A method for measuring surface contamination of alpha-emitting substances at a distance

Abstract

The application discloses a method for measuring surface contamination of long-distance alpha radioactive material, which comprises the steps of measuring long-distance alpha radioactive point source and measuring long-distance alpha surface contamination count rate. The measurement of long-distance alpha radioactive point source comprises intrinsic detection efficiency of alpha particles, collection efficiency calculation and detection efficiency calculation; the measurement of long-distance alpha surface contamination count rate comprises establishing a contamination count rate model, calculating the mean value of point source detection efficiency in each place in a detection area, calculating the expression of the radius R of the detection area D and performing a least square fitting process on the change relation of the radioactive surface source detection efficiency with the opening angle of a shielding body. The application detects fluorescent photons by using a single photon counter, and converts the measured count rate value into the count rate of surface contamination required in the operation intervention level by using the established model.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of nuclear radiation detection, and particularly relates to a method for measuring surface contamination of a long-distance alpha radioactive substance. BACKGROUND

[0002] The measurement of alpha ray surface count rate of radioactive contamination is an important content in nuclear radiation monitoring. In the operation intervention level (OIL) index proposed by the International Atomic Energy Agency (IAEA) in the nuclear accident emergency, the measurement of alpha ray surface contamination count rate is required to determine the response action. When a nuclear facility faces decommissioning, it also needs to be characterized for radioactivity. In the process of radioactive waste disposal, it is required to separate alpha-containing nuclides separately. The commonly used method for measuring alpha surface count rate needs to attach the detector to the contaminated surface within a distance of a few centimeters.

[0003] In recent years, a long-distance measurement method based on alpha-induced fluorescence effect has appeared, which can detect alpha rays at a long distance. The basic principle is that alpha particles interact with nitrogen and oxygen in the air, ionize a large number of secondary electrons, and these secondary electrons continue to react with the components in the air to produce fluorescence. Since the propagation distance of fluorescence photons in the air is far, alpha rays can be detected at a long distance.

[0004] At a long distance from radioactive contamination, the count rate directly measured by the detector is not the count rate of the surface contamination of the alpha radioactive substance, which cannot be directly used to guide the nuclear emergency response action. SUMMARY

[0005] In view of the above problems, the present application aims to provide a method for measuring surface contamination of a long-distance alpha radioactive substance. The present application establishes a measurement model for surface contamination of alpha radioactive substance, and realizes the measurement of surface contamination count rate of alpha radioactive substance by combining theoretical derivation, numerical integration, Geant4 simulation and experimental measurement.

[0006] In order to achieve the above purpose, the technical scheme adopted by the present application is as follows: a method for measuring surface contamination of a long-distance alpha radioactive substance, characterized in that it comprises the following steps:

[0007] Step one: measurement of long-distance alpha radioactive point source

[0008] S1, collection efficiency calculation

[0009] The collection efficiency ε is calculated according to formula (1) col :

[0010]

[0011] N is the number of photons collected by the detector, and N0 is the number of photons emitted by the radioactive source.col The fluorescent photon counting to reach the photocathode;

[0012] S2, intrinsic detection efficiency calculation

[0013] Take the average of the required photons at 300 nm and 400 nm as the inverse of the intrinsic efficiency reference value, see formula (2):

[0014]

[0015] Where, sen 300 and sen 400 represent the number of 300 nm to 400 nm monochromatic photons required to reach the photocathode per count produced by the detector due to radiation;

[0016] Step two: measurement of the far distance alpha surface contamination count rate

[0017] S3, single photon counter alpha particle detection efficiency calculation

[0018] The detection efficiency of the single photon counter for alpha particles is calculated by formula (3):

[0019]

[0020] Where, N PC is the net count value of the single photon counter, N p is the number of emitted alpha particles, ε int is the intrinsic detection efficiency, which is related to the model and properties of the single photon counter, ε col is the collection efficiency, E α is the energy of the alpha rays, MeV; Y air is the number of fluorescent photons generated by 1 MeV energy of alpha rays in air, MeV -1 ;

[0021] S4, establish a contamination count rate model

[0022] The object to be measured is approximated as an alpha radioactive homogeneous radioactive surface source, and the detector surface is placed parallel to the surface of the radioactive surface source, and a conical shield is added at the detector probe. The surface contamination count rate can be calculated by formula (5):

[0023]

[0024] Where, ε PMT_S is the detection efficiency of the detector for the radioactive surface source; the detector is preferably a single photon counter.

[0025] S5, calculation of the average of the point source detection efficiency in the detection area

[0026]

[0027] wherein, ε PMT (r) is the detection efficiency of the detector to the α point source at horizontal distance r; D is a circular area with the center at the vertical projection point of the surface contamination on the detector and with the detection range of the detector to the α surface contamination as the radius; the detection efficiency of the detector to the α point source at horizontal distance r is determined by the product of the collection efficiency and the intrinsic efficiency; the intrinsic efficiency is a certain value for a certain detector and is not affected by the detection model; and the collection efficiency can be obtained by Monte Carlo simulation or point source experiment and the relationship between the collection efficiency and the horizontal distance is fitted by the least square method.

[0028] S6, the expression of the radius R of the detection area D is calculated, see formula (7):

[0029]

[0030] wherein, d is the vertical distance from the detector to the radioactive contamination surface, 1 m, θ is the opening angle of the shielding conical cover, R PMT is the radius of the detector surface;

[0031] S7, the relationship between the detection efficiency of the radioactive surface source and the opening angle of the shielding body is polynomial fitted:

[0032] ε PMT_S = p1·θ 2 + p2·θ + p3 (8)

[0033] wherein, p1, p2, p3 are fitting coefficients; after the detection efficiency of the detector to the surface contamination is obtained, the α-ray surface count rate of the radioactive contamination can be obtained according to formula (5).

[0034] The beneficial effects of the present application are:

[0035] (1) It is not necessary to detect close to the α contaminated surface, and when there is strong α or other types of radioactive contamination in the environment, for example, in the relatively strong α weak γ radiation background of the uranium plutonium processing plant accident, the harm to personnel is reduced;

[0036] (2) The count rate of the α radioactive material surface contamination can be directly obtained and directly used in the operational intervention level (OILs) to guide the response action under the nuclear emergency condition;

[0037] (3) The probe surface of the commonly used alpha detector is plated with a thin film such as ZnS. Due to the existence of the thin film, the detector is more prone to damage, and at the same time, the electronic device can be damaged under strong radiation field. The detector of the present application does not need to be exposed to the radiation field at close range, which can reduce the damage to the equipment and improve the service life of the detector;

[0038] (4) The detector can analyze and process signals from a long distance, so the detection area of the detector is wider at the same position. In the process of nuclear facility decommissioning or large-area alpha radioactivity screening on the ground, the measurement time and cost can be reduced under certain conditions. BRIEF DESCRIPTION OF DRAWINGS

[0039] Figure 1 The measurement model of the surface contamination count rate of the alpha radioactive material at a long distance in the present application.

[0040] Figure 2 The Monte Carlo simulation single photon counter measures the isotropic emitted alpha particles in the present application. Among them, (a) the transport process of alpha particles in air is simulated by Geant4, and a part of the induced fluorescent photons are collected by the detector;

[0041] (b) The red dots represent the detection efficiency of the detector for alpha particles at different distances, the black line represents the least square fitting result of the change rule of the detection efficiency with distance, and the green dots represent the relative error between the simulation and the experimental results.

[0042] Figure 3 The change rule of the detection efficiency of the detector for alpha radioactive point source with distance is measured in the experiment of the present application. Among them, (a) the relative position of the detector and the radioactive source in the dark box; (b) the experimental platform and device; (c) the background is measured for 9000s in the dark room environment; (d) the detection efficiency of the Pu radioactive point source at a distance of 25cm 239 for 600s.

[0043] Figure 4 The measurement of the surface contamination count rate of alpha at a long distance in the present application. Among them, (a) the detector is rotated by 45° and the measurement distance is adjusted to realize the point source detection efficiency measurement experiment with a shielding angle of 90°;

[0044] (b) The simulation and experimental results of the change rule of the detection efficiency of the point source with horizontal distance;

[0045] (c) The simulation and experimental fitting results of the change rule of the detection efficiency of the surface source with the shielding angle. DETAILED DESCRIPTION

[0046] In order to enable those skilled in the art to better understand the technical solutions of the present application, the technical solutions of the present application will be further described in combination with the drawings and examples.

[0047] Referring to the drawings Figures 1-4 The method for measuring surface contamination of a long-distance alpha radioactive material shown in the drawings comprises the following steps:

[0048] Step 1: Measurement of a long-distance alpha radioactive point source

[0049] S1, collection efficiency calculation

[0050] In order to study the collection efficiency of the detector for alpha rays, a single photon counter produced by Hamamatsu Corporation with a model number of CH326 is used as a detector to detect the detection efficiency of alpha particles at different horizontal positions, Figure 1 (a) shows the detection model used for simulation in Geant4. The single photon counter used for simulation has a cross-sectional radius of 2.5 cm, the photocathode uses K2CsSb material, and the energy window uses SiO2 material. Since the influence of the tube body part on the simulation result is small, SiO2 is also used to approximate replacement.

[0051] The single photon counter is placed at different horizontal distances from an alpha particle energy of 5.1 MeV 239 Pu radioactive point source, the simulation obtains the measurement process of the single photon counter for the fluorescent photons generated by the alpha particle excitation, and the collection efficiency ε col can be calculated according to formula (1);

[0052]

[0053] Where N col is the count of fluorescent photons reaching the photocathode.

[0054] S2, intrinsic detection efficiency calculation

[0055] For the intrinsic detection efficiency, according to the radiation sensitivity of the CH326 single photon counter, for monochromatic light of 300 nm and 400 nm, the detector needs to generate 1 count due to radiation. 7.94 and 4.57 photons reach the photocathode, respectively. Since the wavelengths of most of the fluorescent photons generated by the alpha particle excitation of air are concentrated between 300 nm and 400 nm, the average number of photons required for 300 nm and 400 nm is taken as the reciprocal of the reference value of the intrinsic efficiency, see formula (2);

[0056]

[0057] Where sen 300 and sen 400The number of 300-400 nm monochromatic photons that need to reach the photocathode when the detector generates one count per radiation, so the intrinsic detection efficiency of CH326 single photon counter can be calculated as 0.160. According to the quantum efficiency and collection efficiency, the simulation results of the change of the detection efficiency of the detector to alpha particles with the distance between the detector and the radiation source can be calculated by formula (1), see Figure 2 (b).

[0058] In order to verify the accuracy of the simulation results, radioactive point source measurement experiments were carried out in dark conditions. A dark box with a size of 1.5x0.5x0.5m was customized, a single photon counter with model CH326 of Hamamatsu Company was selected as the detector, the detector and the radiation source were placed in the dark box, and the signal line and power line were connected through the lightproof channel for power supply and signal acquisition. The signal processing device is a NIM box produced by ORTEC Company, including main amplifier and multi-channel nuclear electronics plug-in, and the processed data are transmitted into the computer for analysis and further analysis by using MASTRO software.

[0059] The radiation source used 239 Pu point source, the surface emission rate of 4π solid angle is 2x10 6 cpm, and since 239 The half-life of Pu is 24131a, so the influence of the decay of Pu source on activity can be approximately ignored. 239

[0060] The experimental platform and device built are shown in Figure 3 (a)-(b), first the background measurement was carried out, the measurement energy spectrum is shown in Figure 3 (c). When the background measurement time is 9000s, the total count is 211767, i.e. the background count rate is 23.5cps, which is lower than the typical value of dark count rate given in the manual 60cps, so it can be considered that the dark box meets the lightproof condition, and the detector is in normal working state. The radiation source was placed at a distance of 10cm, 25cm, 50cm, 75cm and 1m from the surface of the detector for experiment, taking the measurement energy spectrum at 25cm as an example, the measurement results are shown in Figure 3 (d).

[0061] Since the intensity of the radiation source is calculated from the surface emission rate, the detection efficiency can be calculated by formula (4),

[0062]

[0063] Where n d is the net count rate of single photon counter, cps, n e is the surface emission rate of alpha source, cps. ​

[0064] Therefore, the detection efficiency of the detector can be calculated as 239 The experimental results of the detection efficiency of the Pu radioactive source at different distances from the detector are shown in Figure 2 (b). It can be seen that when the radioactive source is placed at a distance of 10 cm to 1 m from the surface of the detector, the simulation results are close to the experimental results, with a maximum relative error of 13.8%, which proves the effectiveness of the simulation experiment.

[0065] S3, Calculation of the detection efficiency of the single-photon counter for alpha particles

[0066] The detection efficiency calibration is an important part of nuclear radiation detection. According to the detection efficiency, the activity of radioactive contamination and other information can be obtained. This value is often obtained by pre-calibration. The detection efficiency of the single-photon counter for alpha particles is calculated by formula (3):

[0067]

[0068] where N PC is the net count value of the single-photon counter, N p is the number of emitted alpha particles, ε int is the intrinsic detection efficiency, which is related to the model and properties of the single-photon counter, ε col is the collection efficiency, E α is the energy of the alpha rays, MeV; Y air is the number of fluorescent photons generated by 1 MeV of energy of the alpha rays in air, MeV -1 .

[0069] Step two, measurement of the far-distance alpha surface contamination count rate

[0070] S4, Establishment of a contamination count rate model

[0071] At a distance far from radioactive contamination, the results directly measured by the detector are not the surface contamination count rate, which cannot be directly used to guide nuclear emergency response actions. Therefore, the single-photon counter is used to detect the induced fluorescent photons, and the results measured by the single-photon counter are converted into the surface contamination count rate required in the operational intervention level.

[0072] Since the object to be measured is the alpha surface contamination count rate, a reasonable assumption is made for the model, which approximates the object to be measured as a radioactive surface source with uniform alpha radioactivity distribution. This reasonable assumption is a prerequisite for the use of the method. The surface of the detector is placed parallel to the surface of the radioactive surface source, and a conical shield is added at the probe to limit the detection area of the detector. The model for measuring the surface contamination count rate using the far-distance alpha ray detection method can be established as Figure 1As shown in the figure, the vertical distance between the single photon counter as a detector and the surface contamination is d, a conical shield is added in front of the single photon counter, the opening angle of the shield is θ, the extension line of the conical shield encloses a region on the surface contamination with a radius of R, and the radioactive contamination outside the region is limited to interfere with the counting of the single photon counter. The measurement range of the surface contamination is related to the vertical distance d and the opening angle θ of the shield. The surface contamination counting rate can be calculated by formula (5):

[0073]

[0074] wherein ε PMT_S is the detection efficiency of the single photon counter to the radioactive surface source. The detection efficiency of the detector to the radioactive surface source can be obtained by numerical integration.

[0075] S5, mean value calculation of point source detection efficiency in the detection area

[0076] The detection efficiency of the detector to the surface contamination should be the mean value of the point source detection efficiency in the detection area. According to the definition of mean value in probability theory, we have:

[0077]

[0078] wherein ε PMT (r) is the detection efficiency of the detector to the α point source at a horizontal distance of r; D is a circular area with the center at the vertical projection point of the detector center on the surface contamination and with the detection range of the detector to the α surface contamination as the radius. The detection efficiency of the detector to the α point source at a horizontal distance of r is determined by the product of the collection efficiency and the intrinsic efficiency. The intrinsic efficiency is a certain value for a certain detector and is not affected by the detection model. The collection efficiency can be obtained by Monte Carlo simulation or point source experiment, and the relationship between the collection efficiency and the horizontal distance is fitted by the least square method.

[0079] In order to calculate the surface contamination counting rate, according to formula (2), the detection efficiency ε PMT (r) of the detector to the α point source at a horizontal distance of r and the size of the detection area D should be obtained first. In this application, the process of α rays inducing fluorescence in air is simulated by Geant4, and a single photon detection model is established to simulate the relationship between the collection efficiency of the detector to the point source and the horizontal distance when the vertical distance between the single photon detector and the surface contamination is 1 m.

[0080] The parameters of the single photon counter of type CH326 of Hoshin Company are used as the detector model for Monte Carlo simulation. The diameter of the photosensitive area is 25 mm. The simulation results of the detection efficiency of the detector to the α radioactive point source and the horizontal distance are shown in Figure 4(b) As shown in the figure, the horizontal distance r is the distance between the detector's projection point on the radioactive surface source and the boundary of the detection area when the angle of the shielding cone is θ. The variation of the detector's detection efficiency for the radioactive point source with the horizontal distance can be obtained by linear interpolation.

[0081] S6. The expression for the radius R of the detection area D can be calculated based on geometric relationships, as shown in equation (7):

[0082]

[0083] Where d is the vertical distance from the detector to the radioactively contaminated surface, which is selected as 1m in this application, θ is the angle of the shielding cone, and R PMT Let be the radius of the detector surface.

[0084] According to formula (6), the detection efficiency of the detector for the radioactive surface source can be calculated, and the relationship between the detection efficiency of the surface source and the shielding angle can be fitted by polynomial using the least squares method.

[0085]

[0086] Where p1, p2, and p3 are the fitting coefficients; the calculated values ​​are p1 = -1.569e-07, p2 = 5.664e-07, and p3 = 0.003478. The fitted regression coefficients R0 are... 2 =0.9952, indicating a good fitting effect. After obtaining the detector's detection efficiency for surface contamination, the α-ray surface count rate of radioactive contamination can be obtained according to formula (5).

[0087] To verify the accuracy of the simulation results, it is necessary to use... 239 Experimental verification was conducted using a Pu radioactive source. Inside a dark chamber, the detector was placed 1 meter away from the radioactive source, and experiments were conducted at different angles of the shielding to verify the effectiveness of the method for measuring the surface contamination count rate at long distances.

[0088] First, the detection efficiency ε of the detector for a point source α at a horizontal distance of r is measured experimentally. PMT (r), where the radiation source is located at the boundary of the shield's angle. Due to the size limitations of the darkroom, an equivalent measurement can be achieved by rotating the detector and adjusting the position of the radiation source, see [reference needed]. Figure 4 (b). The experimental results are shown in Table 4(c). As can be seen from the figure, the maximum relative error between the simulation and experimental results is 13.5%, which proves that the experimental and simulation results are largely consistent.

[0089] The principle of the application is: the application simulates the process of alpha ray inducing fluorescence in air by Geant4, and establishes a single photon detection model to simulate the change relationship between the collection efficiency of the point source and the horizontal distance when the vertical distance between the single photon detector and the surface contamination is 1m.

[0090] The above shows and describes the basic principles, main features and advantages of the application, and various changes and improvements can be made to the application without departing from the spirit and scope of the application. These changes and improvements all fall within the scope of the application claimed. The scope of protection of the application is defined by the appended claims and their equivalents.

Claims

1. A method for measuring surface contamination of alpha radioactive materials over long distances, characterized in that, The method includes the measurement of a long-range alpha radioactive point source and the measurement of the long-range alpha surface contamination count rate; the measurement of the long-range alpha radioactive point source includes the calculation of alpha particle detection efficiency, collection efficiency, and intrinsic detection efficiency. The measurement of the long-distance α-surface contamination count rate includes establishing a contamination count rate model, calculating the average point source detection efficiency at each location within the detection area, and calculating the detection area. D radius R The expression for the radiation surface source detection efficiency and the least-squares fitting process for the relationship between the shielding angle and the shielding angle; The establishment of the contamination count rate model includes: treating the object to be measured as a radioactive surface source with uniform α-radioactivity, placing the detector surface parallel to the surface of the radioactive surface source, and adding a conical shield at the detector probe. The surface contamination count rate is calculated using formula (5). （5） in, ε PMT_S The detector's efficiency in detecting radioactive surface sources; The average point source detection efficiency at each location within the detection area is calculated as follows: （6） in, ε PMT ( r ) is the detector at a horizontal distance of r The detection efficiency of the α point source; D It is a circular area with the vertical projection point of the detector center onto the surface as the center and the detection range of the detector on the α surface as the radius. The calculated detection area D radius R The expression for this is shown in equation (7): （7） in, d The vertical distance from the detector to the radioactively contaminated surface. θ To shield the angle of the cone-shaped shield, R PMT The radius of the detector surface; The relationship between the detection efficiency of the radioactive surface source and the angle of the shielding body was fitted using a polynomial: （8） in, p 1, p 2, p 3 is the fitting coefficient; after obtaining the detector's detection efficiency for surface contamination, the α-ray surface count rate of radioactive contamination can be obtained according to formula (5).

2. The method according to claim 1, characterized in that, Specifically, the calculation steps are as follows: S1, Collection efficiency calculation The collection efficiency is calculated according to formula (1). ε col : （1） in, N col To count the fluorescent photons reaching the photocathode; S2, Calculation of intrinsic detection efficiency The average value of the photons required at 300 nm and 400 nm is taken as the reciprocal of the intrinsic efficiency reference value, as shown in formula (2): （2） in, sen 300 and sen 400 These represent the number of monochromatic photons at 300nm and 400nm that need to reach the photocathode for each count generated by the detector due to radiation; S3. Calculation of the detection efficiency of a single-photon counter for alpha particles The detection efficiency of alpha particles using a single-photon counter is calculated using formula (3): （3） in, N PC This is the net count value of the single-photon counter. N p The number of alpha particles emitted. ε int The intrinsic detection efficiency is related to the type and properties of the single-photon counter. ε col For collection efficiency, E α The energy of alpha rays is MeV; Y air MeV represents the number of fluorescent photons generated by 1 MeV energy when alpha rays are deposited in air. -1 ; S4. Establish a contamination count rate model. The object to be measured is regarded as a radioactive surface source with uniform α-radioactivity, and the detector surface is placed parallel to the surface of the radioactive surface source. A conical shield is added to the detector probe. The surface contamination count rate is calculated using formula (5): （5） in, ε PMT_S The detector's efficiency in detecting radioactive surface sources; S5. Calculation of the average point source detection efficiency at various points within the detection area. （6） in, ε PMT ( r ) is the detector at a horizontal distance of r The detection efficiency of the α point source; D It is a circular area with the vertical projection point of the detector center onto the surface as the center and the detection range of the detector on the α surface as the radius. S6. Calculate the detection area D radius R The expression for this is shown in equation (7): （7） in, d The vertical distance from the detector to the radioactively contaminated surface. θ To shield the angle of the cone-shaped shield, R PMT The radius of the detector surface; S7. Perform polynomial fitting on the relationship between the detection efficiency of the radioactive surface source and the angle of the shield: （8） in, p 1, p 2, p 3 is the fitting coefficient; after obtaining the detector's detection efficiency for surface contamination, the α-ray surface count rate of radioactive contamination can be obtained according to formula (5).

3. The method according to claim 2, characterized in that: In formula (6), the detector is at a horizontal distance of r The detection efficiency of the α point source is determined by the product of the collection efficiency and the intrinsic efficiency. The intrinsic efficiency is a fixed value for a specific detector and is not affected by the detection model. The collection efficiency is obtained by Monte Carlo simulation or point source experiment and the relationship between the collection efficiency and the horizontal distance is fitted by the least squares method.

4. The method according to claim 3, characterized in that: In formula (7), the vertical distance from the detector to the radioactively contaminated surface is... d The selected value is 1 m.

5. The method according to claim 4, characterized in that: The detector is a single-photon counter.

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