A method for measuring the (n, α) reaction angle differential cross section

Abstract

The present invention discloses a method for measuring ( n , α ) reaction angle differential cross section method, belongs to the field of nuclear data measurement technology. The present invention uses a screen ionization chamber and a gas sample to measure the anode pulse rise time and anode amplitude, and obtains the projection length of the nuclear reaction event track parallel to the neutron incident direction through the anode pulse rise time. One projection length corresponds to two α The particle emission angle; the difference in anode amplitude can distinguish the same projection length but α The events with different particle emission angles are divided into different categories according to the two-dimensional spectrum of anode amplitude-anode pulse rise time through the V-shaped event area on it. α The number of events is obtained by measuring the particle emission angle, and the differential cross section of the reaction angle to be measured can be obtained by combining the number of reaction nuclei to be measured and the corresponding neutron flux. The present invention can be used to measure ( n , α ) reaction, which has the characteristics of a large number of sample nuclei to be tested, high detection efficiency, and is not restricted by the detector placement angle.

Description

Technical Field

[0001] The present invention belongs to the technical field of nuclear data measurement, and in particular relates to a method for measuring ( n , α ) Method of reaction angle differential cross section. Background Art

[0002] Angular differential cross sections are one of the most important types of nuclear data. The detailed information on nuclear reaction mechanisms they provide will be helpful in the evaluation of nuclear data and the study of nuclear reaction theory. n , α ) reaction is one of the important types of neutron nuclear reactions. In the field of nuclear energy applications, ( n , α ) reaction is a neutron absorption reaction, and the accuracy of the cross-section data will directly affect the accuracy of the reactor core physical design. In addition, α Particles can also cause helium embrittlement of materials. Common ( n , α The method for measuring the angular differential cross section of a reaction is to place a detector at the angle to be measured to detect the emitted charged particles. However, this method has low detection efficiency and only provides results at the angle where the detector is located. Summary of the Invention

[0003] In order to overcome the existing n , α ) reaction angle differential cross section measurement has the problems of low detection efficiency, small solid angle coverage, and small number of nuclei to be measured. The present invention proposes a method for measuring ( n , α ) Method of reaction angle differential cross section.

[0004] The basic principle of the method proposed in the present invention is that the anode pulse rise time and anode amplitude can be measured by using a screen ionization chamber in the experiment, wherein the anode pulse rise time can be used to obtain the projection length of the nuclear reaction event track parallel to the neutron incident direction, and one projection length corresponds to two α The particle emission angle; the difference in anode amplitude can distinguish the same projection length but α The event with different particle emission angles can be uniquely determined based on the anode pulse rise time and anode amplitude. α Particle emission angle. After determining the number of reaction nuclei to be measured and the corresponding neutron flux, the angular differential cross section of the reaction can be obtained.

[0005] The details are as follows:

[0006] When conducting experiments using gas samples, the charged particles emitted by nuclear reactions and the remaining nuclei will deposit energy in the sensitive volume of the screen ionization chamber, thus contributing to the signal amplitude. However, due to the PHD (Pulse Height Defect) effect, the energy of heavy charged particles with a high ionization density will not be fully detected. Therefore, the amplitude of the anode signal reflects the total energy of the two, but at the same time, this undetected portion must be deducted, as shown in Equation (1):

[0007] (1)

[0008] In formula (1), A a is the anode amplitude, G a is a constant related to electronics, E α Emitted by nuclear reactions α The energy of the particle, E recoil is the energy of the remaining nuclei emitted by the nuclear reaction, Δ E PHD is the energy that cannot be detected due to the PHD effect. α The energy of the remaining nucleus and the energy that is not detected are different depending on the particle emission angle. The basic rule is that the greater the energy of the remaining nucleus, the greater the energy that cannot be detected, and the lower the anode amplitude of the event.

[0009] different α In addition to the different anode amplitudes, events with different particle emission angles also have significantly different track lengths. When processing the original waveform, the rise time of the anode pulse can be extracted. The rise time of the anode pulse reflects the projected length of the event track parallel to the neutron incidence direction. The rise time of the anode pulse is determined by Equation (2):

[0010] (2)

[0011] In formula (2), T r is the anode pulse rise time, l e is the projection length of the electron cluster parallel to the electric field direction (generated by the ionization of charged particles), v g is the electron drift velocity when the electron passes through the gate, t ga is the drift time of electrons between the gate and the anode.

[0012] From formula (2), we know that v g andt ga Under the premise of T r Calculated l e Therefore, when the electron diffusion effect of the working gas is weak, the projection length of the electron cluster parallel to the electric field direction is l e It is equal to the projection length of the nuclear reaction event track parallel to the electric field direction l In formula (2) l e In nuclear reaction events α The relationship between the particle emission angles is as follows: Figure 2 As shown in (a). l α For emission α The range of the particle, θ α For emission α The particle's exit angle, l recoil is the range of the remaining nuclei, θ recoil is the recoil angle of the remaining nuclei. When the neutron energy is determined, for any θ α , can be uniquely calculated through nuclear reaction kinematics θ recoil and emission α The energy of the particle and the remaining nucleus can be calculated by the energy loss law of charged particles in the gas l α and l recoil .

[0013] Figure 2 (b) shows different θ α nuclear reaction events l e It can be seen that with θ α Get bigger, l e It shows a trend of first getting smaller and then getting bigger, and for a certain range l e There will be two θ α Therefore, in order to uniquely determine θ α , we need to distinguish the same l e Different θ αnuclear reaction events. They can be distinguished by the anode amplitude. Using the anode pulse rise time as the ordinate and the anode amplitude as the abscissa, a two-dimensional spectrum of anode amplitude versus anode pulse rise time can be constructed. Figure 3 This is a typical two-dimensional spectrum of anode amplitude-anode pulse rise time. The V-shaped event area is framed by red lines, and the Figure 2 5 different θ α The position of the nuclear reaction event on the two-dimensional spectrum. Figure 3 It can be seen that the anode pulse rise time provides different l e The ability to distinguish nuclear reaction events, while the difference in anode amplitude can distinguish l e Same but θ α Different events (e.g. Figure 2 Using this information, we can infer the nuclear reaction events ② and ④ of the event through equation (3). α Particle emission angle:

[0014] (3)

[0015] Next, we will explain how to determine v g and t ga . v g and t ga For an experiment, it is almost unchanged, so in order to determine v g and t ga , you can select Figure 3 Two special positions in the V-shape, namely the upper left corner α Particles 180° emission and upper right corner α The position of the particle 0° emission (corresponding to Figure 2 Nuclear reaction events ① and ⑤ in these two locations T r Available from Figure 3 Obtained in l e It can be obtained based on the energy loss law of charged particles in the gas and the kinematics of nuclear reactions. According to formula (2), the equations of the two positions can be solved together to get v g and t ga .

[0016] Finally, we discuss systematic errors caused by range deviation of charged particles and diffusion during electron drift. Range deviation occurs when charged particles move through a gas medium and are ultimately stopped. This can lead to drastic changes in direction due to elastic collisions with gas atoms. However, this interaction typically occurs at the end of the track, resulting in relatively small interference. This interference can be calculated using the Geant4 program package or SRIM software. Diffusion during electron drift occurs because electron clusters are generated along the charged particle track and must drift to reach the gate to generate an anode signal. Therefore, diffusion during the drift process must be considered. Diffusion varies in different gases, so gases with minimal diffusion effects should be selected. The impact of diffusion effects can be calculated using the Garfield++ program package.

[0017] Based on the above principle, the present invention proposes a measurement ( n , α ) A method for determining a reaction angle differential cross section, comprising the following steps:

[0018] 1) Identify the nuclear reaction to be measured, select a gas sample containing the nucleus to be measured, and design a working gas containing this gas sample. This working gas must exhibit a certain PHD effect when tested in a screened ionization chamber, such that the event region of the nuclear reaction to be measured exhibits a V-shape in the two-dimensional spectrum of anode amplitude-anode pulse rise time.

[0019] 2) Fill the screen grid ionization chamber with the working gas containing the gas sample to be measured and conduct an in-beam experiment. Collect the cathode signal, anode signal and fission signal of the screen grid ionization chamber.

[0020] 3) Analyze the collected anode signals to obtain the pulse rise time and anode amplitude of each anode signal, and make a two-dimensional spectrum of anode amplitude-anode pulse rise time.

[0021] 4) On the two-dimensional spectrum of anode amplitude-anode pulse rise time, the event area to be measured appears as a V-shaped area. This V-shaped area is the area of ​​all events to be measured, because α Particle emission angle θ α The difference in the V-shaped area leads to different events at different locations. Therefore, it is necessary to distinguish different θ α The event, that is, the binning method of determining the angle, divides the V-shaped area into multiple bins, and a bin is the event area corresponding to a certain angle interval.

[0022] 5) After determining the angle binning method, use formula (2) to calculate the anode pulse rise time corresponding to the upper and lower boundaries of each bin T r, thus completing the binning.

[0023] 6) Count the number of events in each bin, and make corrections for interference caused by background and / or low-energy neutrons according to the actual experimental conditions, and calculate α Particle detection efficiency.

[0024] 7) Determine the neutron flux and nucleus number in the experiment. The method can refer to the method for determining the neutron flux and nucleus number in gas samples established in the literature (Y. Hu, Y. Gledenov, Z.Cui, et al. Eur. Phys. J. A, 60, 51(2024)) and calculate the angular differential cross section.

[0025] Furthermore, the working gas in step 1) above is mainly composed of an inert gas that has little interference with the nuclear reaction to be measured, and is added with the gas sample containing the nuclear to be measured. For example, in one embodiment of the present invention, the nuclear reaction to be measured is 16 O( n , α ) 13 For the C reaction, the gas sample containing the nuclei to be tested is selected as CO2, and the working gas is 3 atm 96% Kr + 4% CO2.

[0026] After the working gas is introduced into the screened ionization chamber, the electron drift rate and PHD effect are evaluated to see if they meet the requirements. Generally, the electron drift rate is required to be within an appropriate range. In the embodiments of the present invention, a range of 0.6 to 0.8 cm / μs is preferred. This range is neither too slow to prevent electron recombination during the drift process nor too fast to shorten the anode pulse rise time. A certain PHD effect is required, such that the event region of the nuclear reaction to be measured exhibits a V-shaped region in the two-dimensional spectrum of anode amplitude and anode pulse rise time. Generally speaking, the presence of some electronegative gases (such as carbon dioxide and tetrafluoromethane) in the working gas can impart a PHD effect to the working gas.

[0027] With the emission α Particle emission angle θ α From 0° to 180°, the anode pulse rise time T r It will first decrease and then increase. For the V-shaped event area on the two-dimensional spectrum of anode amplitude-anode pulse rise time, it is from the upper right to the middle and then to the upper left. Preferably, in step 4) using T r Divide the bins in the V-shaped event area, from the upper right corner θ α The position of 0° is binned in the order from upper right to middle and then to upper left, and the middle of the V-shaped event area is Tr Smaller positions are grouped into a bin, such as Figure 3 shown.

[0028] In step 5), as mentioned above, use the upper left corner of the V-shaped area α Particles 180° emission and upper right corner α The two positions where the particle exits at 0° are solved by solving the equations together. v g and t ga .because v g and t ga For an experiment, the anode pulse rise time is almost constant. T r Reflects the projection length of the event track parallel to the neutron incident direction.

[0029] In getting parameters v g and t ga Then, for each event in the V-shaped area, according to formula (2), T r Calculate the corresponding l e , and then according to the anode amplitude and formula (3), the emission of the event can be deduced α Particle emission angle θ α Since a bin is the event area corresponding to a certain angle interval, the T r It can be inferred that θ α , so that according to the event T r Divide it into the corresponding bins.

[0030] Furthermore, the background interference described in step 6) can be determined by the following method: enclosing the V-shaped region with a trapezoid, simulating the ratio of background events within the trapezoid to the total number of events within the trapezoid; calculating the background events per unit area within the trapezoid by multiplying the total number of events within the trapezoid by the ratio and dividing the result by the area of ​​the trapezoid; and then multiplying the background events per unit area by the area of ​​each bin to calculate the number of background events per bin. Simultaneously, a liquid scintillator detector is used to monitor the neutron energy spectrum, and the interference of low-energy neutrons is determined through simulation. The low-energy neutron interference is then corrected based on the neutron energy spectrum.

[0031] Furthermore, in step 7), the starting time difference between the anode signal and the cathode signal is used to determine the distance interval from the end of the nuclear reaction event track to the gate, and the ratio of the events occurring in this distance interval to all the events occurring in the main reaction area is determined by simulation, and this ratio is used as α Particle detection efficiency.

[0032] Figure 4 This is a typical experimental device of the present invention. 238 U3O8 sample. The sample is placed on the shielding electrode of the screen ionization chamber close to the neutron source, because 238 U( n , f ) The reaction cross section is between 0.5 MeV and 200 MeV, so 238 U( n , f ) reaction to measure the neutron flux. Figure 4 The main reaction zone is also marked in the figure. The number of nuclei in the experiment is calculated by monitoring the pressure and temperature of the gas during the experiment and combining it with the volume of the main reaction zone. This allows us to obtain the angular differential cross section of the reaction to be measured, as shown in Equation (4).

[0033] (4)

[0034] In formula (4), is the angular differential cross section, θ is the median value of the bin’s angle interval, N event ( θ ) is the number of events in the bin, Φ MRV is the average neutron flux in the main reaction zone. The method can be found in the literature (Y. Hu, Y. Gledenov, Z.Cui, et al. Eur. Phys. J. A, 60, 51(2024)). N sample is the number of cores, and ΔΩ is the solid angle spanned by the angle interval of the bin.

[0035] The present invention utilizes a screen grid ionization chamber and a gas sample to measure ( n , α ) reaction of the anode pulse rise time and anode amplitude, the anode pulse rise time is used to obtain the projection length of the nuclear reaction event track parallel to the neutron incident direction. One projection length corresponds to two α The particle emission angle; the difference in anode amplitude can distinguish the same projection length but α The event with different particle emission angles can be uniquely determined based on the anode pulse rise time and anode amplitude. αThe particle emission angle is determined by dividing the V-shaped event area on the two-dimensional spectrum of anode amplitude-anode pulse rise time into different α The number of events is obtained by combining the number of reaction nuclei to be measured and the corresponding neutron flux to obtain the differential cross section of the reaction angle to be measured. The method of the present invention can be used to measure ( n , α ) reaction, which has the characteristics of a large number of sample nuclei to be tested, high detection efficiency, and is not restricted by the detector placement angle. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 This is the anode signal waveform measured by a typical screen grid ionization chamber of the present invention, in which the anode pulse rise time is marked T r and the start time of the signal.

[0037] Figure 2 (a) shows the projection length of the nuclear reaction event track parallel to the electric field direction. l e and α Particle emission angle θ α The relationship between ; (b) shows the different emission α The projection length of the nuclear reaction event track corresponding to the particle's emission angle is parallel to the direction of the electric field.

[0038] Figure 3 This is a typical two-dimensional spectrum of anode amplitude-anode pulse rise time, where Figure 2 (b) 5 different α Particle emission angle θ α The position of the nuclear reaction event on this two-dimensional spectrum.

[0039] Figure 4 It is a structural schematic diagram of the experimental device of the present invention.

[0040] Figure 5 These are the fission cathode spectrum and the simulated fission cathode spectrum of the 10.45 MeV energy point experiment measured in the embodiments of the present invention.

[0041] Figure 6 This is the neutron energy spectrum measured by the EJ-309 liquid scintillator detector in the embodiment of the present invention.

[0042] Figure 7 is the angular differential cross section of the 10.45 MeV energy point measured in an embodiment of the present invention. DETAILED DESCRIPTION

[0043] The technical solution of the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0044] Oxygen is the most abundant element in the earth's crust and also the most abundant element in the human body. 16 O( n , α ) 13 C reaction cross section information is of great significance in the field of nuclear engineering. Modern power reactors generally use uranium dioxide as nuclear fuel, and accurate 16 O( n , α ) 13 The C reaction cross section will be beneficial for core physical design and evaluation α The damage caused by particles to materials. This reaction also has important applications in the fields of radiobiology, astrophysics, etc. Since nuclear reaction experiments are difficult to carry out and there are few experimental data, theoretical calculations and data evaluations of neutron nuclear reactions are particularly important. 16 For light nuclei such as O, the R-matrix analysis is currently mostly used in theory, and the R-matrix analysis is highly dependent on high-quality experimental data, especially angular differential cross-section data.

[0045] According to the measurement proposed by the present invention ( n , α ) reaction angle differential cross section method, this embodiment uses a screen ionization chamber and a gas sample to measure the 10.45 MeV neutron energy 16 O( n , α 0) 13 C reaction angle differential cross section, the specific steps are as follows:

[0046] 1) Determine the nuclear reaction to be tested 16 O( n , α 0) 13 For the C reaction, CO2 is used as the gas sample containing the target nuclei, and the working gas is 3 atm 96% Kr + 4% CO2, which meets the requirement of an appropriate electron drift rate range. Furthermore, CO2 itself has a certain PHD effect, which can cause the event region of the target nuclear reaction event to exhibit a V-shaped pattern in the two-dimensional spectrum of anode amplitude and anode pulse rise time.

[0047] 2) Fill the screen ionization chamber with the above working gas and conduct the beam experiment. The experiment was carried out on the HI-13 tandem accelerator. The experimental equipment is as follows: Figure 4 The experiment obtains the cathode signal, anode signal and fission signal of the screen grid ionization chamber and collects them.

[0048] 3) Analyze the anode signals collected, obtain the pulse rise time and anode amplitude of each anode signal, and make a two-dimensional spectrum of anode amplitude-anode pulse rise time, such as Figure 3 shown.

[0049] 4) In Figure 3 In the two-dimensional spectrum of anode amplitude and anode pulse rise time, the event area to be measured is V-shaped, which has been framed by a red frame. The V-shaped area includes all the events to be measured, because α Particle emission angle θ α The difference in the V-shaped area leads to different events at different locations. Therefore, it is necessary to distinguish different θ α events, that is, the binning method for determining the angles. Figure 3 The figure shows the event area (i.e., a bin) corresponding to a certain angle interval. The bin is bounded by two black lines, upper and lower, and the red lines that frame the "V-shaped" event area to be measured are the left and right boundaries.

[0050] 5) After determining the angle binning method, use formula (2) to calculate the anode pulse rise time corresponding to the upper and lower boundaries of each bin T r .in l e , v g and t ga Determine using the method described later in formula (3).

[0051] With the emission α Particle emission angle θ α From 0° to 180°, T r It will decrease first and then increase. For the V-shaped event area, it is from the upper right to the middle and then to the upper left. T r Divide the bins in the V-shaped event area, from the upper right corner θ α The position of 0° is binned in the order from upper right to middle and then to upper left. T r Smaller locations are grouped into one bin. Figure 3 The event area (i.e., a bin) corresponding to a certain angle interval has been given in the figure, and the bins of the remaining angle intervals are given in the same way.

[0052] 6) Count the number of events in each bin and correct for interference caused by background and low-energy neutrons based on the actual experimental conditions. After binning, the number of events in each bin can be obtained. Next, correct for background based on the actual experimental conditions. In this embodiment, the main source of interference for the event to be measured is Kr ( n , α ) event (called background event). First, a trapezoidal area containing the V-shaped area is given, such as Figure 3 The dark green line framed area is shown in FIG. First, the Kr in the trapezoidal area is obtained by simulation ( n , α ) events to the total events in the trapezoidal region. The simulation method can be found in the literature (Y. Hu, H. Jiang, Z. Cui et al. Nucl. Sci. Tech. 32(8), 062001(2021), Y.Hu, Y. Gledenov, Z. Cui, et al. Eur. Phys. J. A, 60,51(2024)). The number of background events in the trapezoidal region can be obtained by counting the total number of events in the trapezoidal region and multiplying it by the above ratio. Since the background events are almost uniform in the trapezoidal region, the background events per unit area in the trapezoidal region can be obtained by counting the number of background events in the trapezoidal region and dividing it by the area of ​​the trapezoidal region. Next, the background events per unit area of ​​the trapezoidal region are multiplied by the area of ​​each bin in the two-dimensional spectrum of anode amplitude-anode pulse rise time to calculate the number of background events in each bin. In the experiment, an EJ-309 liquid scintillator detector was used to monitor the neutron energy spectrum, and the low-energy neutron interference was corrected in combination with the neutron energy spectrum. The interference of low-energy neutrons was also determined by simulation. The method can be found in the literature (Y. Hu, H. Jiang, Z. Cui et al. Nucl. Sci. Tech. 32(8), 062001(2021), Y. Hu, Y. Gledenov, Z. Cui, et al. Eur. Phys. J. A, 60, 51(2024)). In this example, the interference of low-energy neutrons is extremely small, so for simplicity, the low-energy neutron interference of each bin is considered uniformly.

[0053] 7) Next, confirm α In order to reduce the impact of charged particles hitting the plate and thus failing to deposit their energy completely in the V-shaped area, only the main reaction area is selected (see Figure 4 ) The reaction to be measured occurs in a part of the area away from the plate. Specifically, the starting time difference between the anode signal and the cathode signal is used. The so-called starting time is the time when the signal starts to rise. Figure 1For example, the anode signal of the event is marked in the figure. The time difference between them is due to the event occurring between the cathode and the grid. Therefore, the cathode signal is generated immediately after the event, while the anode signal does not begin until the electron cluster ionized by the event drifts to the grid. Therefore, the time difference between their onsets is proportional to the distance from the end of the nuclear reaction event track to the grid. In this example, we selected events with time differences between 1000 ns and 5000 ns, corresponding to distances from the end of the nuclear reaction event track to the grid ranging from 7.8 mm to 39.1 mm. The ratio of events occurring between 7.8 mm and 39.1 mm to all events occurring in the main reaction zone was determined using simulations (Y. Hu, H. Jiang, Z. Cui et al. Nucl. Sci. Tech. 32(8), 062001(2021), Y. Hu, Y. Gledenov, Z. Cui, et al. Eur. Phys. J. A, 60, 51(2024)). α Particle detection efficiency.

[0054] 8) Determine the neutron flux and nucleus number in the experiment. The method for determining the neutron flux and nucleus number in a gas sample established in the literature (Y. Hu, Y. Gledenov, Z.Cui, et al. Eur. Phys. J. A, 60, 51(2024)) can be used. 238 U3O8 samples were used to monitor neutron flux. Figure 5 Given the measured 238 U( n , f ) fission cathode spectrum and simulated fission cathode spectrum. The simulation was implemented using the Geant4 program package and uses realistic experimental conditions as input. It can be seen that the simulation results are in good agreement with the measured results, so they can be used to determine the detection efficiency and neutron flux. 16 The number of O nuclei was determined by monitoring the pressure and temperature of the gas during the experiment and combining the main reaction zone (see Figure 4 ) volume. High-precision barometers and thermometers were used in the experiment to ensure the accuracy of the nucleus count. 16 O nuclei also participate in the reaction, and this fraction is also taken into account. The specific proportion is given by the simulation. The main reaction zone consists of the inner area of ​​the ionization chamber collimated by the neutron collimator and the penumbra area at the edge. Most of the reactions to be measured occur in this area. Determining the neutron flux and the number of nuclei allows the calculation of the angular differential cross section. The measured angular differential cross section at the 10.45 MeV energy point is as follows: Figure 7 shown.

[0055] The above embodiments are not intended to limit the scope of the present invention. Any person skilled in the art can, without departing from the spirit of the present invention, utilize the methods and technical contents disclosed above to make many possible changes, modifications, or optimizations to the technical solutions of the present invention. Therefore, any simple transformations, modifications, and improvements made based on the technical essence of the present invention still fall within the scope of protection of the technical solutions of the present invention.

Claims

1. A measurement ( n , α ) A method for reaction angle differential cross section, characterized in that The following steps are involved: 1) Selecting a gas sample containing the nuclei to be measured based on the nuclear reaction to be measured, and designing a working gas containing the gas sample, wherein the working gas is required to exhibit a PHD effect when tested in a screen ionization chamber; 2) Filling the screen grid ionization chamber with the working gas to perform an in-beam experiment, and collecting cathode signals, anode signals, and fission signals of the screen grid ionization chamber; 3) Analyze the collected anode signals to obtain the pulse rise time and anode amplitude of each anode signal, and make a two-dimensional spectrum of anode amplitude-anode pulse rise time; 4) The event area to be measured appears as a V-shaped area on the two-dimensional spectrum of anode amplitude-anode pulse rise time. α Particle emission angle θ α The difference in angles leads to different events at different positions in the V-shaped area. The angle binning method is determined, and the V-shaped area is divided into multiple bins. A bin is the event area corresponding to a certain angle interval. 5) Use formula (2) to calculate the anode pulse rise time corresponding to the upper and lower boundaries of each bin T r , thus completing the binning; （2） in, l e is the projection length of the electron cluster parallel to the electric field direction, v g is the electron drift velocity when the electron passes through the gate, t ga is the drift time of electrons between the gate and the anode; 6) Count the number of events in each bin, and make corrections for interference caused by background and / or low-energy neutrons according to the actual experimental conditions, and calculate α Particle detection efficiency; 7) Determine the neutron flux and number of nuclei in the experiment and calculate the angular differential cross section.

2. The measurement according to claim 1 ( n , α ) A method for reaction angle differential cross section, characterized in that In step 1), the working gas is mainly composed of an inert gas that has little interference with the nuclear reaction to be measured, and is added with the gas sample containing the nucleus to be measured.

3. The measurement according to claim 1 n , α ) A method for reaction angle differential cross section, characterized in that In step 4) use the anode pulse rise time T r Divide the bins in the V-shaped area, starting from the upper right corner θ α The position of 0° is binned in the order from upper right to middle and then to upper left, and the middle of the V-shaped area is T r Smaller locations are grouped into one bin.

4. The measurement according to claim 1 n , α ) A method for reaction angle differential cross section, characterized in that In step 5), the parameters in formula (2) v g and t ga Obtained by the following method: First, according to the upper left corner of the V-shaped area α Particles 180° emission and upper right corner α The two positions where the particle exits at 0° obtain the corresponding T r Then, according to the energy loss law of charged particles in the gas and the kinematics of nuclear reactions, the corresponding l e , by solving the equation (2) corresponding to these two positions, we can get v g and t ga ; Getting parameters v g and t ga Then, for each event in the V-shaped area, according to formula (2) T r Calculated l e , and then infer the emission of the event based on the anode amplitude and formula (3) α Particle emission angle θ α : （3） In formula (3), θ α For emission α The particle's exit angle, l α For emission α The range of the particle, l recoil is the range of the remaining nuclei, θ recoil is the recoil angle of the ejected residual nucleus; Therefore, the T r Infer θ α , divide it into the corresponding bin.

5. The measurement according to claim 1 n , α ) A method for reaction angle differential cross section, characterized in that In step 6), the interference caused by the background is obtained by the following method: a trapezoid is used to frame the V-shaped area, and the ratio of background events in the trapezoidal area to the total events in the trapezoidal area is obtained by simulation; the background events per unit area in the trapezoidal area are obtained by counting the total number of events in the trapezoidal area, multiplying the ratio by the area of ​​the trapezoidal area, and dividing the result by the area of ​​the trapezoidal area; and then the background events per unit area are multiplied by the area of ​​each bin to calculate the number of background events in each bin.

6. The measurement according to claim 1 n , α ) A method for reaction angle differential cross section, characterized in that In step 6), a liquid scintillator detector is used to monitor the neutron energy spectrum, the interference of low-energy neutrons is determined through simulation, and the low-energy neutron interference is corrected in combination with the neutron energy spectrum.

7. The measurement according to claim 1 n , α ) A method for reaction angle differential cross section, characterized in that In step 6), the starting time difference between the anode signal and the cathode signal is used to determine the distance interval from the end of the nuclear reaction event track to the gate. The ratio of the events occurring in this distance interval to all the events occurring in the main reaction area is determined by simulation, and this ratio is used as the α Particle detection efficiency.

8. The measurement according to claim 1 n , α ) A method for reaction angle differential cross section, characterized in that Set on the shielding pole of the screen ionization chamber close to the neutron source 238 U3O8 samples, through monitoring 238 U( n , f ) reaction to obtain a neutron flux.

9. The measurement according to claim 1 n , α ) A method for reaction angle differential cross section, characterized in that The number of nuclei in step 7) is calculated by monitoring the pressure and temperature of the gas during the experiment and combining it with the volume of the main reaction zone.

10. The measurement according to claim 1 n , α ) A method for reaction angle differential cross section, characterized in that In step 7), the angular differential cross section of the reaction to be measured is calculated using formula (4): （4） In formula (4), is the angular differential cross section, θ is the median value of the angle interval of a bin, N event ( θ ) is the number of events in the bin, Φ MRV is the average neutron flux in the main reaction zone, N sample is the number of cores, and ΔΩ is the solid angle spanned by the angle interval of the bin.

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