A method for zero-point correction of neutron time-of-flight spectrum for neutron energy spectrum measurement

Abstract

The present invention relates to a neutron time-of-flight spectrum zero-point correction method for neutron energy spectrum measurement. The method comprises placing a first scintillator detector on a neutron beam behind a target as a start signal detector in the time-of-flight; placing a second scintillator detector at a distance L from the first scintillator detector in a preset angular direction of the neutron beam as a stop signal detector for the neutron time-of-flight; the two scintillator detectors remove gamma signals through pulse shape discrimination, and then provide an initial neutron time-of-flight spectrum through a coincidence method. Since the initial neutron time-of-flight spectrum has a zero-point deviation T_L0, a zero-point correction is required. The method disclosed in the present invention utilizes accompanying gamma in the neutron generation process to correct the zero-point deviation T_L0 of the neutron time-of-flight spectrum when measuring the neutron energy spectrum using a dual-scintillator time-of-flight spectrometer, thereby improving the measurement accuracy of the neutron time-of-flight and, therefore, the accuracy of the neutron energy spectrum measurement.

Description

Technical Field

[0001] The invention belongs to the technical field of nuclear radiation detection, and in particular relates to a neutron time-of-flight spectrum zero-point correction method for neutron energy spectrum measurement. Background Art

[0002] By bombarding a thick heavy metal target (tungsten, lead, etc.) with high-energy protons to induce spallation reactions of heavy metal nuclei, a large number of neutrons can be produced. This is a spallation neutron source, and also a white light neutron source. By bombarding a thin light metal target (lithium, beryllium, etc.) with high-energy protons to induce metal nuclear reactions, a large number of neutrons can also be produced. This is a quasi-monoenergetic neutron source. Before using quasi-monoenergetic and white light neutron sources for scientific and applied research, it is necessary to accurately measure the neutron energy spectrum. Among them, the neutron flight time method is a commonly used neutron energy spectrum measurement method. By measuring the time t that the neutron flies a fixed distance L, the neutron's flight velocity v = L / t is obtained, and the neutron energy is also obtained, that is, Therefore, the prerequisite for measuring neutron energy using the neutron time of flight method is the accurate measurement of the neutron flight time.

[0003] For pulsed neutron sources, the start time of the neutron flight is provided by the pickup signal of the pulsed beam. Only one detector is required to detect the end time of the neutron flight. Subtracting the two provides the neutron flight time. This is the commonly used neutron energy spectrum measurement method at major spallation neutron sources both domestically and internationally. For non-pulsed neutron sources, both the start and end times of the neutron flight are provided by detectors, which generally use scintillators. This is also known as a dual-scintillator neutron time-of-flight spectrometer.

[0004] The key to measuring neutron energy spectra using a dual-scintillator neutron time-of-flight spectrometer is to accurately measure the start and end times of the neutron's flight. In actual experimental operation, the neutron signal detected by the detector probe, passes through the photomultiplier tube, transmits through the transmission cable, is analyzed by the data acquisition device, and finally enters the computer. Each stage requires a certain amount of time. Obviously, the two signals recording the start and end times of the neutron's flight take different times to transmit. To obtain the precise neutron energy, it is necessary to correct for the effect of the transmission time of these two signals on the flight time, that is, to correct the zero point of the neutron time-of-flight spectrum.

[0005] The conventional correction method simply uses cables of strictly identical length to connect the signals of the two scintillator detectors to the data acquisition device, but ignores the fact that the differences between the two scintillators will cause the signal transmission time in the detector to be different, and that the two signals enter different channels of the data acquisition device, and the data processing time of different channels will also be different, thereby affecting the time zero point of the neutron flight time spectrum and causing a systematic shift in the neutron energy. For a 10MeV neutron and a flight distance of 3m, the zero point shift error of the flight time can reach more than ten nanoseconds, resulting in a neutron energy measurement error of up to several MeV. Summary of the Invention

[0006] In response to the defects existing in the prior art, the purpose of the present invention is to provide a neutron time-of-flight spectrum zero-point correction method for neutron energy spectrum measurement. By utilizing the accompanying gamma (whose speed is known to be the speed of light) in the neutron production process, the flight time zero-point deviation inherent in the hardware of the entire dual-scintillator neutron time-of-flight spectrometer is measured and corrected. When the dual-scintillator time-of-flight spectrometer measures the neutron energy spectrum, the zero point of the neutron flight time spectrum is corrected, thereby improving the measurement accuracy of the neutron flight time and further improving the accuracy of the neutron energy spectrum measurement.

[0007] To achieve the above objectives, the present invention adopts a technical solution: a method for zero-point correction of neutron time-of-flight spectrum for neutron energy spectrum measurement, the method comprising the following steps:

[0008] S1. Place the first scintillator detector on the neutron beam behind the target as the starting signal detector during the time of flight;

[0009] S2. Place a second scintillator detector at a distance of L from the first scintillator detector in the direction of the preset neutron beam angle, to serve as a termination signal detector for the neutron time of flight;

[0010] S3. After removing the gamma signal through pulse shape discrimination, the two scintillator detectors provide an initial neutron flight time spectrum through a coincidence method. The initial neutron flight time spectrum is deducted from the zero point deviation T_L0 to perform zero point correction on the neutron flight time spectrum.

[0011] Furthermore, the accompanying gamma rays in the neutron production process are used to calculate the zero-point deviation T_L0 of the neutron time-of-flight spectrum through the following steps:

[0012] S31, placing a first scintillator detector on the neutron beam behind the target, and placing a second scintillator detector at a distance L from the first scintillator detector in the preset angular direction of the neutron beam. When L=L1, both scintillator detectors only record gamma signals, and the gamma flight time T_L1 is obtained by coincidence;

[0013] S32, adjusting the position of the second scintillator detector so that L=L2, both scintillator detectors only record the gamma signal, and obtain the gamma flight time T_L2 through the coincidence;

[0014] S33. With the distance L between the two scintillator detectors as the horizontal axis and the gamma flight time between the two detectors as the vertical axis, perform data fitting and extrapolate the fitting straight line to L=0 to obtain the intercept on the vertical axis, that is, the zero-point deviation T_L0 of the flight time when the distance between the two scintillator detectors is zero.

[0015] Furthermore, using the accompanying gamma in the neutron production process, calculating the zero point deviation T_L0 of the neutron time-of-flight spectrum includes the following steps:

[0016] S31 a. Place a first scintillator detector on the neutron beam behind the target, and place a second scintillator detector at a distance L from the first scintillator detector in the preset angular direction of the neutron beam. When L=L1, both scintillator detectors only record gamma signals, and the gamma flight time T_L1 is obtained by coincidence.

[0017] S32a, adjusting the position of the second scintillator detector so that L=L3, L4...LN, and both scintillator detectors only record gamma signals, and obtain the corresponding gamma flight times T_L3, T_L4...T_LN respectively through conformity;

[0018] S33a. With the distance L between the two scintillator detectors as the horizontal axis and the corresponding gamma flight time between the two detectors as the vertical axis, perform data fitting and extrapolate the fitting straight line to L=0 to obtain the intercept on the vertical axis, that is, the zero-point deviation T_L0 of the flight time when the distance between the two scintillator detectors is zero.

[0019] Furthermore, the second scintillator detector is larger in size than the first scintillator detector, so that the neutron and gamma detection efficiency of the second scintillator detector is higher than that of the first scintillator detector.

[0020] Furthermore, in step S3, the signal T2 detected by the second scintillator detector is used as the start signal start, and the signal T1 of the first scintillator detector is used as the stop signal stop, and an initial neutron time-of-flight spectrum is obtained by a coincidence method.

[0021] Furthermore, the method further comprises the steps of:

[0022] S4. According to the corrected neutron time-of-flight spectrum, the detection efficiency of the two scintillator detectors and the preset angle are taken into consideration to obtain a neutron energy spectrum.

[0023] Furthermore, the first scintillator detector and the second scintillator detector are either liquid scintillator detectors or plastic scintillator detectors.

[0024] The beneficial technical effect of the present invention is that: a neutron time-of-flight spectrum zero-point correction method for neutron energy spectrum measurement disclosed in the present invention is adopted, by measuring the flight time of gamma, performing data fitting and extrapolation to obtain the flight time zero-point deviation inherent in the hardware of the entire dual-scintillator neutron time-of-flight spectrometer when the distance is zero, and deducting this zero-point deviation from the initial neutron time-of-flight spectrum to achieve zero-point correction of the neutron time-of-flight spectrum. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 A schematic diagram of the working principle of a neutron time-of-flight spectrometer in an implementation environment of a neutron time-of-flight spectrum zero-point correction method for neutron energy spectrum measurement according to an embodiment of the present invention;

[0026] Figure 2 A neutron time-of-flight spectrum obtained by using a neutron time-of-flight spectrum zero-point correction method for neutron energy spectrum measurement according to an embodiment of the present invention;

[0027] Figure 3 A schematic diagram of a process for obtaining a correction value using a neutron time-of-flight spectrum zero-point correction method for neutron energy spectrum measurement according to an embodiment of the present invention;

[0028] Figure 4 This is a diagram illustrating an example of correction values ​​obtained by using a neutron time-of-flight spectrum zero point correction method for neutron energy spectrum measurement according to an embodiment of the present invention. DETAILED DESCRIPTION

[0029] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0030] Example 1

[0031] An embodiment of the present invention provides a neutron time-of-flight spectrum zero point correction method for neutron energy spectrum measurement, which uses accompanying gamma rays generated by neutrons to correct the time zero point of the entire detection system.

[0032] Since proton bombardment of a metal target produces neutrons, they are always accompanied by gamma rays. The selected scintillator responds to both neutrons and gamma rays. Neutron and gamma ray discrimination, as well as extraction of either neutron or gamma signals, are achieved through pulse shape discrimination (PSD). The inherent time-of-flight zero-point deviation of the entire dual-scintillator neutron time-of-flight spectrometer hardware is constant, independent of whether neutrons or gamma rays travel between the two scintillator detectors. Therefore, by measuring the gamma ray time-of-flight, the difference in transmission and processing time between the two signal paths of the detector system—that is, the time-of-flight zero-point deviation—can be measured and corrected, thereby correcting the zero point of the neutron time-of-flight spectrum.

[0033] like Figure 1As shown in the figure, when a beam of neutrons is incident on the scintillator of the first scintillator detector, they interact with the atomic nuclei of the scintillator. Some of these neutrons are scattered toward a predetermined angle and detected by the second scintillator detector, generating signal T2. Simultaneously, during the scattering process, the neutrons transfer some of their energy to the first scintillator detector, forming recoil nuclei in its scintillator. The energy deposited by these recoil nuclei is signal T1. There is a corresponding time correlation between the recorded recoil nucleus signal T1 and the scattered neutron signal T2. This time correlation allows for coincidence measurement, obtaining the time difference between the two signals T1 and T2, and thus the neutron time of flight (uncorrected for zero point). Gamma radiation also uses the same principle to measure time of flight.

[0034] At least two experiments are required, with two scintillator detectors placed at two different distances to achieve zero-point correction of the time of flight. The method is as follows:

[0035] (1) At the distance L1, the two detectors only record the gamma signal, and the gamma flight time T_L1 is obtained by coincidence.

[0036] (2) At the distance L2, the two detectors only record the gamma signal, and the gamma flight time T_L2 is obtained by coincidence.

[0037] (3) With the distance L between the two scintillator detectors as the horizontal axis and the gamma flight time between the two detectors as the vertical axis, data fitting is performed and the fitting straight line is extrapolated to L = 0 to obtain the zero-point deviation T_L0 of the gamma flight time when the distance is zero.

[0038] (4) Conduct neutron flight experiments, take the correction T_L0 into account when processing data, and perform zero-point correction on the neutron time spectrum.

[0039] (5) Based on the corrected neutron flight time spectrum and taking into account the detection efficiency, the energy spectrum of the neutrons scattered by the first scintillator detector is obtained.

[0040] (6) Considering the preset angle, the energy spectrum of the neutrons before being scattered by the first scintillator detector is calculated.

[0041] In this embodiment, a neutron time-of-flight spectrum zero-point correction method for neutron energy spectrum measurement disclosed in an embodiment of the present invention is illustrated by taking a 100 MeV proton cyclotron as an example.

[0042] High-energy protons bombard a tungsten target, producing white-light neutrons. A liquid scintillator detector (LSD) is placed behind the target, along the neutron beam, to signal the start of the neutron flight. Another high-efficiency LSD is placed at a distance L, 45° from the proton beam, to signal the end of the neutron flight time. The LSD has excellent neutron and gamma-ray discrimination capabilities. After removing the gamma ray signal through pulse shape discrimination (PSD), the two LSDs are combined to produce a neutron time spectrum using a coincidence method. Finally, the neutron energy spectrum is obtained by combining the time spectrum with the detector efficiency.

[0043] The detectors used are two EJ301 liquid scintillator detectors, measuring Φ2"x2" and Φ5"x5", respectively. The Φ2"x2" detector is a neutron scattering detector. For ease of description, the Φ2"x2" and Φ5"x5" liquid scintillator detectors are named Liquid Scintillator 1 and Liquid Scintillator 2, respectively. The data acquisition system uses a CAEN DT5720 digital waveform digitizer, controlled by DPP-PSD (Digital Pulse Processing-Pulse Shape Discrimination) software.

[0044] like Figure 2 As shown in the figure, in the signal coincidence process, since liquid scintillator 2 detects neutrons scattered by liquid scintillator 1, the effective signal of liquid scintillator 2 is much less than that of liquid scintillator 1. A more reliable approach is to use the liquid scintillator 2 signal (T2) as the start and the liquid scintillator 1 signal (T1) as the stop. This is exactly the opposite of the actual situation. Therefore, the neutron flight time spectrum obtained after coincidence is in the negative direction of the time axis, as shown in the figure. Figure 3 As shown, Figure 3 (a) and Figure 3 (b) The gamma-ray time-of-flight spectra when the distance between the two detectors is L2 and L1, respectively. Figure 3 (c) is the intercept of the ordinate obtained by extrapolating the distance between the two detectors to 0m through data fitting, which is the zero-point deviation of the flight time. After zero-point correction, the final neutron flight time spectrum is obtained by inverting to the positive direction of time.

[0045] like Figure 4 As shown, Figure 4 (a) and Figure 4 (b) The gamma-ray time-of-flight spectra when the distance between the two detectors is 5m and 3m, respectively. Figure 4(c) is the intercept of the ordinate obtained by extrapolating the distance between the two detectors to 0 m during data fitting, which is the zero-point deviation of the time-of-flight. Therefore, using the zero-point correction method for the neutron time-of-flight spectrum, the corrected neutron time-of-flight value is 19.53 ns. For a 10 MeV neutron, the time-of-flight at 3 m is 68.84 ns. Without zero-point correction for the time-of-flight, the error would be as much as 28%.

[0046] An embodiment of the present invention provides a method for correcting the zero point of a neutron time-of-flight spectrum for neutron energy spectrum measurement, the method comprising the following steps:

[0047] S1. Place the first liquid scintillator detector on the neutron beam behind the target as the starting signal detector during the time of flight;

[0048] S2. Place a second liquid scintillator detector at a distance of L from the first liquid scintillator detector in the direction of the preset neutron beam angle, to serve as a termination signal detector for the neutron time of flight;

[0049] S3. After removing the gamma signal through pulse shape discrimination, the two liquid scintillator detectors give an initial neutron time-of-flight spectrum through a coincidence method. The zero-point deviation T_L0 is deducted from the initial neutron time-of-flight spectrum to perform zero-point correction on the neutron time-of-flight spectrum.

[0050] The neutron correction zero point T_L0 is calculated by the following steps:

[0051] S31, placing a first liquid scintillator detector on the neutron beam behind the target, and placing a second liquid scintillator detector at a distance L from the first liquid scintillator detector in the preset angular direction of the neutron beam. When L = S1, both liquid scintillator detectors only record gamma signals, and the gamma flight time T_L1 is obtained by coincidence;

[0052] S32, adjusting the position of the second liquid scintillator detector so that L=L2, both liquid scintillator detectors only record the gamma signal, and obtain the gamma flight time T_L2 through the coincidence;

[0053] S33. With the distance L between the two scintillator detectors as the horizontal axis and the gamma flight time between the two detectors as the vertical axis, perform data fitting and extrapolate the fitting straight line to L=0 to obtain the intercept on the vertical axis, that is, the zero point T_L0 of the flight time when the distance between the two detectors is zero.

[0054] It should be noted that by adjusting the position of the second scintillator detector to change the gamma flight distance, at least two gamma flight times at distances are required, namely T_L1 and T_L2. There can also be more than two distance adjustments to obtain T_L3, T_L4, ..., and more than two gamma flight times can still be fitted with a straight line and extrapolated to obtain the zero-point deviation T_L0 of the flight time.

[0055] The size of the second liquid scintillator detector should be larger than that of the first liquid scintillator detector, so that the neutron and gamma detection efficiency of the second liquid scintillator detector is greater than that of the first liquid scintillator detector.

[0056] In this embodiment, the preset angle is 45°, but in fact, there is no limitation to this and it can also be other angles.

[0057] In this embodiment, the sizes of the first liquid scintillator detector and the second liquid scintillator detector are Φ2"x2" and Φ5"x5", respectively. There is no limitation on this and other sizes are also possible.

[0058] In this embodiment, both detectors are liquid scintillator detectors, but this is not limited to other detectors. Other scintillator detectors can also be used as long as they have the ability to detect and discriminate neutrons and gamma rays. From the above embodiment, it can be seen that the present invention discloses a method for correcting the zero point of a neutron time-of-flight spectrum for neutron energy spectrum measurement. The method comprises placing a first liquid scintillator detector on the neutron beam behind the target as a start signal detector in the time-of-flight; placing a second liquid scintillator detector at a distance L from the first liquid scintillator detector in the direction of the neutron beam at a preset angle as a stop signal detector for the neutron time-of-flight; after the two liquid scintillator detectors remove the gamma signal through pulse shape discrimination, they give an initial neutron time-of-flight spectrum through a coincidence method. The initial neutron time-of-flight spectrum is deducted from the zero point deviation T_L0 to perform zero point correction on the neutron time-of-flight spectrum. By using the method disclosed in the present invention, the zero point of the neutron time-of-flight spectrum is corrected when the dual scintillator time-of-flight spectrometer measures the neutron energy spectrum, which can improve the measurement accuracy of the neutron time-of-flight, thereby improving the accuracy of the neutron energy spectrum measurement.

[0059] The method described in the present invention is not limited to the embodiments described in the specific implementation manner. Those skilled in the art may derive other implementation manners based on the technical solution of the present invention, which also fall within the scope of the technical innovation of the present invention.

Claims

1. A method for zero-point correction of a neutron time-of-flight spectrum for neutron energy spectrum measurement, the method comprising the following steps: S1. Place the first scintillator detector on the neutron beam behind the target as the starting signal detector during the time of flight; S2. Place a second scintillator detector at a distance of L from the first scintillator detector in the direction of the preset neutron beam angle, to serve as a termination signal detector for the neutron time of flight; S3. After removing the gamma signal through pulse shape discrimination, the two scintillator detectors provide an initial neutron time-of-flight spectrum through a coincidence method. The zero-point deviation T_L0 is deducted from the initial neutron time-of-flight spectrum to perform zero-point correction on the neutron time-of-flight spectrum. Utilizing accompanying gamma rays in the neutron production process, and calculating the zero-point deviation T_L0 of the neutron time-of-flight spectrum through the gamma flight times at at least two distances, the method comprises the following steps: S31, placing a first scintillator detector on the neutron beam behind the target, and placing a second scintillator detector at a distance L from the first scintillator detector in the preset angular direction of the neutron beam. When L=L1, both scintillator detectors only record gamma signals, and the gamma flight time T_L1 is obtained by coincidence; S32, adjusting the position of the second scintillator detector so that L=L2, both scintillator detectors only record the gamma signal, and obtain the gamma flight time T_L2 through the coincidence; S33. With the distance L between the two scintillator detectors as the horizontal axis and the gamma flight time between the two detectors as the vertical axis, perform data fitting and extrapolate the fitting straight line to L=0 to obtain the intercept on the vertical axis, that is, the zero-point deviation T_L0 of the flight time when the distance between the two scintillator detectors is zero.

2. A neutron time-of-flight spectrum zero point correction method for neutron energy spectrum measurement according to claim 1, characterized in that: Step S32 is replaced by: S32a. Adjust the position of the second scintillator detector so that L=L3, L4...LN. Both scintillator detectors only record gamma signals, and obtain the corresponding gamma flight times T_L3, T_L4...T_LN respectively through conformity.

3. A method for zero-point correction of neutron time-of-flight spectrum for neutron spectrum measurement according to any one of claims 1 or 2, characterized in that: The second scintillator detector is larger in size than the first scintillator detector, so that the neutron and gamma detection efficiency of the second scintillator detector is higher than that of the first scintillator detector.

4. The method for zero-point correction of neutron time-of-flight spectrum for neutron spectrum measurement according to claim 1, characterized in that: In step S3 , the signal T2 detected by the second scintillator detector is used as the start signal start, and the signal T1 of the first scintillator detector is used as the stop signal stop, and an initial neutron time-of-flight spectrum is obtained by a coincidence method.

5. The method for zero-point correction of neutron time-of-flight spectrum for neutron spectrum measurement according to claim 1, wherein: The method further comprises the steps of: S4. According to the corrected neutron time-of-flight spectrum, the detection efficiency of the two scintillator detectors and the preset angle are taken into consideration to obtain a neutron energy spectrum.

6. The method for zero-point correction of neutron time-of-flight spectrum for neutron spectrum measurement according to claim 1, characterized in that: The first scintillator detector and the second scintillator detector are at least one of liquid scintillator detectors and plastic scintillator detectors.

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