenomenon is that the cross-section library in superMC is different, and there are certain difficulties in simulating photoelectric effect and Compton scattering in superMC.

[0073] Since there are certain difficulties in superMC simulation, Geant4 software is used to study the γ rejection capability of the straw tube. Geant4 is an object-oriented Monte Carlo software package developed by the European Nuclear Center, and is widely used in nuclear and medical fields. Compared with superMC, Geant4 has the advantages of open source code, easy modification, and installation of database verified by experiments. Users can modify and expand the program according to their own requirements, so it is widely used in particle simulation research. In Geant4, the model of the straw tube is first established, and then the neutron source is set to emit parallelly towards the straw tube, as shown in Figure 8 ​As shown, each emitted gamma particle is able to enter the sensitive volume of the detector.

[0074] For this simulation, 400800000 gamma particles of 662KeV were emitted into the straw tube. Unlike superMC, Compton scattering and photoelectric effect were not considered because Geant4 simulates the whole process of each particle from creation to disappearance, including the secondary particles produced in the process. Thus, the secondary electrons produced by Compton effect or photoelectric effect will be tracked and recorded, and the deposited energy will be counted. The deposited energy particles were plotted into an energy spectrum diagram using CERN ROOT software, as shown in Figure 9.

[0075] As can be seen from the figure, most of the deposited energy is below 10KeV, and as the energy increases, the number of deposited particles decreases, and the maximum energy deposition is around 60KeV. In order to more conveniently select the discrimination threshold and calculate the gamma rejection ratio, we used matlab software to count the number of particles with deposited energy in the range of 0-100KeV with a step of 0.5KeV, as shown in Figure 10. Figure 10 The gamma ray detection efficiency under different discrimination thresholds was calculated as shown in Table 2.

[0076] Table 2 Gamma ray detection efficiency under different discrimination thresholds

[0077]

[0078] As can be seen from the table, the straw tube detector has very good gamma discrimination ability, and when the discrimination threshold is set to 60KeV, the gamma rejection ratio can reach 10 -7个 orders of magnitude.

[0079] Example 3: Performance research of boron-coated straw tube neutron detector

[0080] The performance of a single boron-coated straw tube neutron detector was tested and experimentally studied, including the background, operating voltage, detection efficiency, and decay time. Experiments were carried out using the existing experimental conditions of the spallation neutron source. The electronic components of the detector were configured as three modular components: a current-sensitive preamplifier module in the detector housing, a custom-designed CAMAC unit containing a shaper, summing amplifier, delay line limiting circuit, and discriminator circuit (post-amplifier module), and a digital converter module containing time-to-digital conversion (TDC) and analog-to-digital conversion (ADC) channels. At one end of the detector, each row of straw tubes was connected together through a terminating resistor, and at the other end, each column of straw tubes was connected together in a similar manner. Then each column or row of straw tubes was connected to a tap on the delay line, and the current-sensitive preamplifier read out from both ends of the delay line. The relative time difference between the signals at both ends of the delay line was then used to determine which row or column the event occurred in. Combining this information with the longitudinal position measured by the charge division method, the complete three-dimensional position of each detection event was obtained. To achieve this, the analog timing signal was directly passed to a pair of time-to-digital converters (TDCs) in the next digital module. In a parallel circuit, the delay line signal was added, the delay line was limited to reduce pile-up, and a custom integrated circuit was used for shaping, then passed to the analog channel of the digital converter module. The resulting 12-bit digital value was passed as a serial data stream following the Fast Ethernet protocol. Then, using an Ethernet hub, the serial streams from a large number of digitizers were combined into a single stream and passed to a PC or PC cluster, as shown in the specific electronic readout system Figure 11

[0081] Experimental objectives:

[0082] This study focuses on the testing needs of the detector performance and builds a test platform for the boron-coated straw tube detector system. Considering the actual use of the detector, noise level, detection efficiency, and gamma suppression ability are important indicators to measure the performance of the detector, so this study focuses on testing these three aspects to comprehensively evaluate its performance. Among them, the noise level and detection efficiency are tested at the No. 20 beam line of the spallation neutron source, and the gamma suppression ability is tested using a 252Cf neutron source.

[0083] Experimental apparatus and test conditions:

[0084] 1) Straw tube detector

[0085] ​The straw tube detector used in this experiment is produced by Proportional Technologies, Inc. (PTI) in the United States. The boron-coated straw tube detector is manufactured by PTI in the United States. It has a 5-layer structure, and each layer is composed of 7 or 6 large tubes. Each large tube is composed of 7 straw tubes, which contain micron-level metal wires as anodes and are filled with argon and carbon dioxide mixed gases for gas multiplication. The large tube has a diameter of 25.4mm and a length of about 1000mm. The tube wall is made of aluminum alloy. A large tube is equipped with 7 copper foils as neutron conversion layers, called small tubes. Their axes are parallel to the axis of the large tube and are arranged in a regular hexagon on the cross section of the large tube. The small tube is based on copper with a thickness of 0.05mm. A layer of PVD method is used to cover it. 10 The B4C is rolled into a 7.5mm diameter cylinder, serving as the detector's cathode. A 20μm diameter, 4.3kΩ / m resistance wire is attached to its axis, serving as the detector's anode. The large tube is filled with a mixture of Ar and CO2 at an operating pressure of 0.7 bar. The effective length of the detector used in the experiment along the wire is 900mm.

[0086] Each layer of the boron-coated straw tube has an independent current-sensitive preamplifier, which uses the charge distribution method to read the Z-direction position. The signal from the preamplifier is connected to the neutron imaging DAQ (DAQ is a data acquisition board) to receive data from the detector to the connected computer. The functions of the front and back of the data acquisition board are as follows Figure 12 As shown. Four signal wires with push-pull SMB connectors should be connected to the corresponding connectors on the front of the DAQ board. Two cables go to the straw tube decoder port, and the other two go to the tube decoder port of the detector module. Ethernet cables connected to the back of the board transmit panel data from the board to a connected Ethernet switch. The Ethernet switch combines the data strings from all five panels into a single data string, which is then sent to the computer via another Ethernet cable connected at its other end to a properly configured Ethernet port on the computer. The chopped signal (T0 signal) from the system can be connected to a BNC port, where the DAQ system generates time-stamped data for each collected neutron event (with a resolution of 20 nanoseconds) for energy characterization. The five data acquisition boards are powered by a low-voltage power supply (5V) through a distribution box. A high-voltage power supply is connected to the preamplifier port to complete the boron-coated straw tube wiring.

[0087] 2) China Spallation Neutron Source BL20 test beamline

[0088] The noise level and detection efficiency of the straw tube detector were tested at the BL20 test beamline of the China Spallation Neutron Source. BL20 uses a decoupled narrowed liquid hydrogen moderator to provide neutron beams with different time pulse structures and neutron wavelength ranges. Figure 13 The following are floor plans and interior photos of BL20.

[0089] The experiment of γ suppression ability of the straw tube detector was carried out in the 252 Cf neutron source, which was 252 Cf with an activity of 6.3 MBq (measured activity of 20 MBq, 252 Cf half-life of 2.645 years). The thermal neutron flux was about 1.5 n / (s*cm -2 ), and the photon flux was one order of magnitude higher than the neutron flux. 252 The neutron spectrum of Cf was provided by the empirical equation of MattSpectrum, see Figure 14 , and the average neutron energy was 2.3 MeV. The shielding structure of the neutron source was divided into three layers, from inside to outside, lead, paraffin and borax. In paraffin and borax, a hole with an inner diameter of 10 cm was opened in the horizontal direction, which communicated with the lead shielding layer, as a neutron beam exit hole.

[0090] Experimental instruments and equipment:

[0091] 1) Digital oscilloscope

[0092] The digital oscilloscope is used to determine whether the T0 signal will be attenuated after passing through a 15m long signal line, and whether the attenuation amplitude of the T0 signal with different impedance matching is normal. The Tektronix MDO3052 digital oscilloscope used in the experiment has a sampling rate of 2.5GS / s, a bandwidth of 500MHz, and can realize impedance matching of 50Ω, 75Ω and 1MΩ, with double analog input channels, which can well meet the requirements of waveform viewing during the test.

[0093] 2) Low-voltage power supply

[0094] The low-voltage power supply is GPC-3030DNGWINSTEK linear DC power supply, with three groups of output (two groups adjustable 0-30V / 0-3A, one group fixed voltage 5V / 3A Max), which can monitor the current and voltage in real time, mainly for the electronics of the boron-coated straw tube detector to provide ±5V DC low-voltage power supply.

[0095] 3) High-voltage power supply

[0096] The high-voltage power supply used in the experiment is the high-voltage power supply of iseg, which can provide a maximum of 4kV high voltage in double channels, with a maximum current of 3mA, and can be remotely controlled through a computer. In the beam test room, the operation of adding high voltage is realized by connecting the internal host computer and then remotely controlling it. The maximum voltage of the boron-coated straw tube detector in this experiment does not exceed 1100V, and the current does not exceed 50nA. This power supply can meet the high-voltage requirements.

[0097] Experimental content:

[0098] 1) Detector performance test in the BL20 beamline of the spallation neutron source

[0099] a. Detector performance test platform

[0100] The schematic diagram and physical diagram of the connection of the boron-coated straw tube detector electronics system are shown in Figure 15 , the detector has five layers, each layer shares one electronics module, and each module has a corresponding DAQ data acquisition system, which uses high and low voltage power supplies to power the detector and the electronics system respectively, as shown in Figure 16 (Left). In the BL20 beamline of the spallation neutron source, the physical diagram of the completed detector system is shown in Figure 16 (Right), a special bracket is designed to fix the position of the detector, the neutron channel of this beamline is a circular channel with a diameter of 20mm, and a laser is used to adjust the position of the detector before the experiment to ensure that the neutron beam can be incident on the center of the detector, as shown in Figure 16 (Right).

[0101] b. Detector noise background test

[0102] Noise level is an important indicator for measuring detector electronics. In the noise test of the boron-coated straw tube detector, the noise of each layer of boron-coated straw tube was tested separately, and then the whole detector was tested. The basic working parameters of the boron-coated straw tube detector were also recorded during the noise test of the boron-coated straw tube detector. When each layer was separately powered, the current stabilized at about 3 nanoamperes when the high voltage reached 1050V. When the high voltage of the 5-layer detector was 1050V and the current stabilized at about 15 nanoamperes, the detector noise was recorded. The test time was 1962.8 seconds. The 5-layer straw tube detector started taking data at the same threshold value, and the results are shown in Figure 17 and 18 .

[0103] From the result graph, it is still not difficult to see that under the same threshold conditions, the noise of the third layer is not as good as the noise test results of the first two layers. The results of separate testing of each layer also differ little from the overall test results. The reason is that the processing technology of the boron-coated straw tube detector is not the same, resulting in a small difference in noise test results. However, from the comprehensive evaluation and analysis of the test results, the noise level of the detector used in the experiment is only 2.5 cps, which is relatively small. This is mainly due to the position of the preamplifier and the subsequent electronics analysis system.

[0104] BL20 beamline time-of-flight spectrum test:

[0105] The measurement of time-of-flight spectrum is mainly to verify the performance of the boron-coated straw tube detector and the ordinary3 The difference of the detection efficiency of He tube detector. The internal trigger of the electronics was changed to external T0 trigger before the time-of-flight spectrum test. T0 is a 25 Hz square wave, which has the same frequency as the neutron beam. The consistency of T0 after passing through the 15 m signal line under different impedance matching was tested by oscilloscope first. The T0 test results are shown in Fig. Figure 19

[0106] From the waveform diagram, it can be found that the amplitude of T0 signal is not the same under different impedance matching, but the voltage is still 3.3 V under 1 MΩ impedance matching. For the electronics, the time-of-flight spectrum related to the neutron beam can be obtained by using T0 external trigger, and then compared with the spectrum of the neutron beam. 3 The test results of time-of-flight spectrum of He tube detector under the same conditions were compared. The test results of time-of-flight spectrum of the two detectors are shown in Fig. Figure 20

[0107] Under the condition of straight-through beam, the time-of-flight spectrum of He tube was compared with 3 The neutron counts in 5-8 ms increased obviously, and there were grooves in 13-14 ms and 16 ms, which was consistent with the time-of-flight spectrum of 3 He tube. Due to the distance between the detector and the beam hole, the test time and the detection efficiency were not consistent, so the counts were not the same, but the basic time-of-flight spectrum was consistent.

[0108] Detection efficiency test and evaluation:

[0109] According to the data provided by PTI, the working voltage of this straw tube detector is 1000 V. The neutron detection efficiency of a small tube under different neutron energies (different neutron energies correspond to different neutron wavelengths) is shown in Table 3.

[0110] Table 3 Neutron detection efficiency of a small tube of straw tube detector

[0111]

[0112] In order to obtain the detection efficiency of the 5-layer boron-coated straw tube detector, a test system was built, i.e. 20 atm standard 3 He detectors were placed at the same position of the boron-coated straw tube detector in Figure 16 (right), and the same time was tested under the same conditions. Under the same conditions, the counts of the detectors were obtained as Figure 21 Test. The simulation results of the detection efficiency of 20 atm standard 3 He tube detector for different wavelength neutrons by Geant4 simulation are shown in Figure 20 (left), and the detection efficiency of the 5-layer boron-coated straw tube detector is shown in Figure 21 ​​The detection efficiency of the boron-coated straw tube detector under different layer numbers and wavelength conditions can be obtained by fitting. Figure 22 It was observed that when the number of detector layers is 5, the detection efficiency for neutrons with a wavelength of 1.8 angstroms is 60%.

[0113] Using 252Cf neutron source to test the detector's gamma suppression capability:

[0114] 1) Build a detector gamma suppression capability test platform

[0115] In order to test the γ suppression capability of the detector, an experiment was designed. The straw tube detector used in the experiment used 7 large tubes arranged in a row. The detailed experimental settings and experimental parameters are shown in Table 4.

[0116] Table 4 Experimental settings and experimental parameters

[0117]

[0118]

[0119] 2)γ-inhibitory ability test

[0120] γ suppression capability is an important indicator of neutron detectors. 252 The photon fluence rate of the Cf neutron source is one order of magnitude higher than that of thermal neutrons. The pulse amplitude spectrum obtained by the detector can be used to evaluate the gamma suppression capability of this detector. Figure 23 The histogram on the right side of the figure is the logarithmic display of the histogram on the left side, and the figure below is the result of removing the counts on the left side of the red line from the upper figure. There is an obvious peak in the lower channel of the two upper figures. After passing this peak, the spectrum begins to rise and several less obvious peaks appear. The distribution in the higher range of this channel is similar to 10 The reaction product of B and thermal neutrons, 7 The distribution of Li and α caused by energy deposition in the detector is close. Considering that the energy deposited by photons in the detector is relatively low, the peak on the low track in the figure is mainly contributed by photons. PTI also uses 252 The Cf Neutron Source tested the gamma-ray suppression capabilities of straw-tube detectors. The results of this paper are consistent with those of the previous study, which also assumes that the red left side of the image represents photons and the red right side represents neutrons. Overall, the straw-tube detector used in the experiment is able to separate photons from neutrons when the photon fluence rate is an order of magnitude higher than the neutron fluence rate. A clear boundary between the photon and neutron spectra can be seen in the pulse amplitude spectrum. This indicates that the straw-tube detector has excellent gamma-ray suppression capabilities.

[0121] Experimental summary:

[0122] Before the experiment, the background noise of the detector was tested and analyzed at the BL20 test beamline of the China Spallation Neutron Source. It was found that the noise level was only 2.5 cps, which means the detector was at a good noise level. The detection efficiency was tested and the results are as follows: Figure 21 In addition, using 252 The simultaneous existence of neutrons and photons in the Cf neutron source was used to test the γ suppression capability of the detector. From its pulse amplitude spectrum, it can be seen that there is a clear boundary between the distributions of photons and neutrons, such as Figure 23 , so it is believed that the detector has good γ suppression capability.

[0123] Example 4: Design of Boron-Coated Straw Tube Neutron Detector System

[0124] Using computer simulation technology and Monte Carlo particle transport program, we have carried out optimization research on the model, quantity, layout and other issues of the neutron measurement system based on boron-coated straw tube detectors to accurately measure the quality properties of nuclear materials. We have solved the optimization problems of parameters such as detection efficiency, decay time, and time resolution of the neutron measurement system, and realized the optimization scheme of the neutron detection system based on boron-coated straw tubes.

[0125] First, we study the AWCC detector model and use the Model Carlo software to build the AWCC detector. The model is constructed as follows Figure 24 , used to measure the mass of nuclear materials. In this process, we understand how neutron detectors use a small amount of known information to derive mass attribute information, become familiar with the principles of neutron multiplicity counting methods, and be able to derive neutron multiplicity equations. We use simulated neutron transport results to perform actual calculations, analyze the results and causes of errors, and propose optimization solutions. Then, in the simulation, the AWCC system is 3 The He neutron tube was replaced with a straw tube, and the data was processed using the same method. The performance differences between them were then studied and compared to provide theoretical guidance for subsequent experiments.

[0126] The method mainly simulates the sample to generate a neutron pulse sequence in the counter, then processes it through the simulated shift register, and finally solves the measurement equation and other actual measurement processes. It can be seen that the simulation measurement process of uranium materials is relatively complex. The quality deviation of the simulation measurement results is about 10%, which has a certain error. Experimental measurement is needed to improve the simulation process. 3 He is consistent, every 3 He can be seen as a bundle of straw tubes, such as Figure 25 .

[0127] The nuclear material quality is calculated, and the tube structure arrangement is optimized to improve the detection efficiency, so as to improve the accuracy of the nuclear material quality calculation.

[0128] The above specific embodiments are only an explanation of the present application, and are not a limitation of the present application. Those skilled in the art can make modifications to the embodiments without creative contribution after reading the specification, and the modifications are protected by the patent law as long as they are within the scope of the claims of the present application.

Claims

1. A neutron detection method based on boron-coated straw tubes, characterized by: The method comprises the following steps: S1, using metallic Al as the substrate of the boron-coated straw tube, and electroplating B4C coating inside the substrate; S2, when neutrons enter the detector, the neutrons react with 10 B generates secondary charged particles 7Li and α particles, and the reaction equation is: When S3, charged particles of 0KeV-20MeV pass through the target material, they interact with the target material in four types through the Coulomb force, and interact with the electrons outside the target nucleus, transferring energy to the electrons, changing the energy state of the material atoms, and causing ionization or excitation; S4. Connect the central anode of the boron-coated straw tube to high voltage, and the wall of the boron-coated straw tube to ground. The detector counts in the proportional counting area. The ionized electrons drift toward the anode and electron avalanche occurs near the anode wire. Then the electrical signal is collected on the anode wire to complete the detection of neutrons.

2. The neutron detection method based on boron-coated straw tubes according to claim 1, characterized in that: In step S2, the nuclear reaction generates two groups of charged particles with different energy combinations: α_1:1775.45KeV(6.1%)、 7 Li_1: 1014.5KeV (6.1%)、α_2: 1470KeV (93.9%) and 7 Li_1: 840KeV (93.9%).

3. The neutron detection method based on boron-coated straw tubes according to claim 1, characterized in that: In step S3, the interaction includes an inelastic collision between the charged particle and the electron outside the target nucleus, an inelastic collision between the charged particle and the target nucleus, an elastic collision between the charged particle and the electron outside the target nucleus, and an elastic collision between the charged particle and the target nucleus; Among them, the inelastic collision between charged particles and electrons outside the target nucleus is the main way of energy loss.

4. The neutron detection method based on boron-coated straw tubes according to claim 2, characterized in that: In step S4, the specific method for detecting neutrons is as follows: each time a nuclear reaction occurs, two types of charged particles generated in the B4C coating are emitted in any two opposite directions, and some of the particles emitted toward the inside of the boron-coated straw tube pass through the coating and enter the detection gas, and the charged particles deposit energy by ionizing gas molecules and atoms in the working gas; when the central anode of the boron-coated straw tube is connected to high voltage and the wall of the boron-coated straw tube is grounded, the detector operates in the proportional counting area, the ionized electrons drift toward the anode, and electron avalanche occurs in a very small area near the anode wire, and then the electrical signal is collected on the anode wire to complete the detection of neutrons.

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