CN116256789A - Method for measuring surface contamination of remote alpha radioactive substance - Google Patents

Method for measuring surface contamination of remote alpha radioactive substance Download PDF

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CN116256789A
CN116256789A CN202310183741.7A CN202310183741A CN116256789A CN 116256789 A CN116256789 A CN 116256789A CN 202310183741 A CN202310183741 A CN 202310183741A CN 116256789 A CN116256789 A CN 116256789A
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alpha
detector
radioactive
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程金星
吴泽乾
王庆波
绪梅
吴友朋
温伟伟
于艾
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23 Units Of Chinese People's Liberation Army 96901 Force
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Abstract

The application discloses a method for measuring surface contamination of a remote alpha radioactive substance, which comprises the steps of measuring a remote alpha radioactive point source and measuring a remote alpha surface contamination count rate. The measurement of the remote alpha radioactive point source comprises the calculation of the intrinsic detection efficiency and the collection efficiency of alpha particles and the calculation of the detection efficiency; the measurement of the remote alpha surface contamination count rate comprises the steps of establishing a contamination count rate model, calculating the average value of detection efficiency of point sources at all positions in a detection area, calculating an expression of the radius R of the detection area D, and carrying out a least square fitting process on the change relation of the detection efficiency of the radioactive non-point source along with the opening angle of the shielding body. The present application utilizes a single photon counter to detect fluorescent photons and utilizes an established model to convert measured count values to count rates of surface contamination required in operational intervention levels.

Description

Method for measuring surface contamination of remote alpha radioactive substance
Technical Field
The application relates to the technical field of nuclear radiation detection, in particular to a method for measuring surface contamination of a long-distance alpha radioactive substance.
Background
Measurement of the alpha ray surface count rate of radioactive contamination is an important piece of nuclear radiation monitoring. In nuclear accident emergency, the measurement of the alpha surface contamination count rate is required in the Operational Intervention Level (OIL) index set forth by the international atomic energy organization (IAEA) to determine the responsive action taken. The nuclear facilities also need to be radioactively characterized when they are in retirement. In the radioactive waste disposal process, separate separation of alpha-containing nuclides is required. The currently common method of measuring the alpha surface count rate requires that the detector be positioned within a few centimeters of the contaminated surface.
In recent years, a remote measurement method based on an alpha-induced fluorescence effect has appeared, and detection of alpha rays can be realized in a remote distance. The basic principle is that alpha particles interact with nitrogen and oxygen in the air, ionization generates a large number of secondary electrons, and the secondary electrons continue to react with components in the air to generate fluorescence. Since the propagation distance of the fluorescent photons in the air is long, the detection of alpha rays at a long distance can be achieved.
At a distance from the radioactive contamination, the count rate measured directly by the detector results in a count rate that is not the count rate of alpha radioactive material surface contamination, which is not directly useful for directing nuclear emergency response actions.
Disclosure of Invention
Aiming at the problems, the application aims to provide a method for measuring the surface contamination of the alpha radioactive substance in a long distance, establishes a model for measuring the surface contamination of the alpha radioactive substance, and realizes the measurement of the surface contamination count rate of the alpha radioactive substance by combining theoretical derivation, numerical integration and Geant4 simulation with experimental measurement.
In order to achieve the above purpose, the technical scheme adopted in the application is as follows: a method for measuring surface contamination of a remote alpha radioactive substance, comprising the steps of;
step one: measurement of remote alpha radioactive point sources
S1, calculating collection efficiency
Calculating the collection efficiency epsilon according to the formula (1) col ：
Figure BDA0004103091330000021
Wherein N is col Counting fluorescent photons reaching the photocathode;
s2, intrinsic detection efficiency calculation
Taking the average value of photons required by 300nm and 400nm as the reciprocal of the intrinsic efficiency reference value, see formula (2):
Figure BDA0004103091330000022
wherein sen 300 And sen 400 Representing the number of 300nm to 400nm monochromatic photons which need to reach the photocathode when the detector generates 1 count due to radiation;
step two: measurement of remote alpha surface contamination count rate
S3, calculating the detection efficiency of the single photon counter to alpha particles
The detection efficiency of alpha particles using a single photon counter is calculated by equation (3):
Figure BDA0004103091330000023
wherein N is PC N is the net count value of the single photon counter p Epsilon for the number of alpha particles emitted int Epsilon is related to the model and nature of the single photon counter for intrinsic detection efficiency col For collecting efficiency E α Energy of alpha rays, meV; y is Y air The number of fluorescence photons generated for depositing 1MeV energy in air for alpha rays, meV -1 ；
S4, establishing a contamination count rate model
The object to be measured is approximately regarded as a radioactive surface source with uniformly distributed alpha radioactivity, the surface of the detector is placed in parallel with the surface of the radioactive surface source, a conical shielding cover is additionally arranged at the position of the detector probe, and the surface contamination count rate can be calculated by using the formula (5):
Figure BDA0004103091330000031
wherein ε PMT_S The detection efficiency of the detector on the radioactive non-point source is achieved; the detector is preferably a single photon counter.
S5, calculating average value of detection efficiency of point sources at all positions in detection area
Figure BDA0004103091330000032
Wherein ε PMT (r) is the detection efficiency of the detector for the alpha point source at a horizontal distance r; d is a circular area surrounded by taking a vertical projection point of the detector, which is stained on the surface, as a center and taking the detection range of the detector for staining the alpha surface as a radius; the detection efficiency of the detector on the alpha point source with the horizontal distance r is determined by the product of the collection efficiency and the intrinsic efficiency; the intrinsic efficiency is a determined value for a particular 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 change relation of the collection efficiency with the horizontal distance is fitted through a least square method.
S6, calculating an expression of the radius R of the detection area D, wherein the expression is shown in the formula (7):
Figure BDA0004103091330000033
wherein d is the vertical distance from the detector to the radioactive contamination surface, 1m, θ is the opening angle of the shielding cone, R PMT Is the radius of the detector surface;
s7, performing polynomial fitting on the change relation of the detection efficiency of the radioactive non-point source along with the opening angle of the shielding body:
ε PMT_S ＝p 1 ·θ 2 +p 2 ·θ+p 3 (8)
wherein p is 1 ，p 2 ，p 3 Fitting coefficients; after the detection efficiency of the detector for surface contamination is obtained, the count rate of the alpha-ray surface of the radioactive contamination can be obtained according to the formula (5).
The beneficial effects of this application are:
(1) Detection is not required to be carried out at the position close to the alpha pollution surface, and when stronger alpha or other types of radioactive pollution exist in the environment, the harm to personnel is reduced under the radiation background of relatively strong alpha, weak gamma such as accidents of uranium and plutonium treatment factories;
(2) The counting rate of the surface contamination of the alpha radioactive substance can be directly obtained and directly used in the Operation Intervention Level (OILs) to guide the response action under the nuclear emergency condition;
(3) The probe surface of the commonly used alpha detector is plated with a layer of film such as ZnS, and the detector is easy to damage due to the existence of the film, meanwhile, under a strong radiation field, an electronic device can be damaged, and the detector does not need to be exposed to the radiation field in a short distance, so that the damage to equipment can be reduced, and the service life of the detector is prolonged;
(4) The detector can analyze and process signals from a long distance, so that the range of a detection area of the detector at the same position is wider, and the time and cost of measurement can be reduced under certain conditions when nuclear facility retirement or surface large-area alpha radioactivity screening process is carried out.
Drawings
FIG. 1 is a model of the measurement of the surface contamination count rate of a remote alpha radioactive substance of the present application.
Fig. 2 is a diagram of a monte carlo simulated single photon counter of the present application measuring isotropically emitted alpha particles. Simulating the transportation process of alpha particles in the air by using Geant4, and collecting a part of fluorescence photons generated by induction by a detector;
(b) Red dots represent the detection efficiency of the detector on alpha particles at different distances, black lines represent the least squares fitting result of the change rule of the detection efficiency along with the distance, and green dots represent the relative error of simulation and experimental results.
Fig. 3 shows the law of the detection efficiency of the experimental measurement detector for the alpha radioactive point source according to the distance. Wherein, (a) the relative position of the detector and the radioactive source in the camera bellows; (b) an experimental platform and apparatus; (c) measuring background 9000s in a darkroom environment; (d) Measuring 25cm from the detector 239 Pu radioactive point source 600s.
Fig. 4 is a measurement of the remote alpha surface contamination count rate of the present application. Rotating the detector by 45 degrees and adjusting the measuring distance to realize a point source detection efficiency measuring experiment with the opening angle of the shielding cover of 90 degrees;
(b) Simulation and experimental results of the change rule of the point source detection efficiency along with the horizontal distance;
(c) Simulation and experimental fitting results of the change rule of the surface source detection efficiency along with the opening angle of the shielding body.
Detailed Description
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 are further described below with reference to the accompanying drawings and examples.
A method of remote alpha radioactive material surface contamination measurement as described with reference to figures 1-4, comprising the steps of:
step one: measurement of remote alpha radioactive point sources
S1, calculating collection efficiency
To study the collection efficiency of the detector for alpha rays, a single photon counter model CH326 manufactured by Geant4 simulation bingo was used as the detection efficiency of the detector for alpha particles at different horizontal positions, and fig. 1 (a) shows a detection model for simulation in Geant 4. The section radius of a single photon counter adopted in the simulation is 2.5cm, and a photocathode adopts K 2 CsSb material, siO is adopted as energy window 2 Since the material has less influence on the simulation result in the pipe body part, siO is also used 2 Approximate substitution.
The single photon counter was placed at a distance of 5.1MeV from the alpha particle energy 239 The measurement process of fluorescent photons generated by alpha particle excitation by a single photon counter is obtained through simulation at different horizontal distances of the Pu radioactive point source, and the collection efficiency epsilon can be calculated according to the formula (1) col ；
Figure BDA0004103091330000061
Wherein N is col To count fluorescent photons reaching the photocathode.
S2, intrinsic detection efficiency calculation
For intrinsic detection efficiency, according to the radiation sensitivity of the CH326 single photon counter, for 300nm and 400nm monochromatic light, the detector needs to reach 7.94 and 4.57 photons, respectively, for every 1 count due to radiation. Since most of photons of the radioactive fluorescence effect generated by exciting air by alpha particles have wavelengths concentrated between 300nm and 400nm, taking the average value of photons required by 300nm and 400nm as the reciprocal of an intrinsic efficiency reference value, and the average value is shown in a formula (2);
Figure BDA0004103091330000062
wherein sen 300 And sen 400 Representing the number of 300nm to 400nm monochromatic photons that need to reach the photocathode each time the detector generates 1 count due to radiation, respectively, it can be calculated that the intrinsic detection efficiency is 0.160 for the CH326 single photon counter. According to the quantum efficiency and the collection efficiency, the simulation result of the change rule of the detection efficiency of the detector to alpha particles along with the distance between the detector and the radioactive source can be calculated by the formula (1), and the simulation result is shown in fig. 2 (b).
In order to verify the accuracy of the simulation results, radioactive point source measurement experiments were performed in dark conditions. A camera bellows with the size of 1.5 multiplied by 0.5m is customized, a single photon counter with the size of CH326 of Binsong company is selected as a detector, the detector and a radioactive source are placed in the camera bellows, and a signal line and a power line are connected through a light-shielding channel for power supply and signal acquisition. The signal processing device is an NIM cabinet produced by ORTEC company and comprises a main amplifier and a plurality of two nuclear power plug-ins, and the processed data are transmitted into a computer to be analyzed, controlled and further analyzed by MASTRO software.
The radioactive source adopts 239 Pu point source with surface emissivity of 2 x 10 at 4 pi solid angle 6 cpm, and due to 239 Pu has a half-life of 24131a and is therefore approximately negligible 239 Effect of decay of Pu radiation source on activity.
The built experimental platform and device are shown in fig. 3 (a) - (b), background measurement is firstly carried out, and the measurement energy spectrum is shown in fig. 3 (c). When the background measurement time is 9000s, the total count is 211767, namely the background count rate is 23.5cps, which is lower than the typical value of the dark count rate of 60cps given in the instruction manual, and the dark box can be considered to meet the light-shielding condition, and the detector is in a normal working state. Experiments were performed by placing the radioactive source at distances of 10cm, 25cm, 50cm, 75cm and 1m from the detector surface, respectively, taking the measurement spectrum at 25cm as an example, and the measurement results are shown in fig. 3 (d).
Since the intensity of the radiation source is calculated from the surface emissivity, the detection efficiency can be calculated by equation (4),
Figure BDA0004103091330000071
wherein n is d Net count rate of single photon counter, cps, n e Is the surface emissivity of the alpha radiation source, cps.
Thus, it can be calculated that when 239 The experimental results of the detection efficiency of Pu sources at different distances from the detector are shown in fig. 2 (b). It can be seen that the simulation results were close to the experimental results with a maximum relative error of 13.8% when the radiation source was placed at a distance of 10cm to 1m from the detector surface, which demonstrates the effectiveness of the simulation experiment.
S3, calculating the detection efficiency of the single photon counter to alpha particles
The detection efficiency scale is an important content of nuclear radiation detection, and according to the detection efficiency, information such as activity of radioactive contamination can be obtained, and the value is often obtained through the advance scale. The detection efficiency of the single photon counter for alpha particles is calculated by formula (3):
Figure BDA0004103091330000081
wherein N is PC N is the net count value of the single photon counter p Epsilon for the number of alpha particles emitted int Epsilon is related to the model and nature of the single photon counter for intrinsic detection efficiency col For collecting efficiency E α Energy of alpha rays, meV; y is Y air The number of fluorescence photons generated for depositing 1MeV energy in air for alpha rays, meV -1 。
Step two, measuring the contamination count rate of the long-distance alpha surface
S4, establishing a contamination count rate model
At a distance from the radioactive contamination, the result measured directly by the detector is not a count rate of surface contamination, which is not directly used to guide nuclear emergency response actions. The present application thus utilizes a single photon counter to detect the induced fluorescent photons and converts the results measured by the single photon counter to the count rate of surface contamination required in the level of operational intervention.
Since the object to be measured is the alpha surface contamination count rate, a reasonable assumption is made on the model that the object to be measured is approximately regarded as a radioactive source with uniformly distributed alpha radioactivity, and the reasonable assumption is a use premise of the method. And placing the surface of the detector in parallel with the surface of the radioactive non-point source, and additionally arranging a conical shielding cover at the probe for limiting the detection area of the detector. A model for measuring the surface contamination count rate by using a remote alpha ray detection method can be established as shown in a figure 1, wherein a single photon counter is used as a detector, the vertical distance between the detector and alpha surface contamination is d, a conical shielding cover is additionally arranged at the front section of the single photon counter, the opening angle of the shielding conical cover is theta, the radius of an area surrounded by an extension line of the conical shielding cover on the surface contamination is R, and the radiation contamination outside the area is limited to interfere the counting of the single photon counter. The measurement range of surface contamination is related to the vertical distance d and the opening angle θ of the shield cone. The surface contamination count rate can be calculated using equation (5):
Figure BDA0004103091330000091
wherein ε PMT_S For the detection efficiency of the single photon counter to the radioactive surface source, the calculation of the detection efficiency of the detector to the radioactive surface source can be obtained by a numerical integration method.
S5, calculating average value of detection efficiency of point sources at all positions in detection area
The detection efficiency of the detector on surface contamination is the average value of detection efficiency of point sources at all positions in a detection area, and the average value is defined in a probability theory to be:
Figure BDA0004103091330000092
wherein ε PMT (r) is the detection efficiency of the detector for the alpha point source at a horizontal distance r; and D is a circular area surrounded by taking a vertical projection point of the detector, which is stained on the surface, as a center and taking the detection range of the detector for staining the alpha surface as a radius. The detection efficiency of the detector on the alpha point source with the horizontal distance r is determined by the product of the collection efficiency and the intrinsic efficiency, and the intrinsic efficiency is a determined value for a specific detector and is not influenced by a detection model; the collection efficiency can be obtained by Monte Carlo simulation or point source experiment, and the change relation of the collection efficiency with the horizontal distance is fitted through a least square method.
In order to calculate the surface contamination count rate, it is necessary to first obtain the detection efficiency ε of the detector for the α -point source at a horizontal distance r according to equation (2) PMT (r) and the size of the detection area D. The application utilizes Geant4 to simulate the process of generating fluorescence by alpha rays induced in air, establishes a single photon detection model and simulates the single photon detectionWhen the vertical distance of the detector from the surface contamination is 1m, the collection efficiency of the detector to the point source is changed along with the horizontal distance.
The parameters of a single photon counter with the model CH326 of Binsong company are adopted as a detector model of Monte Carlo simulation, the diameter of a photosensitive area is 25mm, and the simulation result of the change rule of the detection efficiency of the detector to the alpha radioactive point source along with the horizontal distance is shown in fig. 4 (b). As can be seen in the figure, the horizontal distance r is the distance between the projection point of the detector on the radioactive surface source and the boundary of the detection area when the opening angle of the shielding conical cover is θ. The change rule of the detection efficiency of the detector on the radioactive point source along with the horizontal distance can be obtained through a linear interpolation method.
S6, calculating an expression of the radius R of the detection area D according to the geometric relation, wherein the expression is shown in the formula (7):
Figure BDA0004103091330000101
wherein d is the vertical distance from the detector to the radioactive contamination surface, the value chosen in this application is 1m, θ is the opening angle of the shielding cone, R PMT Is the radius of the detector surface.
Then according to the formula (6), the detection efficiency of the detector on the radioactive non-point source can be calculated, and polynomial fitting is carried out on the change relation of the detection efficiency of the non-point source along with the opening angle of the shielding body by using a least square method;
Figure BDA0004103091330000102
wherein p is 1 ，p 2 ，p 3 Fitting coefficients; calculating the p 1 ＝-1.569e-07，p 2 ＝5.664e-07，p 3 = 0.003478. Regression coefficient of fitting R 2 = 0.9952, the fitting effect was seen to be good. After the detection efficiency of the detector for surface contamination is obtained, the count rate of the alpha-ray surface of the radioactive contamination can be obtained according to the formula (5).
To simulateVerification of the accuracy of the results requires the use of 239 Pu radiation source was experimentally verified. In the camera bellows, the detector is placed at a position 1m away from the radioactive source, experiments are carried out according to different opening angles of the shielding case, and the effectiveness of the measuring method of the remote alpha surface contamination count rate is verified.
Firstly, measuring the detection efficiency epsilon of a detector to an alpha point source with a horizontal distance r through experiments PMT (r) at which point the representative source is located at the boundary of the opening angle of the shield. Due to the limitations of the camera size, equivalent measurements can be achieved by rotating the detector through an angle and adjusting the position of the radiation source, see fig. 4 (b). The experimental results are shown in Table 4 (c). The maximum relative error between the simulation and the experimental result is 13.5%, which proves that the experimental and the simulation result are consistent.
The principle of this application is: the method utilizes Geant4 to simulate the process of generating fluorescence by alpha rays in air, establishes a single photon detection model, and simulates the change relation of the collection efficiency of the detector on the point source along with the horizontal distance when the vertical distance of the single photon detector from the surface is 1m.
The foregoing has shown and described the basic underlying principles, main features and advantages of the application, but there are numerous variations and modifications of the application which fall within the scope of the application as claimed, without departing from the spirit and scope of the application. The scope of the application is defined by the appended claims and equivalents thereof.

Claims (5)

1. A method for measuring surface contamination of a remote alpha radioactive substance, comprising the steps of measuring a remote alpha radioactive point source and measuring a remote alpha surface contamination count rate; the measurement of the remote alpha radioactive point source comprises detection efficiency calculation, collection efficiency calculation and intrinsic detection efficiency calculation of alpha particles;
the measurement of the remote alpha surface contamination count rate comprises the steps of establishing a contamination count rate model, calculating the average value of detection efficiency of point sources at all positions in a detection area, calculating an expression of the radius R of the detection area D, and carrying out a least square fitting process on the change relation of the detection efficiency of the radioactive non-point source along with the opening angle of the shielding body.
2. The method according to claim 1, characterized in that it comprises in particular the following calculation steps:
s1, calculating collection efficiency
Calculating the collection efficiency epsilon according to the formula (1) col ：
Figure FDA0004103091320000011
Wherein N is col Counting fluorescent photons reaching the photocathode;
s2, intrinsic detection efficiency calculation
Taking the average value of photons required by 300nm and 400nm as the reciprocal of the intrinsic efficiency reference value, see formula (2):
Figure FDA0004103091320000012
wherein sen 300 And sen 400 Representing the number of 300nm to 400nm monochromatic photons which need to reach the photocathode when the detector generates 1 count due to radiation;
s3, calculating the detection efficiency of the single photon counter to alpha particles
The detection efficiency of alpha particles using a single photon counter is calculated by equation (3):
Figure FDA0004103091320000021
wherein N is PC N is the net count value of the single photon counter p Epsilon for the number of alpha particles emitted int Epsilon is related to the model and nature of the single photon counter for intrinsic detection efficiency col For collecting efficiency E α Energy of alpha rays, meV; y is Y air For alpha raysThe number of fluorescence photons generated by depositing 1MeV energy in air, meV -1 ；
S4, establishing a contamination count rate model
The object to be measured is approximately regarded as a radioactive surface source with uniformly distributed alpha radioactivity, the surface of the detector is placed in parallel with the surface of the radioactive surface source, a conical shielding cover is additionally arranged at the position of the detector probe, and the surface contamination count rate can be calculated by using the formula (5):
Figure FDA0004103091320000022
wherein ε PMTS The detection efficiency of the detector on the radioactive non-point source is achieved;
s5, calculating average value of detection efficiency of point sources at all positions in detection area
Figure FDA0004103091320000023
Wherein ε PMT (r) is the detection efficiency of the detector for the alpha point source at a horizontal distance r; d is a circular area surrounded by taking a vertical projection point of the detector, which is stained on the surface, as a center and taking the detection range of the detector for staining the alpha surface as a radius;
s6, calculating an expression of the radius R of the detection area D, wherein the expression is shown in the formula (7):
Figure FDA0004103091320000024
where d is the vertical distance from the detector to the radioactive contamination surface, θ is the opening angle of the shielding cone, R PMT Is the radius of the detector surface;
s7, performing polynomial fitting on the change relation of the detection efficiency of the radioactive non-point source along with the opening angle of the shielding body:
ε PMT_S ＝p 1 ·θ 2 +p 2 ·θ+p 3 (8)
wherein p is 1 ，p 2 ，p 3 Fitting coefficients; after the detection efficiency of the detector for surface contamination is obtained, the count rate of the alpha-ray surface of the radioactive contamination can be obtained according to the formula (5).
3. The method according to claim 2, characterized in that: in the formula (6), the detection efficiency of the detector on the alpha point source with the horizontal distance r is determined by the product of the collection efficiency and the intrinsic efficiency; the intrinsic efficiency is a determined value for a particular 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 change relation of the collection efficiency with the horizontal distance is fitted through a least square method.
4. A method according to claim 3, characterized in that: in equation (7), the vertical distance d from the detector to the radioactive contamination surface is chosen to be 1m.
5. The method according to claim 4, wherein: the detector is a single photon counter.
CN202310183741.7A 2023-02-24 2023-02-24 Method for measuring surface contamination of remote alpha radioactive substance Pending CN116256789A (en)

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