In order to facilitate the calibration and scale of pyroelectric detectors for environmental dose monitoring, a new simple standard irradiation device was designed. Through Monte Carlo software, the uniformity of radiation field, absorbed dose of pyroelectric detector and scattering contribution rate of this device are simulated. The simulation results show that the new simple standard irradiator satisfies the national standards and can fulfill the calibration and scaling requirements of pyroelectric detectors for environmental dose monitoring.
In the field of personal and environmental dose monitoring, dose monitoring is mainly carried out using thermoluminescence detectors (TLD), which are more convincing in the evaluation of environmental radiation levels because of their cumulative dose function, which allows them to be placed for a long period of time in the radiation field of interest and to record the radiation dose for the entire monitoring time period [
Combined with the use of demand and relevant standards, the irradiator needs to be able to irradiate a large number of TLD at one time, and to meet the requirements of the standard specifications in terms of safety, uniformity, scattering contribution rate and other indicators. The designed irradiator mainly consists of radiation source, scattering chamber, collimator, shielding chamber, pyroluminescent pallet, low-speed motor and arc attenuator. Among them, the shielding chamber is a cylinder with a height of 620 mm, an inner diameter of 258 mm, a thickness of 100 mm, and a material of Pb; the scattering chamber has an inner diameter of 58 mm, a depth of 50 mm, an outer diameter of 118 mm, a height of 80 mm, and a material of Cu; the collimator reference GB/T 12162. 1-2000 recommended collimator design, using seven layers of 8 mm after the tungsten alloy grating (W89%, Ni7%, Cu4%) composed of density of 17 g/cm3 [
The structure of the irradiation device is shown in
According to GB/T 17568-2019 [
K a 0 = A Γ k / R 2 . (1)
K a 1 = A Γ k e − μ x / R 2 . (2)
Equation (1) is the dose rate calculation for the reference point without shielding and Equation (2) is the dose rate calculation for the reference point with shielding [
In the equation, A is the activity of the radioactive source with the value of 2 × 108 Bq, Γ k is the air specific kinetic energy constant of 137Cs with the value of 0.079 μGy・h−1・m2・MBq−1, R is the measurement distance with the value of 0.3 m, μ is the linear attenuation coefficient of the shielding material, x is the thickness of the shielding material, K a 0 is the air specific kinetic energy rate without shielding, and K a 1 is the air specific kinetic energy rate after the shielding. The specific kinetic energy rate of air at 30 cm is 175.6 μGy/h by Equation (1), and the minimum shielding thickness is 3.3 cm according to the maximum reference dose value of 2.5 μGy/h after shielding. The design thickness of the lead chamber is 10 cm, which meets the relevant requirements.
The TLD shall meet the charged particle equilibrium when in calibration. The thermoluminescent pallet material is plexiglass, which is mainly used for placing the TLD in calibration and provides the condition of charged particle equilibrium. According to the beta particle range equation
R = 0.412 E ( 1.265 − 0.0954 ln E ) . (0.01 MeV < E < 2.5 MeV) (3)
where R is the mass range of electrons in low z matter (g・cm−2) [
According to the requirements of JJG 593-2016 [
The radius of the irradiation field is divided into ten equal parts, and the specific kinetic energy rate of air release at each point is calculated according to Equation (1), and the homogeneity of the reference radiation field is finally obtained. The results are shown in
The role of the arc attenuator is used to improve the uniformity of the reference radiation field, as well as to reduce the low-energy scattering of the reference ray beam, taking into account the functional requirements of the arc attenuator and the material requirements, graphite is the main material of the arc attenuator. Meanwhile, considering the processing difficulty of the arc graphite and the new scattered rays brought by the over-thick graphite, an attenuator with the edge of the arc attenuator 1mm thick is used. According to the Equation (2) the radiation field dose value is corrected to the same (the dose value of the edge 1 mm), the thickness of each correction point of the arc attenuator is shown in
Based on the required attenuator thickness at each reference point, the attenuator is fitted to a hemisphere with a radius of 90.76 mm using matlab to obtain the curved attenuator.
According to the requirements of GB/T 12162.1-2000, the contribution of scattered radiation from the reference radiation field should not exceed 5% of the total air-specific kinetic energy rate, and the variation of the air-specific kinetic energy rate should not exceed 5% over the entire sensitive volume of the detector at each test point. As well as in accordance with the requirements of JJG 593-2016, the homogeneous area of the radiation beam at the calibration point should be able to completely cover the examined dosimeter, and the inhomogeneity of this area should not exceed 5%.
The following simulations will be carried out in terms of the air specific kinetic energy of the reference radiation field as well as the absorbed dose of pyroelectric light to verify the uniformity of the radiation field and the scattering contribution, respectively.
The Monte Carlo software is used to model the dimensions and materials designed in “Heading 2”, and simulate the properties of scattering and uniformity in the irradiation field of the reference radiation field with reference to GB/T 12162.1-2000, and analyze the results to optimize the design. The design model is shown in
| Position of the detection point from the center of the ray beam (mm) | Relative Air Specific Kinetic Energy |
|---|---|
| 0 | 100.00% |
| 4.3 | 99.99% |
| 8.6 | 99.96% |
| 12.9 | 99.90% |
| 17.2 | 99.82% |
| 21.5 | 99.72% |
| 25.8 | 99.60% |
| 30.1 | 99.46% |
| 34.4 | 99.30% |
| 38.7 | 99.11% |
| 43.0 | 98.91% |
| Inhomogeneity | 1.09% |
| Radial Position (mm) | Attenuator Thickness (mm) |
|---|---|
| 0 | 1.67 |
| 4.3 | 1.67 |
| 8.6 | 1.64 |
| 12.9 | 1.61 |
| 17.2 | 1.57 |
| 21.5 | 1.51 |
| 25.8 | 1.43 |
| 30.1 | 1.34 |
| 34.4 | 1.24 |
| 38.7 | 1.13 |
| 43.0 | 1.00 |
The air specific kinetic energy of the reference radiation field is simulated using Monte Carlo software. Since the whole model is a symmetrical structure, only one side of the irradiation field needs to be simulated and it extends beyond the irradiation field until it reaches the inner side of the shielded room. The simulation results are shown below.
1) Inhomogeneity
The inhomogeneity of the reference radiation field is 1.18% obtained from the calculation results, which meets the relevant requirements. The dose values of each reference point of the radiation field without attenuator are shown in
2) Scattering
Radiation field scattering is modeled in two ways. One, the scattering is analyzed within the irradiation field. Second, the scattering was analyzed outside the irradiation field. The results are shown in
It can be concluded from the results that the scattering contribution is less than 5% both in the irradiation and in the field, which meets the design requirements.
| Reference Point within the Irradiation Field | Reference Point outside the Irradiated Field | ||
|---|---|---|---|
| Distance from Center (cm) | Normalized Count | Distance from Center (cm) | Normalized Count |
| 0.0 | 2.92E-05 | 4.5 | 1.37E-07 |
| 0.5 | 2.92E-05 | 5.0 | 1.14E-07 |
| 1.0 | 2.92E-05 | 5.5 | 1.09E-07 |
| 1.5 | 2.92E-05 | 6.0 | 1.08E-07 |
| 2.0 | 2.92E-05 | 6.5 | 1.06E-07 |
| 2.5 | 2.91E-05 | 7.0 | 1.05E-07 |
| 3.0 | 2.91E-05 | 7.5 | 1.04E-07 |
| 3.5 | 2.90E-05 | 8.0 | 1.03E-07 |
| 4.0 | 2.89E-05 | 8.5 | 1.02E-07 |
| Reference Point within the Irradiation Field | Reference Point outside the Irradiated Field | ||
|---|---|---|---|
| Distance from Center (cm) | Scattering Contribution | Distance from Center (cm) | Scattering Contribution |
| 0.0 | 3.56% | 4.5 | 2.49% |
| 0.5 | 3.55% | 5.0 | 2.22% |
| 1.0 | 3.52% | 5.5 | 1.80% |
| 1.5 | 3.46% | 6.0 | 1.43% |
| 2.0 | 3.38% | 6.5 | 1.16% |
| 2.5 | 3.28% | 7.0 | 0.98% |
| 3.0 | 3.17% | 7.5 | 0.84% |
| 3.5 | 3.06% | 8.0 | 0.74% |
| 4.0 | 2.95% | 8.5 | 0.67% |
For the simulation of the absorbed dose of the TLD, the thermoluminescent grating element is arranged on one side in the thermoluminescent pallet and extends beyond the irradiation field until it reaches the edge of the thermoluminescent pallet.
1) Inhomogeneity
The simulation results are shown in
The results of the calculations show that the inhomogeneity of the radiation field within the irradiation field reaches 1.81%. It meets the requirements of the relevant standards.
2) Scattering
The dose values detected by the TLD in the irradiated field were all contributed by scattered rays, and the contribution of scattered rays was 0.97% of the average value of the dose values in the irradiated field. The requirements of the relevant standards have been met.
The following section focuses on describing the corrections to the specific kinetic energy of the air in the radiation field with the addition of an attenuator.
1) Inhomogeneity
The results of the dose value of each reference point of the radiation field after adding the attenuator are shown in
2) Scattering
The changes in radiation field scattering in the irradiated field after the addition of the attenuator are shown in
| Reference Point within the Irradiation Field | Reference Point outside the Irradiated Field | ||
|---|---|---|---|
| Distance from Center (cm) | Normalized Count | Distance from Center (cm) | Normalized Count |
| 0.0 | 2.85E−05 | 4.5 | 1.40E−07 |
| 0.5 | 2.85E−05 | 5.0 | 1.16E−07 |
| 1.0 | 2.85E−05 | 5.5 | 1.12E−07 |
| 1.5 | 2.85E−05 | 6.0 | 1.10E−07 |
| 2.0 | 2.85E−05 | 6.5 | 1.08E−07 |
| 2.5 | 2.85E−05 | 7.0 | 1.07E−07 |
| 3.0 | 2.85E−05 | 7.5 | 1.05E−07 |
| 3.5 | 2.84E−05 | 8.0 | 1.03E−07 |
| 4.0 | 2.84E−05 | 8.5 | 1.01E−07 |
| No Attenuator | With Attenuator | ||
|---|---|---|---|
| Distance from Center (cm) | Scattering Contribution | Distance from Center (cm) | Scattering Contribution |
| 0.0 | 3.56% | 0.0 | 3.54% |
| 0.5 | 3.55% | 0.5 | 3.54% |
| 1.0 | 3.52% | 1.0 | 3.50% |
| 1.5 | 3.46% | 1.5 | 3.46% |
| 2.0 | 3.38% | 2.0 | 3.37% |
| 2.5 | 3.28% | 2.5 | 3.28% |
| 3.0 | 3.17% | 3.0 | 3.17% |
| 3.5 | 3.06% | 3.5 | 3.06% |
| 4.0 | 2.95% | 4.0 | 2.96% |
It can be obtained from the results that the low-energy scattering of the radiated field is partially shielded after the addition of the attenuator. But the shielding share is small.
The results show that the shielding performance, radiation field uniformity, scattering contribution rate and other indexes of the simple standard irradiator meet the requirements of the standards, and can calibrate more than 50 thermoluminescent detectors at one time. The designed simple standard irradiator can meet the calibration and scale requirements of pyroelectric detectors for environmental dose monitoring.
The authors gratefully acknowledge the financial support from the Sichuan Province Ecological Environmental Protection Science and Technology Program (NO. 2021HB11).
The authors declare no conflicts of interest.
Li, X.X., Gu, Y. and Song, B.L. (2023) Design and Monte Carlo Simulation of a Novel Irradiation Device. Open Access Library Journal, 10: e10886. https://doi.org/10.4236/oalib.1110886