An optimized design method for real-time measurement of alpha particle detector based on air scintillation

Abstract

The application discloses a kind of real-time alpha particle detector optimization design method based on air scintillation, with the lowest detection sensitivity as design goal, through Monte Carlo analysis method, carry out multi-parameter optimization design, specifically through detector modeling, threshold and coincidence logic setting, the detection efficiency calculation of alpha particle, cosmic-ray background and the accidental coincidence count rate calculation of PMT and measurement time setting etc. Steps optimize the system parameters of detector, according to the lowest detection sensitivity lower limit principle, determine optimal detector model, threshold and coincidence logic and measurement time etc. The method is simple and easy to understand, high universality, can solve the problem of low detection efficiency of alpha particle and low signal-to-noise ratio of real-time alpha particle detector based on air scintillation due to low air light yield, reach the lowest detection sensitivity lower limit of alpha particle allowed in actual situation, applicable to the efficient real-time measurement of alpha particle concentration / activity under the condition of low radioactivity level with air scintillation.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of nuclear radiation detection, and particularly relates to an optimization design method of an air scintillation-based real-time alpha particle detector. BACKGROUND

[0002] On-site continuous monitoring of artificial radioactive aerosols in nuclear technology application sites and inside and outside nuclear facilities is required for nuclear safety, and the monitoring can ensure timely and automatic early warning in the case of nuclear leakage accidents, so as to quickly take emergency measures. The main representative of artificial radioactive aerosols is uranium, plutonium and other alpha aerosols, and the radioactive alpha aerosols formed by the long-lived alpha radionuclides and their decay daughters are the main source of internal exposure of workers, which seriously threatens the occupational health and safety of workers. Therefore, on-site rapid, real-time and non-contact measurement of alpha aerosol concentration has important value for nuclear facility operation and maintenance, protection of nuclear personnel and the public safety, and provision of accident rescue.

[0003] The detection and measurement of uranium and plutonium alpha aerosols is essentially the detection and measurement of alpha particles. Since the range of alpha particles is very short (for example, the range of 5 MeV alpha particles in air is about 3.5 cm), the measurement efficiency of current detectors (ionization chambers, solid scintillators, semiconductors, etc.) is not high, resulting in low detection sensitivity. In addition, the traditional measurement method generally requires sampling, and the operation steps are complex, which cannot meet the on-site real-time and non-contact measurement requirements. These shortcomings bring difficulties to the prevention of nuclear accidents, the rapid development of rescue after nuclear accidents, and the protection of nuclear personnel from high-dose internal exposure.

[0004] Studies have shown that the deposition energy of alpha particles in air causes excitation of nitrogen in air, and the nitrogen de-excitation emits ultraviolet photons, more than 95% of which are in the range of 310 nm-400 nm (mainly in the ultraviolet A band). In addition, alpha particles release (19±3) ultraviolet photons per 1 MeV of energy in air, and the ultraviolet photons in this band have a decay length of more than 1,000 meters in air, which is much larger than the range of alpha particles in air, and have strong penetration. Therefore, the advantages of the real-time alpha particle detector based on air scintillation are: ① air is a detection sensitive material, and no sample preparation is required, so real-time and rapid measurement can be achieved; ② alpha particles are converted into ultraviolet light, and ultraviolet light has strong penetration, which can improve the measurement efficiency and thus improve the detection sensitivity.

[0005] However, the current mechanism does not consider the low light yield, low detection efficiency, cosmic ray background and PMT noise, etc. The reason is that the light yield of the scintillator is low, the signal is weak, and the signal may be buried in the noise, which may cause the experimental data to be unable to be measured, so noise reduction and efficiency improvement are the key to the problem, and the detection sensitivity can reflect the noise reduction effect and efficiency, and is often used as an index for detector design, so the existing technology cannot design the detector by the detection sensitivity, thereby affecting the detection efficiency and signal-to-noise ratio of the real-time alpha particle detector based on air scintillation due to low air light yield. SUMMARY

[0006] In order to overcome the above-mentioned shortcomings of the prior art, the purpose of the present application is to provide an optimized design method of a real-time alpha particle detector based on air scintillation, to solve the technical problems of low detection efficiency and low signal-to-noise ratio of the real-time alpha particle detector based on air scintillation due to low air light yield.

[0007] In order to achieve the above-mentioned purpose, the present application adopts the following technical solutions:

[0008] The optimized design method of a real-time alpha particle detector based on air scintillation disclosed by the present application comprises the following steps: step 1, modeling the detector, determining the volume of the detector, setting the incident particles as alpha particles and muons, simulating the number of photons reaching the photocathode, and sampling the number of photoelectrons according to the PMT quantum efficiency curve for each photon;

[0009] Step 2, setting a threshold and coincidence logic for the number of photoelectrons generated by the incident particles set in step 1, and performing noise reduction processing;

[0010] Step 3, counting the alpha particles that meet the threshold and coincidence logic, calculating the detection efficiency of the alpha particles, counting the muons that meet the threshold and coincidence logic, calculating the cosmic ray background count rate, and calculating the dark noise count rate of the PMT under the same threshold and coincidence logic;

[0011] Step 4, setting the measurement time according to the actual measurement requirement and the detection sensitivity requirement of the alpha particles;

[0012] Step 5, calculating the detection sensitivity of the alpha particles, comparing and judging whether the detection sensitivity of the alpha particles under different detector parameters is the minimum value among all values:

[0013] If yes, the alpha particle detector parameters are determined, and the optimization design is completed;

[0014] If not, return to step 1 until the optimization design is completed.

[0015] Preferably, step 4 sets the measurement time operation can be performed before or between steps 1, 2, 3, that is, the operation step of setting the measurement time does not affect the final design result.

[0016] Further preferably, the real-time alpha particle detector based on air scintillation in the application is a pulse counting detector: when the half-life of a certain nuclide that can decay alpha particles is longer than the measurement time, the statistical error of pulse counting decreases with the increase of the measurement time; when the half-life of a certain nuclide that can decay alpha particles is shorter than the measurement time, the measurement is too long and the detection sensitivity is reduced. Therefore, in the design process, the measurement time needs to be set according to the actual measurement requirements combined with the detection sensitivity requirements of alpha particles. The initial value is set to t i = 300 s.

[0017] Preferably, in step 1, modeling the detector includes: setting the geometry and material of the detector, setting the radioactive source, setting the physical process, setting the air light parameter, and setting the material optical parameter.

[0018] Further preferably, the geometry and material are set: the structure of the air scintillation real-time measurement alpha particle detector includes a measurement cavity, a reflective film, a measurement cavity shell and a PMT, and the geometric parameters include the shape of the detector, the size of the measurement cavity, the thickness of the measurement cavity reflective film, the thickness of the measurement cavity shell and the size of the PMT photocathode, etc. Generally, the shape of the measurement cavity chamber is cuboid, cylinder, sphere, etc.; the corresponding size is: the length, width and height of the cuboid, the diameter and height of the cylinder, the diameter of the sphere, etc. In terms of material: the measurement cavity is air; the reflective film is aluminum, chromium, silver, etc.; the measurement cavity shell is aluminum, acrylic, etc.; the PMT simulation modeling mainly considers its incident window and photocathode, and the incident window material is generally quartz, borosilicate glass, violet glass, etc., and the photocathode is generally a double-alkali photocathode surface, a multi-alkali photocathode surface, an alkali halide photocathode surface, etc.

[0019] More preferably, the initial values of the geometry and material parameters of the detector are set according to typical values: an aluminum spherical chamber, an air measurement cavity diameter of 10.00 cm, a measurement cavity aluminum foil reflective film thickness of 0.09 cm, a measurement cavity aluminum shell thickness of 0.30 cm, a PMT incident window of a quartz cylinder (diameter 2.80 cm, thickness 2.20 mm), and a photocathode of a double-alkali cylinder (diameter 2.80 cm, thickness 20.00 nm).

[0020] It should be noted that: (1) In the actual processing process, the production difficulty, portable design or fixed design (equipment net weight) requirements for the shape and size of the detector need to be considered. (2) The selection principle and arrangement method of the PMT. (3) The setting of air pressure.

[0021] Therefore, further preferably, in step 1, the number of photons is sampled by a PMT quantum efficiency curve, specifically: the PMT is selected according to the following principle:

[0022] First, the spectral response curve of the PMT should match the air scintillation spectrum to ensure a high effective quantum efficiency QE eff , and the calculation formula is as follows: ①

[0023] QE eff =∑I(λ)*QE(λ)……………………①

[0024] Where I(λ) is the normalized light intensity of wavelength λ ultraviolet light generated by α particles exciting air; QE(λ) is the quantum efficiency of the PMT for incident light of wavelength λ.

[0025] Second, the PMT photocathode coverage area should be as large as possible. The PMT arrangement refers to the arrangement of different numbers of PMTs in different chamber shapes, including symmetric and asymmetric arrangements of different numbers.

[0026] Further, the PMT can be selected from the products of domestic and foreign companies that mainly produce PMTs on the market, such as Beiqing Night Vision Technology Co., Ltd., Beijing High-tech Baisen Optoelectronic Counting Institute, Hamamatsu Group, PHOTEK, ET Enterprises, etc. The quantum efficiency of the PMT in the simulation is input according to the quantum efficiency curve provided by the manufacturer. The initial value of the PMT is set to the PMT of the High-tech Baisen GDB23, which has a photocathode diameter of 28 mm, is an end window type, has a double alkali photocathode, and an effective photosensitive surface diameter of 25 mm, and a spectral response range of 300-650 nm.

[0027] Further, regarding the air pressure, according to the ideal gas state equation, the air pressure is set, i.e. the air density under the corresponding pressure is set. Increasing the air pressure in the chamber can reduce the range of α particles, so that α particles near the inner edge of the chamber can deposit more energy in the air to generate more ultraviolet photons, which is beneficial to improve the detection efficiency of α particles. However, during the pressurization of the chamber, the maximum pressure that the PMT photocathode surface and the chamber structure can withstand needs to be considered. The initial value is set to the normal air density under standard conditions (i.e. one standard atmosphere), and the chamber pressure can be increased step by step in the subsequent screening and evaluation process.

[0028] Further preferably, a radioactive source is set: the α source can be a solid source or an aerosol source. The particle emission direction and vertex distribution of the solid source are set according to the actual application. The aerosol case is determined, and an aerosol is taken as an example: the radioactive source is uniformly distributed in the measurement chamber, and the direction is isotropic, and the energy and proportion are set according to the energy and proportion of α particles decayed by a certain nuclide.

[0029] Further preferably, the physical processes of the scintillation photons are set, including the generation and absorption of the scintillation photons, reflection, refraction, etc.; the physical processes of the electrons are multiple scattering, ionization, δ-ray generation, and bremsstrahlung, etc.; and the physical processes of the ions are ionization and multiple scattering, etc.

[0030] Further preferably, the air scintillation parameters are set, i.e., the scintillation parameters of the air. According to the literature, the air scintillation parameters are set as follows: the refractive index is 1.00, the light emission decay time is 7 ns, the light yield is 20 ph / MeV, and the relative intensity of the fast component is 1.0.

[0031] Further preferably, the material optical parameters are set, including the reflectivity of the reflective film and the refractive index of the PMT entrance window, etc. The reflectivity of the reflective film is 0% to 98% in the light emission spectrum range of the air after being excited by the α particles. It should be noted that the increase of the reflectivity of the reflective film will reduce the loss of the ultraviolet photons when they are reflected on the reflective film, thereby increasing the number of the ultraviolet photons hitting the PMT, which is conducive to improving the detection efficiency and signal amplitude of the α particles. According to the parameters provided by the manufacturer, the initial value of the reflectivity of the reflective film is set as 80%; and the refractive index of the PMT entrance window is set as 1.50 according to the parameters provided by the manufacturer.

[0032] Further preferably, in step 2, the threshold and coincidence logic are set by using the N out of M coincidence logic. The detection sensitivity of the detector will be affected by the background count (mainly the cosmic ray background count and the PMT dark noise background count), and the threshold and coincidence are set to reduce the noise. The threshold setting means that when the amplitude of the input signal is lower than a given value, there is no output signal; and when it exceeds the given value, a fixed amplitude signal with a certain width is output. This given value is called the threshold value (in simulation, the threshold value corresponds to the number of photoelectrons). The noise signal of each PMT is random and generally has no time correlation. The coincidence logic method refers to the method of using coincidence circuit logic to select coincidence events. Different coincidence logic circuits are used to judge the simultaneity or correlation of two or more events in time (i.e., the logic method of judging a true event as a true event when M or more PMTs out of a total of N PMTs have threshold signals at the same time within the coincidence time range, which is referred to as the N out of M scheme, denoted as M / N). By selecting a reasonable N out of M scheme, the noise signal of accidental coincidence can be reduced while ensuring the efficiency, thereby improving the signal-to-noise ratio and the detection sensitivity of the measurement. The initial value is set as 2 out of 2, and the threshold value is 1 pe.

[0033] It should be noted that when the threshold and coincidence logic scheme are set, the detection efficiency of the α particles, the cosmic ray background count rate, and the accidental coincidence count rate of the PMT should be considered to ensure the most sensitive detection sensitivity for the α particle concentration.

[0034] Further preferably, in step 3, the alpha particles satisfying the threshold value and the coincidence logic are counted, and the detection efficiency of the alpha particles is calculated as follows:

[0035] The geometric model determined in step 1 is modeled by Monte Carlo software, and the setting of the radiation source is as described above. Finally, the detection efficiency ε of the alpha particles is calculated α , and the calculation formula is as follows:

[0036]

[0037] , wherein ε is the detection efficiency of the alpha particles; N is the number of alpha particles detected after setting the threshold value and the coincidence logic, and the unit is number; N is the number of alpha particles incident into the measurement cavity in the simulation, and the unit is number. α α,RX α,TX

[0038] Further, in step 3, the muons satisfying the threshold value and the coincidence logic are counted, and the cosmic-ray background counting rate is calculated as follows:

[0039] About 90% of the secondary charged cosmic rays on the ground are muons, and muons also contribute most to the background. Since muons have super strong penetration ability, it is difficult for general shielding bodies to effectively shield muons, so muons will probably enter the detector measurement cavity and deposit energy to cause background counting. Therefore, the geometric model in step 1 is modeled, the threshold value and the coincidence logic are the same as in step 2, and the incident particles in step 1 2) are changed: the incident particles are muons, and the muon energy and muon incident angle are set according to the cosmic-ray energy spectrum distribution and the cosmic-ray zenith angle distribution on the sea level (here, the detector is considered as a radiation-exposed body placed on the sea level, and the cosmic rays are equivalent to a circular surface source (the area of this surface source is generally selected to be much larger than the maximum cross-sectional area of the detector, such as 10 times or more, which covers the highest point of the detector and remains horizontal. The zenith angle θ of the cosmic rays is in the range of 0° to 90°, and is a non-uniform sampling distribution, which is approximately cos 2 θ distribution); the muon detection efficiency ε μ is simulated, and the cosmic-ray background counting rate R μ is calculated, and the calculation formula is as follows:

[0040]

[0041] , wherein ε is the muon detection efficiency; N is the number of muons detected after setting the threshold value and the coincidence logic, and the unit is number; N is the number of muons incident into the measurement cavity in the simulation, and the unit is number. μ μ,RX μ,TX

[0042] ​​​​​​

[0043] where R μ is the cosmic ray background count rate, unit Hz; r is the radius of the cosmic ray surface source in the simulation, unit cm; is the flux of muons measured in the environment, unit cm -2 s -1 .

[0044] Further, in step 3, the dark noise count rate of the PMT is calculated under the same threshold and coincidence logic as step 2. The dark noise of the PMT is the dark current generated by the PMT during use. The dark current is a inherent property of the PMT and cannot be eliminated by design. In addition, the dark noise count rate of different types of PMT is different, and the dark noise count rate of the same type of PMT may be different due to individual differences and different use environments. Therefore, in the calculation process, the accidental coincidence count rate R n of the PMT needs to be measured or estimated.

[0045] It includes two methods, including:

[0046] The first method is to cover the PMT photocathode with a black cloth, then apply high voltage to the PMT, and in the absence of a detection radiation object, use the same threshold and coincidence logic as in step 3, count the background counts output by the PMT through the counter, and directly measure the accidental coincidence count rate. The calculation formula is and the relative error formula is:

[0047]

[0048] where R n is the accidental coincidence count rate of the PMT under the same threshold and coincidence logic in step 3, unit Hz; N n is the PMT dark noise count detected after setting the threshold and coincidence measurement, unit number; t n is the measured time, unit s.

[0049]

[0050] where σ r,n is the relative error of R n in formula 5, unitless; N n is the PMT dark noise count detected after setting the threshold and coincidence measurement in formula 5, unit number;

[0051] As can be seen from formula 6, the longer the measurement time, the more PMT dark noise counts, and the smaller the relative error of R n .

[0052] Another, can be according to the performance parameters of PMT provided by the manufacturer of the theoretical estimate, its calculation formula for ⑦:

[0053]

[0054] Where, R n The same threshold value and the same threshold value in step 3 and the accidental coincidence counting rate of PMT under the condition of meeting the logic, unit: Hz; T is the typical width of the threshold value output signal, unit: s; F is the typical noise counting rate of PMT under different threshold value, unit: Hz; The number of combinations of i objects in a group of N objects.

[0055] Preferably, in step 5, the different detector parameters described refer to the detector volume V required for calculating the detection sensitivity of alpha particles (obtained from step 1), the detection efficiency of alpha particles ε α (First for noise reduction, step 2 is executed, and finally obtained from step 3), the detection efficiency of muons ε μ (First for noise reduction, step 2 is executed, and finally obtained from step 3), the calculation of PMT accidental coincidence counting rate (first for noise reduction, step 2 is executed, and finally obtained from step 3) and measurement time t, these 5 parameters. Since it is to find the best detection sensitivity of alpha particles (i.e. the minimum detection sensitivity of alpha particles), the 5 parameters need to be adjusted constantly until the best detection sensitivity of alpha particles (i.e. the minimum detection sensitivity of alpha particles) is found.

[0056] Further preferably, in step 5, in the process of calculating the detection sensitivity of alpha particles, the background calculation method includes two kinds:

[0057] The first kind: the detection sensitivity refers to the lowest lower limit that the detector can detect, which reflects the detection performance of the detector. For real-time alpha particle detectors based on air scintillation, the lower limit of its detection sensitivity is affected by alpha particle detection efficiency and background. Its calculation formula is ⑧⑨:

[0058] N b = (R n + R μ )*t i ……………………⑧

[0059] Where, N b The total background count, unit: number; R n The accidental coincidence counting rate of PMT under N-M condition, unit: Hz; R μ The cosmic ray background counting rate, unit: Hz; t i The measurement time, unit: s.

[0060] Method two: N bThe calculation of R can also be directly measured by experiment μ The calculation of R can also be directly measured by experiment n The actual measurement process is as follows: first, the PMT is assembled in the detector, then the PMT is applied with high voltage, in the absence of a detection radiation object, the same threshold and coincidence logic as in step 3 are used, and the background count of the PMT output is counted by the counter, and the total background count is directly measured.

[0061] The calculation method of the detection sensitivity of α particles is as follows:

[0062] L Q = 50[1 + (1 + 0.08N b ) 1 / 2 ] … … ⑨

[0063] Where L Q is the lower limit of quantification (confidence of 95%, relative error of measurement less than 10%), unit: number; N b is the total background count, unit: number.

[0064]

[0065] Where C0 is the initial concentration of α particles entering the chamber, corresponding to the detection sensitivity, unit: Bq / m 3 ; λ is the decay coefficient of the nuclide that can decay α particles, unit: s -1 ; t i is the measurement time, unit: s; L Q is the lower limit of quantification, unit: number; N b is the total background count, unit: number; ε α is the α particle detection efficiency; V is the measurement cavity volume, unit: m 3 .

[0066] In addition, the measurement time and the length of the half-life will affect the calculation of C0, the specific content is as follows:

[0067] 1) When the measurement time is much smaller than the half-life, the formula ⑧⑨ is substituted into the formula ⑩, and the calculation formula can be approximated as

[0068]

[0069] Where C0 is the initial concentration of α particles entering the chamber, corresponding to the detection sensitivity, unit: Bq / m 3 ; t i is the measurement time, unit: s; R n is the accidental coincidence count rate of PMT, unit: Hz; R μ is the cosmic background count rate, unit: Hz; ε αC0 is the initial concentration of alpha particles entering the chamber, corresponding to the detection sensitivity, unit: Bq / m 3 .

[0070] Obviously, in this case, the longer the measurement time, the lower the detection sensitivity.

[0071] 2) When the measurement time is not much smaller than the half-life, formula 8 and 9 are substituted into formula 10, and the calculation formula is

[0072]

[0073] Where C0 is the initial concentration of alpha particles entering the chamber, corresponding to the detection sensitivity, unit: Bq / m 3 ; λ is the decay coefficient of the nuclide that can decay alpha particles, unit: s -1 ; t i is the measurement time, unit: s; R n is the accidental coincidence count rate of PMT, unit: Hz; R μ is the cosmic background count rate, unit: Hz; ε α is the alpha particle detection efficiency, unitless; V is the volume of the measurement chamber, unit: m 3 .

[0074] Obviously, in this case, the measurement detection sensitivity is not monotonically decreased with the increase of t i , and the optimal measurement time needs to be determined according to the minimum point of the formula.

[0075] Compared with the prior art, the present application has the following beneficial effects:

[0076] The optimization design method of the real-time measurement alpha particle detector based on air scintillation disclosed by the present application can realize the optimization of the real-time measurement alpha particle detector based on air scintillation by optimizing the detector design parameters and the measurement method. Specifically, the lowest detection sensitivity is taken as the design target, the Monte Carlo analysis method is used for multi-parameter optimization design, the system parameters of the detector are optimized through the steps of detector modeling, threshold and coincidence logic setting, alpha particle detection efficiency calculation, cosmic background and PMT accidental coincidence count rate calculation, and measurement time setting, so as to determine the optimal detector model. Therefore, the problem of low detection efficiency of alpha particles and low signal-to-noise ratio of the real-time alpha particle detector based on air scintillation due to low air light yield can be solved, the lowest detection sensitivity limit of alpha particles allowed in actual situation is reached, and the high-efficiency real-time measurement of alpha particle concentration / activity under the condition of low radioactivity level and air scintillation is applicable. BRIEF DESCRIPTION OF DRAWINGS

[0077] Figure 1 is the design method logic diagram of the present application;

[0078] Figure 2 Figure 1 is a schematic diagram of a chamber structure of a prototype device according to the principles of the present application; wherein (a) is a front view of the structure; and (b) is a sectional view;

[0079] Figure 3 Figure 2 is a schematic diagram of an air scintillation spectrum distribution and a quantum efficiency spectrum of a PMT;

[0080] Figure 3 is a schematic diagram of a prototype device according to the principles of the present application; wherein: 1 is a measurement cavity housing; 2 is a pressure baffle; 3 is a PMT; 4 is an air inlet; and 5 is an air outlet. DETAILED DESCRIPTION

[0081] In order to enable persons skilled in the art to better understand the present application, the technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only some of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by persons skilled in the art without creative work should fall within the scope of protection of the present application.

[0082] It should be noted that the terms "first", "second", and the like in the specification and claims of the present application and the above-described drawings are used to distinguish similar objects, and do not necessarily indicate a specific order or a chronological sequence. It should be understood that the data thus used can be interchanged under appropriate circumstances, so that the embodiments of the present application described herein can be implemented in an order other than that illustrated or described herein. In addition, the terms "include" and "have" and any variations thereof are intended to cover non-exclusive inclusion, for example, a process, method, system, product or device that includes a series of steps or units does not necessarily have to be limited to those steps or units clearly listed, but can include other steps or units that are not clearly listed or inherent to the process, method, product or device.

[0083] The present application will be described in further detail below with reference to the drawings:

[0084] As shown in Figure 1 Figure 1, the present application is a real-time alpha particle detector optimization design method based on air scintillation. The method takes the lowest detection sensitivity as the design target, and optimizes the system parameters of the detector through Monte Carlo analysis method, specifically through the steps of detector modeling, threshold and coincidence logic setting, calculation of detection efficiency of alpha particles, calculation of cosmic ray background and accidental coincidence rate of PMT, and measurement time setting, etc. According to the principle of the lowest detection sensitivity limit, the optimal detector model, threshold and coincidence logic, and measurement time, etc. are determined.

[0085] In view of the effectiveness and reliability of the embodiments, according to the actual situation, the Figure 1After optimizing the method design, for ease of implementation through examples... Figure 1 The optimization design method described here uses fixed parameter settings for the detector model (which can be changed according to actual conditions; this is just an example for convenience). A specific example will be used to illustrate this further:

[0086] Step 1: Considering that the spherical measuring cavity has better resistance to internal and external pressures than the cuboid and cylindrical chambers (in order to increase the pressure in the measuring cavity), and that the sphere has the smallest surface area and the smallest net weight of the device under the same volume conditions, the detector measuring cavity is set as a sphere.

[0087] 1) Because this detector is for low-light detection, the measurement cavity needs to have a large volume. The detector's geometric and material parameters are set as follows: aluminum spherical chamber, air measurement cavity diameter 21.00cm, measurement cavity aluminum foil reflective film thickness 0.10cm, measurement cavity aluminum shell thickness 0.30cm, and the chamber pressure is set to 1 atmosphere (the reason is: after conducting a national standard pressure test on the North Night Vision N4021PMT, the absolute pressure inside the measurement cavity can be increased to 0.2MPa, but to ensure the safety of the alpha particle detector, the absolute pressure of the chamber is set to 1 atmosphere). Figure 2 As shown, the gas to be measured enters through inlet 4 and exits through outlet 5; the PMT entrance window is a quartz cylinder (diameter 4.60cm, thickness 2.20mm); and the photocathode is a double-alkali cylinder (diameter 4.60cm, thickness 20.00nm). Based on the PMT setup principles, considering 1) the size of the central chamber, six North Night Vision N4021 PMTs are selected; considering full-coverage PMTs, the arrangement is that the six PMTs are evenly distributed in the +X, -X, +Y, -Y, +Z, and -Z directions of the detection cavity.

[0088] Figure 2 This is a schematic diagram of the chamber structure of the prototype device of the present invention, wherein (a) is a front view of the structure; (b) is a cross-sectional view; the device mainly includes: 1-measuring chamber shell (made of pure aluminum); 2-pressure isolation baffle (made of acrylic, fixedly connected to the measuring chamber shell through a flange); 3-PMT; 4-air inlet; 5-air outlet. The measuring chamber shell 1 is assembled together by two hemispherical probe hemispheres to form a spherical probe chamber shell, and the hemispheres are sealed with sealing rings and sealing grooves.

[0089] 2) Setting up the radiation source: The radiation source is uniformly distributed within the measurement cavity; its direction is isotropic; its energy and proportion are based on... 222 The three types of alpha particles produced by Rn decay are set with energies of 5.5 MeV, 6.1 MeV, and 7.8 MeV, respectively, in a ratio of 1:1:1.

[0090] 3) Set the physical process: the physical processes involved in the scintillation photons are the generation and absorption of scintillation photons, reflection, refraction, etc.; the physical processes involved in the electrons are multiple scattering, ionization, δ-ray generation and bremsstrahlung, etc.; the physical processes involved in the ions are ionization and multiple scattering, etc. The above physical processes are set by Monte Carlo software.

[0091] 4) Set the air luminescence parameters: according to the literature, the scintillation parameters of air are set as follows: the refractive index is 1.00, the light emission decay time is 7 ns, the light yield is 20 ph / MeV, and the relative intensity of the fast component is 1.0.

[0092] 5) Set the material optical parameters: the reflectivity of the reflective film is set to 0.95. The reason is that according to the manufacturer's SMESR090 model reflective film reflectivity parameter is 0.98 or more, to ensure its reliability, the reflective film reflectivity is set to 0.95; the refractive index of the PMT entrance window is set to 1.50 according to the manufacturer's parameters.

[0093] Step 2: On the basis of step 1, set the threshold values of pe>0, pe>1, pe>2, pe>3, and select the coincidence logic measurement scheme of 2 / 6, 3 / 6, 4 / 6, 5 / 6 and 6 / 6.

[0094] Step 3: Calculate the detection efficiency of alpha particles under different threshold values and coincidence logic by formula ②, as shown in Table 1 below:

[0095] Table 1 Detection efficiency of alpha particles under different threshold values and coincidence logic

[0096]

[0097] Step 4: In the simulation, the radius of the cosmic ray circular surface source is set to 33.20 cm, and the number of muons is 10 million (due to the limitation of particle number and assumption conditions in the simulation, in order to ensure the reliability of the results, when no muons are detected, it is calculated as 1, i.e. at this time ε μ = 0.00001%, R μ = 0.0000058 Hz), according to the energy distribution of cosmic rays and the zenith angle distribution of cosmic rays at sea level, the energy of muons, the incident angle of muons, etc. are set, and the cosmic ray background count rate under different threshold values and coincidence logic is calculated by formula ③④, as shown in Table 2 below:

[0098] Table 2 Cosmic ray background count rate under different threshold values and coincidence logic

[0099]

[0100] Step 5: Calculate the accidental coincidence count rate of PMT by formula ⑦ using the theoretical value (f is the typical noise rate of a single PMT, unit: Hz, as shown in Table 3 below), as shown in Table 4 below:

[0101] Table 3 Typical noise count rate of PMT under different threshold

[0102]

[0103] Table 4 Accidental coincidence count rate of PMT under different threshold and coincidence logic

[0104]

[0105] Step 6: The selection of measurement time is optimized according to the actual environment of the measured object and the actual demand combined with the detection sensitivity of alpha particles. According to the empirical value, the measurement time is set to 20 minutes, that is, t i = 1200s;

[0106] Step 7: The detection sensitivity of alpha particles under different threshold and coincidence logic is calculated by formula (10), as shown in the following table 5:

[0107] Table 5 Detection sensitivity of alpha particles under different threshold and coincidence logic

[0108]

[0109]

[0110] It can be seen that by the design method of the present application, using Monte Carlo analysis means, with the minimum detection sensitivity as the design target, under the condition of the same detector size (the diameter of the measuring cavity is 21 cm, the thickness of the measuring cavity shell is 0.3 cm), the same reflectivity (both are 0.95), and the fixed air pressure (1 bar) and the measurement time (1200s), the detector threshold and coincidence logic design under the minimum sensitivity are determined, that is, the 6-to-2 coincidence logic and the threshold of 1pe, and the alpha particle detection sensitivity is 18.14Bq / m 3 .

[0111] In summary, the optimization design method disclosed by the present application solves the problem of low detection efficiency of alpha particles and low signal-to-noise ratio of real-time alpha particle detector based on air scintillation due to low air light yield, and can meet the lowest detection sensitivity limit of alpha particles that can be designed according to the demand, and is suitable for efficient real-time measurement of the concentration / activity of alpha particles at low radioactivity level by air scintillation method. The detection sensitivity of the detector refers to the lowest lower limit that can be detected by the detector, which reflects the sensitivity of the detector. In this method, it is found that the detection sensitivity of alpha particles is affected by the detection efficiency of alpha particles, the volume of the measuring cavity, the background count and the measurement time.

[0112] The detection efficiency of the detector for alpha particles is affected by the processes of light generation, light collection and photoelectric conversion:

[0113] ①Light generation process is that alpha particles deposit energy in air, nitrogen in air is excited, and nitrogen de-excitation generates ultraviolet photons, in this process, the detection efficiency of alpha particles is affected by the range of alpha particles, the smaller the range, the more energy deposition, the more ultraviolet photons generated in the chamber, the detection efficiency of alpha particles rises; ②Light collection process is that the photons generated in the chamber are reflected by the reflecting film in the chamber wall and then enter the photomultiplier (PMT) photocathode, in this process, the detection efficiency of alpha particles is affected by the chamber geometry, PMT arrangement position, measurement cavity reflectivity, under the condition of optimized chamber geometry, PMT arrangement mode and high reflectivity of reflecting film, the number of ultraviolet photons hitting the PMT will increase, and the loss of ultraviolet photons on the reflecting film is less, the ultraviolet photon receiving efficiency is improved, and the detection efficiency of alpha particles rises; ③Light conversion process is that the photoelectric effect occurs on the PMT photocathode, and the photons are converted into photoelectrons, in this process, the detection efficiency of alpha particles is affected by the quantum efficiency of PMT and the area of PMT photocathode, under the condition that the quantum efficiency spectrum of PMT matches the light spectrum after air excitation by alpha particles, the PMT with larger photocathode area can improve the detection efficiency of alpha particles.

[0114] The background influence is noise background count and radiation background count. The noise background mainly refers to the noise of electronic instruments, electromagnetic interference, dark current and noise of the detector, leakage current and breakdown of insulators, damage of the detector, and noise caused by improper selection of working condition parameters of the detector. For the detector of the present application, the main source of noise background is PMT dark noise. The radiation background count mainly refers to the cosmic ray background, because muons have super strong penetration ability, it is difficult for general shielding body to effectively shield muons, therefore muons will enter the detector measurement cavity with a high probability and deposit energy to cause background count. In view of the above background count influence, the threshold and coincidence logic method is often used for noise reduction, under the condition of appropriate threshold and coincidence logic, the size of the chamber, detection efficiency, background count rate and measurement time are comprehensively considered to realize the lower limit of the detection sensitivity of alpha particle concentration.

[0115] The present application takes the lowest detection sensitivity as the design target, and optimizes the design parameters of the detector through Monte Carlo analysis method, specifically through the steps of detector modeling, threshold and coincidence logic setting, detection efficiency calculation of alpha particles, cosmic ray background and PMT accidental coincidence count rate calculation and measurement time setting, etc. According to the principle of the lower limit of the lowest sensitivity, the optimal detector model, threshold and coincidence logic and measurement time are determined. The core of the present application is to optimize the real-time measurement of air scintillation alpha particle detector by optimizing the design parameters of the detector and the measurement method, which can meet the lowest sensitivity lower limit that can be achieved according to the demand design.

[0116] The above merely illustrates the technical idea of the present application, and cannot limit the protection scope of the present application. Any modification made according to the technical idea of the present application, on the basis of the technical scheme, falls within the protection scope of the present application.

Claims

1. An optimized design method for an alpha particle detector based on real-time air scintillation measurement, characterized in that, Includes the following steps: Step 1: Model the detector, determine the detector volume, set the incident particles as alpha particles and muons, simulate the number of photons reaching the photocathode, and sample each photon according to the PMT quantum efficiency curve to obtain the number of photoelectrons; Step 2: Set a threshold and conformal logic for the number of photoelectrons generated by the incident particles set in Step 1, and perform noise reduction processing; Step 3: Statistically analyze alpha particles that meet the threshold and logical consistency, and calculate the detection efficiency of alpha particles; Statistically analyze muons that meet the threshold and logical consistency, and calculate the background count rate of cosmic rays; Under the same threshold and consistent logic, calculate the dark noise count rate of PMT; Step 4: Set the measurement time according to the actual measurement requirements and the sensitivity requirements for detecting alpha particles; Step 5: Calculate the detection sensitivity for alpha particles, and compare and determine whether the detection sensitivity for alpha particles under different detector parameters is the minimum value among all values: The method for calculating the detection sensitivity of alpha particles is as follows: L Q <50[1+(1+0.08N b ) 1 / 2 ]……………………⑨ Among them, L Q The lower limit of quantitation is defined as a 95% confidence level and a relative measurement error of less than 10%, expressed in units of counts; N b This represents the total count against the background, expressed in units of count. Where C0 is the initial concentration of alpha particles entering the chamber, corresponding to the detection sensitivity, in units of Bq / m³. 3 λ is the decay coefficient of a nuclide that can decay into an alpha particle, measured in s. -1 ;t i For measuring time, the unit is seconds (s); L Q This is the lower limit of quantitation, expressed in units of number; N b The total number of elements is the background count; ε α α particle detection efficiency; V is the measurement cavity volume, in m³. 3 ; The measurement time and the length of the half-life affect the calculation of the initial concentration of alpha particles entering the chamber. When the measurement time is much shorter than the half-life, the longer the measurement time, the lower the detection sensitivity. The calculation formula is approximately: Where C0 is the initial concentration of alpha particles entering the chamber, corresponding to the detection sensitivity, in units of Bq / m³. 3 ;t i For measuring time, the unit is seconds; R n R is the random coincidence count rate of PMT, in Hz; μ ε is the background count rate of cosmic rays, measured in Hz. α The alpha particle detection efficiency is dimensionless; V is the volume of the measurement cavity, in meters. 3 ; When the measurement time is not much less than the half-life, the measurement detection sensitivity is not monotonically affected by t. i As the value increases, the value decreases; the optimal measurement time is determined based on the minimum point of this formula. Where C0 is the initial concentration of alpha particles entering the chamber, corresponding to the detection sensitivity, in units of Bq / m³. 3 λ is the decay coefficient of a nuclide that can decay into an alpha particle, measured in s. -1 ;t i For measuring time, the unit is seconds; R n R is the random coincidence count rate of PMT, in Hz; μ ε is the background count rate of cosmic rays, measured in Hz. α The alpha particle detection efficiency is dimensionless; V is the volume of the measurement cavity, in meters. 3 ; If so, then the alpha particle detector parameters are determined, and the optimization design is completed; If not, return to step 1 until the optimization design is complete.

2. The optimized design method for an alpha particle detector based on real-time air scintillation measurement according to claim 1, characterized in that, Step 1, modeling the detector includes: setting the detector's geometry and materials, setting the radiation source, setting the physical processes, setting the air emission parameters, and setting the material optical parameters.

3. The optimized design method for an alpha particle detector based on real-time air scintillation measurement according to claim 1, characterized in that, In step 1, the PMT quantum efficiency curve is sampled to measure the number of photons. Specifically, the PMT is selected, and the spectral response curve of the PMT should match the air scintillation emission spectrum, while ensuring the coverage area of ​​the PMT photocathode. The calculation formulas involved are as follows: THAT eff =∑I(λ)*QE(λ)……………………① Among them, QE eff λ represents the effective quantum efficiency; I(λ) is the normalized luminous intensity of ultraviolet light with wavelength λ generated by α-particle excitation of air; QE(λ) is the quantum efficiency of PMT for incident light with wavelength λ.

4. The optimized design method for an alpha particle detector based on real-time air scintillation measurement according to claim 1, characterized in that, In step 2, the threshold and conformance logic settings adopt N-to-M conformance logic.

5. The optimized design method for an alpha particle detector based on real-time air scintillation measurement according to claim 1, characterized in that, In step 3, the detection efficiency of α particles that meet the threshold and conform to the logic is calculated statistically, as follows: Where, ε α For the detection efficiency of alpha particles; N α,RX To set the threshold and the number of alpha particles detected after conformal logic measurement, in units of number; N α,TX The number of alpha particles incident into the measurement cavity during the simulation is expressed in terms of number.

6. The optimized design method for an alpha particle detector based on real-time air scintillation measurement according to claim 1, characterized in that, In step 3, the muons that meet the threshold and conform to logic are statistically analyzed to calculate the background count rate of cosmic rays, as follows: Where, ε μ For muon detection efficiency; N μ,RX To set the threshold and the number of muons detected after a coincident logic measurement, in units of count; N μ,TX The number of muons incident into the measurement cavity during the simulation is expressed in units of number. Among them, R μ denoted as cosmic ray background count rate in Hz; r is the radius of the cosmic ray surface source in the simulation in cm. To measure the flux of muons under environmental conditions, the unit is cm. -2 s -1 .

7. The optimized design method for an alpha particle detector based on real-time air scintillation measurement according to claim 1, characterized in that, In step 3, under the same threshold and conformal logic as in step 2, the dark noise count rate of the PMT is calculated, including two methods, wherein: Method 1: Cover the PMT photocathode with a black cloth, then apply high voltage to the PMT. Without detecting any radiation target, use the same threshold and coincidence logic as in step 2. Statistically count the background count output of the PMT using a counter to directly measure the random coincidence count rate. The calculation formula is as follows: [Formula omitted for brevity] and the relative error formula is as follows: Among them, R n The random coincidence count rate of PMT under the same threshold and logical conditions as in step 2, in Hz; N n To set the threshold in step 2 and the PMT dark noise count detected after the measurement, in units of count; t n The measured time is in seconds. Where, σ r,n For R in formula ⑤ n The relative error, unitless; N n Set the threshold and the PMT dark noise count detected after the measurement in formula ⑤, in units of number; Method 2: Theoretical estimation based on the performance parameters of the PMT provided by the manufacturer. The calculation formula is ⑦: Among them, R n The random coincidence count rate of PMT is calculated theoretically under the same threshold and logical conditions as in step 2, in Hz; T is the typical width of the over-threshold output signal, in s; f is the typical noise count rate of PMT under different thresholds, in Hz. Let i be the number of combinable objects among a set of N objects.

8. The optimized design method for an alpha particle detector based on real-time air scintillation measurement according to claim 1, characterized in that, In step 5, during the calculation of the detection sensitivity for alpha particles, two methods are used to calculate the background: Method 1: N b =(R n +R μ )*t i ……………………⑧ Where, N b R represents the total number of items against the background. n R is the random coincidence count rate of PMT in the case of N-choose-M, in Hz; μ The background count rate of cosmic rays, measured in Hz; t i For measuring time, the unit is seconds (s); Method 2: Measure R as a whole through experiments. μ With R n The actual measurement process was as follows: the PMT was installed inside the detector, and then a high voltage was applied to the PMT. Without detecting any radiation target, the same threshold and logic as in step 2 were used, and the background count output by the PMT was statistically analyzed by a counter to directly measure the total background count.

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