Multi-ball spectrometer response function calibration method

Abstract

The invention relates to the technical field of neutron measurement, and discloses a multi-ball spectrometer response function calibration method, and the basic principle of the method is that through the establishment of the ratio relation between two sets of multi-ball spectrometer response functions, one set of multi-ball spectrometer is used for completing the calibration experiment of a response function single-energy neutron field; the response function of another set of multi-ball spectrometer can be obtained according to the ratio relation without experimental calibration, so that the establishment time of the response function is shortened. According to the method, the problem of long time consumption of response function calibration of a low-neutron detection efficiency multi-sphere spectrometer in the prior art can be solved, and the response function of the multi-sphere spectrometer can be quickly verified.

Description

Technical Field

[0001] The invention belongs to the technical field of neutron measurement, and in particular relates to a multi-sphere spectrometer response function calibration method. Background Art

[0002] Multi-sphere spectrometers are usually composed of multiple spherical moderators with different diameters and thermal neutron counters located in the moderators. They have a wide range of measurable neutron energies (up to 10 -9 MeV-20MeV), strong anti-γ background, almost isotropic response and simple operation, etc., are widely used in wide neutron spectrum measurement research. Among them, the multi-sphere spectrometer based on the thermal neutron counter with low neutron detection efficiency (such as the multi-sphere spectrometer based on diamond detector) can meet the online measurement requirements of the neutron field with high flux, wide energy range and complex energy spectrum structure around nuclear facilities such as tokamaks, spallation neutron sources and reactors, and provide key measurement data for the operation status of the device in the nuclear facility, the radiation protection shielding design and the exposure dose of the staff.

[0003] The response function of a multi-sphere spectrometer reflects the relationship between the thermal neutron counter counts in moderators of different diameters and the neutron energy spectrum of the neutron radiation field to be measured. Therefore, the neutron energy spectrum finally calculated depends on the reliability of the response function. At present, the response function of a multi-sphere spectrometer is usually calculated using the Monte Carlo method. During the calculation process, factors such as the geometry, density and material composition of the moderator will affect the accuracy of the response function. In addition, even if exactly the same multi-sphere spectrometer geometric model is used, the relative deviation between the response functions calculated by different simulation programs may reach >25%. Therefore, it is necessary to use a standard monoenergetic neutron field covering an energy range of up to 20MeV to verify the simulated response function and complete the calibration. However, the sensitivity of a multi-sphere spectrometer with low neutron detection efficiency is lower than that of a traditional multi-sphere spectrometer (based on 3 He positive ratio counters) are 3-4 orders of magnitude lower, which makes building their response functions very time-consuming and expensive. Summary of the invention

[0004] In view of this, the present invention provides a calibration method for the response function of a multi-sphere spectrometer to overcome the problem of long time consumption in the prior art for the calibration of the response function of a multi-sphere spectrometer with low neutron detection efficiency, and to realize rapid verification of the response function of the multi-sphere spectrometer. The basic principle of the method is to establish a ratio relationship between the response functions of two sets of multi-sphere spectrometers, use one set of multi-sphere spectrometers to complete the calibration experiment of the response function of the monoenergetic neutron field, and obtain the response function of the other set of multi-sphere spectrometers according to the ratio relationship without the need for experimental calibration, thereby shortening the establishment time of the response function.

[0005] The present invention uses a multi-sphere spectrometer based on a diamond detector and a 3The proposed method is introduced by using two multi-sphere spectrometers, a multi-sphere spectrometer with a He proportional counter. The specific steps of the method mainly include: first, using the Monte Carlo program MCNP6 to calculate the response functions of the two multi-sphere spectrometers under different diameters of moderators. Then, a boron carbide spherical shell material is used to wrap the diamond detector, and the response function shape of the multi-sphere spectrometer based on the diamond detector is optimized to make it consistent with the response function of the multi-sphere spectrometer based on the diamond detector. 3 The response functions of the multi-sphere spectrometers of the He proportional counter are consistent in shape, so a ratio relationship is established between the response functions of the two multi-sphere spectrometers that does not change with the incident neutron energy. Finally, the specific response ratio of the two multi-sphere spectrometers is experimentally determined in an arbitrary neutron radiation field, and combined with the response ratio based on 3 The response function of the multi-sphere spectrometer of the He proportional counter after calibration is calculated to obtain the response function suitable for the multi-sphere spectrometer based on the diamond detector.

[0006] Furthermore, the multi-sphere spectrometer based on diamond detector and the 3 The multi-sphere spectrometer of the He proportional counter shares 9 identical spherical polyethylene moderators.

[0007] Furthermore, the diameters of the nine identical spherical polyethylene moderators are 2.5, 3.0, 3.5, 4.0, 5.0, 6.0, 8.0, 10.0, and 12.0 inches, respectively, and the average density is 0.96 g / cm 3 .

[0008] Further, based on 3 The multi-sphere spectrometer of the He proportional counter is mainly composed of stainless steel cladding, ceramic insulator, borosilicate glass and other materials.

[0009] Furthermore, the multi-sphere spectrometer based on diamond detectors uses a coated 6 Single crystal diamond detector with LiF neutron conversion layer. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] In order to more clearly illustrate the specific implementation methods of the present invention or the technical solutions in the prior art, the drawings required for use in the specific implementation methods or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some implementation methods of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0011] Figure 1 It is the process of the method of the present invention;

[0012] Figure 2 It is a diamond detector with LiF conversion layer. 3 Self-shielding factor of He proportional counter;

[0013] Figure 3 shows the diamond detector and 3 Normalized moderated neutron energy spectrum in the sensitive material of He proportional counter, with incident neutron energies of 0.0253 eV (a) and 1 MeV (b);

[0014] Figure 4 shows the diamond detector and the diamond detector after adding boron carbide material under a 2.5" sphere. 3 Normalized moderated neutron energy spectrum in the sensitive material of He proportional counter, with incident neutron energies of 0.0253 eV (a) and 1 MeV (b);

[0015] Figure 5 The results are as follows: the multi-sphere spectrometer based on diamond detector and the multi-sphere spectrometer based on 3 The response function shape of the He proportional counter multi-sphere spectrometer; DETAILED DESCRIPTION

[0016] In order to make the purpose, technical solution and advantages of the present invention clearer, the technical solution of the present invention will be described clearly and completely below. Obviously, the described embodiments 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 creative work belong to the scope of protection of the present invention.

[0017] 1. Introduction to Multi-sphere Spectrometer

[0018] Diamond detector-based multi-sphere spectrometer and 3 The multi-sphere spectrometer of the He proportional counter shares 9 identical spherical polyethylene moderators to eliminate the response function calculation errors caused by inconsistent moderator parameters. The diameters of the moderators are 2.5, 3.0, 3.5, 4.0, 5.0, 6.0, 8.0, 10.0, and 12.0 inches, respectively, with an average density of 0.96 g / cm 3 .

[0019] based on 3 The multi-sphere spectrometer of the He proportional counter uses a spherical SP9 3 The He proportional counter is mainly composed of stainless steel shell, ceramic insulator, borosilicate glass and other materials. The sensitive volume of the detector is about a sphere with a diameter of 33 mm. 3 He reaction with thermal neutrons 3 He(n,p) 3 The number of secondary charged particles produced by H is used to obtain the number of incident neutrons. The manufacturer's nominal pressure is 697Kpa (at 20°C), and the thermal neutron reference radiation field is determined. 3 The density of He atoms is 1.4824×10 20 atoms / cm3 .

[0020] The diamond detector-based multisphere spectrometer uses a coated 6 Single crystal diamond detector for LiF neutron conversion layer. The effective area of ​​the diamond crystal is 3.8×3.8mm2, the thickness is 0.5mm, and it is installed at the bottom of an aluminum barrel with a diameter of 1cm. The thickness is 5μm, the diameter is 7mm, and the enrichment is 90% 6 The LiF material of Li is evenly coated on the surface of a 0.5mm thick aluminum substrate, which also serves as the barrel cover. 6 There is a 3mm air gap between the LiF neutron converter and the diamond crystal. 6 The Li(n,T)α reaction records the number of incident thermal neutrons. The energies of the tritium particles and alpha particles in the reaction products are 2.73MeV and 2.06MeV, respectively, and are emitted in opposite directions. The presence of the air gap will cause the tritium particles emitted at a large angle relative to the diamond crystal surface to lose more energy, thereby removing the low-energy part of the tritium particles in the pulse height spectrum. In addition, since the alpha particles lose most of their energy in the LiF neutron conversion layer and the air gap, in the pulse height spectrum, the tritium particle energy peak is significantly independent of the background gamma-ray and alpha particle energy peaks. Therefore, when the number of recorded tritium particles is used to reflect the number of incident neutrons, the multi-sphere spectrometer based on the diamond detector has a high suppression ability for background gamma-rays.

[0021] 2. Calculation of multi-sphere spectrometer response function

[0022] The geometric structures of the two multi-sphere spectrometers were constructed in detail according to the above parameters, and the response function was calculated in the Monte Carlo program MCNP6. The response function of the multi-sphere spectrometer is defined as: the number of nuclear reactions occurring in the sensitive volume of the detector when a unit neutron is incident, the response R of the multi-sphere spectrometer with a diameter of d to a monoenergetic neutron with an energy of E d (E) can be calculated by the following formula,

[0023]

[0024] Where a is the incident neutron source area (cm 2 ); c is the ratio of the number of nuclear reactions recorded by the detector to the number of nuclear reactions that occurred; N is the number of atoms of the neutron sensitive material in the detector; Φ(E') is the neutron fluence energy distribution in the sensitive volume of the detector (MeV -1 cm -2 ), the effect of self-shielding factor needs to be considered; σ(E') is the reaction cross section of nuclear reaction (cm 2 ). The energy range of the neutron source in the program is 10 -9MeV-20MeV, and the calculation is performed using a parallel source method.

[0025] 3. Comparison of response functions of two multi-sphere spectrometers

[0026] The simulation results compare the multi-sphere spectrometer based on diamond detector and the 3 The response functions of the multi-sphere spectrometer of the He proportional counter are similar in shape but there is still a difference in the specific response values. In the MeV neutron energy region and the neutron energy region below eV, the maximum deviation of the normalized response functions of the 2.5" sphere of the two is about 15%. When the sphere size increases from 2.5" to above 3.5", the deviation drops from 15% to less than 5%. For spheres larger than 3.5", it can be considered that the response functions of the two are similar in shape within the range of less than 5% deviation. Due to the difference in the response functions of the two multi-sphere spectrometers, the response ratio changes with the neutron energy. Therefore, it is necessary to adjust the response function of the multi-sphere spectrometer based on the diamond detector and establish a response ratio based on the diamond detector. 3 The ratio relationship between the response functions of the multi-sphere spectrometer of the He proportional counter does not change with the neutron energy.

[0027] 4. Factors affecting the response function of multi-sphere spectrometer

[0028] In a multi-sphere spectrometer, incident neutrons slowed down by a moderator will absorb and scatter with the counter material before reacting with the thermal neutron sensitive material. The self-shielding factor generated by the material self-absorption will cause the average flux in the counter to be lower than the neutron flux incident on the counter. The self-shielding factors of the two multi-sphere spectrometers are as follows: Figure 2 As shown. In addition to the self-shielding factor, it must be noted that the thermal neutron counter is placed in a limited area in the center of the moderator to detect neutrons. After being moderated by the moderator, the incident neutron field will interact with the thermal neutron counter material itself, and then further interact with the moderator material and enter the counter again. Under the condition of different energy neutron incidence, the normalized moderated neutron energy spectrum at the thermal neutron sensitive material in the 2.5" sphere of the two multi-sphere spectrometers is shown in Figure 3. The results have taken the self-shielding factor into account. Although 6 Li and 3 The neutron absorption cross section of He changes according to the 1 / v law and has a similar shape, but the neutron energy spectrum shape of the reaction with thermal neutron sensitive materials is different, which ultimately leads to different response function shapes of the two multi-sphere spectrometers. Therefore, if the response function shapes of the two sets of multi-sphere spectrometers are to be consistent, the structural design of the diamond detector must be optimized to make the type and degree of interaction between neutrons, especially neutrons in the thermal neutron energy region, and the diamond detector consistent. 3 The consistency of the He counter, that is, Φ in formula (1) th (E') Close to or identical.

[0029] 5. Response function optimization based on diamond detection multi-sphere spectrometer

[0030] Because based on 3 The neutron sensitive material of the multi-sphere spectrometer of the He proportional counter is 3 He mainly absorbs neutrons and has a very small scattering cross section. Therefore, materials with a strong neutron absorption cross section and a low neutron scattering cross section can be used to make them into spherical shells and fill the remaining space between the diamond detector and the polyethylene moderator, thereby optimizing the response function shape of the multi-sphere spectrometer based on diamond detection. 10 B material. Boron carbide material ( 10 The boron carbide content is 19.8%), which is made into a spherical shell to wrap the diamond detector. During the response function simulation of the 2.5" sphere, the thickness of boron carbide is gradually adjusted and the obtained response value is compared with the response value based on 3 The response values ​​of the He proportional counter multi-sphere spectrometer are compared, and the boron carbide thickness with the smallest difference between the two is selected as the design basis. When the boron carbide shell thickness is 140μm, the response function shapes of the two multi-sphere spectrometers are most consistent. At this time, the normalized moderated neutron energy spectrum in the thermal neutron sensitive material is shown in Figure 4.

[0031] Table 1 Maximum relative deviation and relative standard deviation of the response ratio of the two multi-sphere spectrometers under different thicknesses of boron carbide shells

[0032]

[0033]

[0034] 6. Calculation of the response function ratio of the two multi-sphere spectrometers after optimization

[0035] In 10 -9 90 logarithmically equidistant points were selected between MeV and 20MeV, and the response values ​​of the two optimized multi-sphere spectrometers at each neutron energy point were calculated. The relative uncertainty of the simulation results was controlled within 2%. Table 2 shows the response ratio of the selected neutron energy points under the same diameter moderator (the response value of the multi-sphere spectrometer based on the diamond detector divided by the corresponding energy based on the 3 The average value of the response value of the multi-sphere spectrometer of the He proportional counter) and the maximum relative deviation between the response ratios. Under the same moderator, the multi-sphere spectrometer composed of a diamond detector with a boron carbide shell is different from the multi-sphere spectrometer based on 3 The response function shape of the multi-sphere spectrometer of the He proportional counter is basically the same in the -20MeV energy region of thermal neutrons. The response values ​​have a fixed ratio relationship and do not change with the neutron energy, such as Figure 5 shown.

[0036] Table 2 Comparison of the multi-sphere spectrometer composed of diamond detectors with boron carbide shells added under different sphere sizes and the 3 Response function ratios of multi-sphere spectrometers for He proportional counters and the maximum relative deviation between response ratios

[0037]

[0038]

[0039] 7. Calculate the response function of the multi-sphere spectrometer based on the diamond detector according to the response function ratio

[0040] After the above optimization, the response function ratio of the two multi-sphere spectrometers does not change with the incident neutron energy. Therefore, the response function ratio of the multi-sphere spectrometer based on diamond detector and the multi-sphere spectrometer based on diamond detector can be obtained under any neutron field. 3 The relative counting rate of the He proportional counter multi-sphere spectrometer is determined to determine the ratio between the two. 3 The He proportional counter multi-sphere spectrometer completes the experimental calibration of the response function, and based on the obtained ratio relationship, the response function suitable for the multi-sphere spectrometer based on the diamond detector can be obtained. Specifically, it can be expressed as:

[0041]

[0042] In the formula, R SCVD-BSS With R 3He-BSS They represent the multi-sphere spectrometer based on diamond detector and the 3 The response functions of the He proportional counter multi-sphere spectrometer; C SCVD-BSS With C 3He-BSS They represent the difference between the multi-sphere spectrometer based on diamond detector and the multi-sphere spectrometer based on diamond detector and the multi-sphere spectrometer based on diamond detector respectively in the same neutron radiation field. 3 The counting rates of the He proportional counter multi-sphere spectrometer.

Claims

1. A method for calibrating a multi-sphere spectrometer response function, characterized in that: By establishing a ratio relationship between the response functions of two sets of multi-sphere spectrometers and using one of the multi-sphere spectrometers to complete a calibration experiment of the response function of a monoenergetic neutron field, the response function of the other set of multi-sphere spectrometers can be obtained according to the ratio relationship without the need for experimental calibration, thereby shortening the time for establishing the response function.

2. A multi-sphere spectrometer response function calibration method according to claim 1, characterized in that: The two sets of multi-sphere spectrometers include: a multi-sphere spectrometer based on diamond detectors and a multi-sphere spectrometer based on 3 Two types of multi-sphere spectrometers: multi-sphere spectrometer with He proportional counter.

3. A multi-sphere spectrometer response function calibration method according to claim 2, characterized in that: Diamond detector-based multi-sphere spectrometer and 3 The multi-sphere spectrometer of the He proportional counter shares 9 identical spherical polyethylene moderators.

4. A multi-sphere spectrometer response function calibration method according to claim 3, characterized in that: The diameters of the 9 identical spherical polyethylene moderators are 2.5, 3.0, 3.5, 4.0, 5.0, 6.0, 8.0, 10.0, and 12.0 inches, respectively, with an average density of 0.96 g / cm 3 .

5. A multi-sphere spectrometer response function calibration method according to claim 2, characterized in that: based on 3 The multi-sphere spectrometer of the He proportional counter is mainly composed of stainless steel cladding, ceramic insulator, borosilicate glass and other materials.

6. A multi-sphere spectrometer response function calibration method according to claim 2, characterized in that: The diamond detector-based multisphere spectrometer uses a coated 6 Single crystal diamond detector with LiF neutron conversion layer.

7. A multi-sphere spectrometer response function calibration method according to claim 1, characterized in that: The response functions of the two multi-sphere spectrometer geometries were calculated in a Monte Carlo procedure.

8. A multi-sphere spectrometer response function calibration method according to claim 1, characterized in that: Multi-sphere spectrometer based on diamond detector, select 10 Material B is made into a spherical shell shape to wrap the diamond detector, thereby optimizing the response function shape of the multi-sphere spectrometer based on diamond detection.

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