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  • Accomplishing practical tasks in radiation processing

    1. Analysis of EB and X-ray dose distribution formation in flat product

    1.1. Influence of scanner regimes on EB dose distribution formaion. Mode RTL

    ModeRTL was used for analysis and simulation with Monte Carlo (MC) and Analytical methods of electron beam (EB) dose distribution in flat targets (see description of ModeRTL operations and methods of data input in the Chapter 4.1). The geometrical arrangement of EB source, scanner, moving conveyor and target with packing box, irradiated with triangular scanned EB and with non-divergent (parallel rays) scanned EB is shown in Fig. 50a-b (see Chapter 3.2). Input data for analysis and comparison of EB absorbed dose distributions (ADD) in flat polyethylene (PE) targets irradiated with non-divergent and triangular scanned EB are as follows:

    • Source. Electron beam energy: 5MeV, Beam current: 1mA, Angular spread: Mono direct, Space spread: pencil beam.

    • Scan. Scanning horn: Height: 70 cm, Frequency: 100 Hz. Conveyer speed: 1cm/sec.

    • Scanning method. Non-diverging/Triangular.

    • Geometry. Distance scan-conveyer: 20cm, Width of scanning: 90 cm.

    • Target and cover. Target material: PE with density 0.94 g/cm3; target width: 80 cm. Cover (Container) material: Vacuum/air/Aluminum.

    • Irradiation regimes. One/two sided.

    All output data (2D view and 3D view) of the EB absorbed dose distributions into flat targets calculated with ModeRTL by MC or Analytical methods are represented in Chapter 6.2. The sequence of operations with ModeRTL for performing a comparison analysis is the following:

    • Calculate with MC or Analytical methods, one or two sided irradiation, for non-diverging and triangular scanned EB:

      • electron ADDs in a plane, which across the target center in the direction of conveyer movement;
      • electron ADDs near the boundaries of target with vacuum/air or package material at the end of scan beam direction;

    • Enter the selected curves of the ADDs in the module «Comparison» for analysis.

    The comparison results of ADDs in flat PE target irradiated with non-divergent and triangular scanned EB are presented in Fig.154a. EB dose mapping within PE target for optimal target thickness with two sided irradiation is shown in Fig. 154b. As it is seen from Fig. 154a, good agreement between depth dose distributions in a plane, across the target center calculated by analytical method (curve 2) and simulated by MC method (curve 1) is observed. It allows use analytical method for fast optimization of irradiation regimes for various radiation processing and integrates it in a control system of radiation facility [6, 17].

    The obtained simulation results show that values and profiles of the absorbed dose distributions differ for:

    • cases of target irradiated with non-diverging or triangular scanned EB, i.e. the values of the ADD for non-diverging scanned EB are greater in comparison with triangular scanned EB (see Fig. 1a);

    • in the target center and near the target boundary with air, i.e. the values of the ADD in the target center are greater in comparison with ADD near the target boundary with air (see Fig. 1a-b). Such differences essentially influence on the value of dose uniformity ratio (DUR) in targets irradiated with scanned EB [6,16,17].

    FIG. 1. Comparison results: Non-divergent/Triangular scanned EB. Two sided irradiation. Optimal target thickness for PE with density 0.94 g/cm3 under two sided irradiated with 5 MeV electrons is 4.55 cm. Scaling dose: 30.61 kGy, Scaling X (along target thickness): 4.55 cm.
    a) Curves 1,2,3-Non-divergent scanned EB. Curves 4,5 - Triangular scanned EB.
    Curve 1 - depth dose distribution in a plane, across the target center in the direction of conveyor movement, MC simulation. Curve 2 - depth dose distribution in the target center, analytical calculation. Curve 3 - depth dose distribution near the target boundary with air in direction of EB scanning, MC simulation.
    Curve 4 - depth dose distribution in the target center, MC simulation.
    Curve 5 - depth dose distribution near the target boundary with air, MC simulation.
    b) 3D-view of the absorbed dose distribution along target depth (axis X) and target width (EB scan direction, axis Y) for optimal target thickness, two sided irradiation with triangular scanned EB.

    Above example shows that for the calculation of the EB dose uniformity ratio the dose non-uniformity not only in the target center but also on the boundaries of an irradiated target along direction of EB scanning must be taken into consideration.

    1.2. EB dose distribution near the target boundary with cover

    Cover materials can influence the absorbed dose distributions near the target boundary with the cover [16, 17]. Analysis of the anomalies in dose distributions near the target boundary with cover will be performed with PE target and EB irradiation regimes which are the same as in the Chapter 10.1.1. Comparison and analysis will be performed for target irradiated in vacuum, in air and within an aluminum box. Input data for target and cover:

    • Target is irradiated in vacuum, when cover in the frame for input data «Target and Cover» is absent - default mode.

    • Input data for Air-Cover: Air simulated with nitrogen (density - 0.001 g/cm3). Cover thickness is 0,01cm. Add cover thickness 15 cm. Open cover. Material is Nitrogen. Data in the Table «Correct for a cover»: Atomic number -7, Weight part - 1.

    • Input data for Al - Cover: Cover thickness 0.01cm. Density - (0.63-2.7) g/cm3.

    Add cover - Al, thickness (0.5-2) cm. Open cover.
    The comparison results of dose distributions in flat PE target with various cover irradiated with non-divergent and triangular scanned EB are presented in Fig. 2a-b. The values of DUR calculated on the base of Dmax and Dmin for dose distributions which are shown in Fig. 2a-b are represented in the Table 1.

    FIG. 2. Comparison results: Influence of cover materials on dose distribution near the target boundary with cover. EB energy 5 MeV. Two sided irradiation. Scaling dose: 30.61 kGy, Scaling X (along target thickness): 4.55 cm. MC simulation.
    a) Non-divergent scanned EB. Curve 1 - depth dose distribution in the target center.
    Curve 2 - depth dose distribution near the target boundary with vacuum. Curve 3 - depth dose distribution near the target boundary with air. Curve 4 - depth dose distribution near the target boundary with Al box, wall thickness 2cm, density 2.7 g/cm3. Curve 5 - depth dose distribution near the target boundary with Al box, wall thickness 0,5 cm, density 0,6 g/cm3
    b) Triangular scanning. Curve 1 - depth dose distribution in the target center. Curve 2 - depth dose distribution near the target boundary with vacuum/air. Curve 3 - depth dose distribution near the target boundary with Al box, wall thickness 0.5 cm, density 0,6 g/cm3

    Table 1. Anomalies of the ADD near the boundary with cover materials

    N

    Regimes irradiation (Scanner)

    DUR (vacuum)

    DUR
    (air)

    DUR (Al box)
    2 cm, 2.7g/cm3

    DUR (Al box)
    0,5 cm, 0.6 g/cm3

    1

    Non-divergent scanned EB

    2.52

    2.14

    2.28

    1.70

    2

    Triangular scanned EB

    3.28

    3.28

     

    1.65

    The analysis of presented results (Fig. 155 and Table 13) have shown that anomalies of the ADD near the boundary with cover materials can be reduced when the density of cover materials is less than the density of target materials. It is reached due to lateral highlighting of the target boundary with primary and secondary electrons which are released from cover materials [16, 17]. This effect is increased with increasing atomic number of cover materials.

    The simulation results for ModeRTL show that non-uniformity equalization for EB dose distribution along a direction of scanning can be made with cover materials. ModeRTL can be used as predictive tools for EB dose mapping, for determination of locations Dmin, Dmax, calculation of DUR in the target volume, optimal target thickness for one/two sided irradiation for an acceptable value of DUR in the certain radiation processing applications.

    2. Analysis of X-ray dose distribution in flat targets. Mode XR

    ModeXR was used for simulation and analysis of the X-ray dose distribution in flat targets using the Monte Carlo (MC) method. See description of ModeXR operations and methods of data input in Chapter 4.1. Geometrical arrangement of EB source, X-ray converter, scanner, moving conveyor and target with packing box irradiated with triangular scanned X-ray, and with non-divergent (parallel rays) scanned X-ray is shown in Fig. 54, see Chapter 3.2.
    Input data for analysis and comparison of X-ray absorbed dose distributions (ADD) in flat polyethylene (PE) target irradiated with triangular scanned X-ray beam are as follows:

    • Source. Electron beam energy: 5 MeV, Beam current: 15 mA, Angular spread: Mono direct, Space spread: point beam.

    • Scan. Scanning horn: Height: 100cm, Frequency: 100 Hz. Conveyer speed: 0,4 cm/sec.

    • Scanning method. Triangular.

    • Geometry. Distance scan-conveyer -30cm, Width of scanning: 90 cm.

    • Target and cover. Target: Material - PE with density 0.94 g/cm3. Target width 80 cm. Cover: Air. Enter of input data for Air-Cover see in Chapter 10.1.2.

    • Regimes of irradiation: One/two sided.

    All output data (2D view and 3D view) of the X-ray absorbed dose distributions in flat targets calculated with ModeXR by MC methods are represented in Chapter 6.2. The X-ray beam was generated by scanned electron beam with electron energy 5 MeV in a tungsten converter. Fig. 3a presents the X-ray converter which includes the wolfram target plate with thickness 1.1 mm, the cooling water channel (5mm), and the steel backing plate (1.0 mm). X-ray spectrum for 5 MeV electron beam in the wolfram converter is presented in Figs.3b. The X-ray yield in the forward direction for 5 MeV electrons is 8.72 %.

    FIG. 3. X-ray converter construction (a); X-ray spectrum for 5 MeV electron in the wolfram converter (b)

    The comparison of the X-ray depth dose distributions in the center and near the boundaries of PE target with air are presented in Fig.4. The PE target with optimal thickness 22 cm was two sided irradiated with triangular scanned X-ray beam. The optimal thickness hopt for the PE target (relative to the dose distribution at the center of the target) was calculated with ModeXR. The following criterion was used for selection of the optimum thickness value hopt: the value of absorbed dose (Dmax) near the entrance surface of X-ray into the center of PE target differs by 20% from the value of absorbed dose (Dmin) at 1/2 hopt. X-ray dose mapping within a PE plate for optimal target thickness at two sided irradiation for saw-tooth and special forms of current in scan magnet are shown in Fig.158a and Fig. 158b respectively.

    As it is seen from Fig. 4, Fig. 5a and Fig. 5b, the X-ray depth dose distribution within PE plate has a minimum value on the boundaries of the target along the direction of the scanning X-ray beam and maximal value at plane across the target center. For the target center the dose uniformity ratio DUR = Dmax/Dmin is 1.27. For the target boundaries the DUR is 1.41. As a result, the X-ray dose uniformity for all irradiated volume of PE plate DURV is 1.94. The significant dose gradient in the irradiated target volume can be decreased by the selection of a special current shape in the scan magnet. Changing the current shape in scan magnet results in a time dependent electrons intensity along a direction of scanning in front of an X-ray converter [6]. The special shape of current in the scan magnet which provides the maximum uniformity of the X-ray dose distribution in the irradiated target was determined by ModeXR.

    FIG. 4. Comparison results. Equalization of the X-ray absorbed dose distribution along X-ray beam scanning. Scaling dose: 11.61 kGy, Scaling X (along target thickness): 22cm. MC simulation. Triangular scanned X-ray beam.
    Curve 1 - depth dose distribution in a plane, across the PE target center in the direction of conveyor movement. Saw-tooth current form in scan magnet.
    Curve 2 - depth dose distribution near the PE target boundary with air in direction of X-ray beam scanning. Saw-tooth current form in scan magnet.
    Curve 3 - depth dose distribution in the PE target center for the special forms of current in scan magnet.
    Curve 4 - depth dose distribution near the PE target boundary with air for the special forms of current in scan magnet.

    Fig. 5 and Table 2 shows the time dependencies of the scan magnet current in for a conventional saw-tooth shape (curve 1) and the special shape of current (round points). The X-ray dose distributions determined by the special current shape (round points) are shown in Fig. 4 (red and blue curves) and in Fig. 5b. For case of the usage the special shape of current in scan magnet (see Table 2), the X-ray dose uniformity for the whole irradiated PE plate volume of was improved to DURV = 1.61.

    FIG. 5. X-ray dose mapping within PE target with optimal target thickness for the saw-tooth form of current (a), and the special form of current (b) in scan magnet.  

    FIG. 6. Time dependence of current for the saw-tooth (curve1) and special shape of current (round points) in scan magnet (a). «Scanning system» frame of the software ModeXR which allows simulate various form of current in scan magnet of EB scanning system of electron accelerator. The frame «Correct table» comprises the data of round points in the first quadrant for Time of current from 0 to 1 (b).

    Table 2. Time dependence of current in scan magnet for saw-tooth and special form

    Time of current in scan magnet, relative units

    0

    0.25

    0.5

    0.75

    1

    1.25

    1.5

    1.75

    2

    Current value, saw-tooth form, relative units

    -1

    -0.75

    -0.5

    -0.25

    0

    0.25

    0.5

    0.75

    1

    Current value, special form, relative units

    -0.1

    -0.9

    -0.75

    -0.4

    0

    0.4

    0.75

    0.9

    0.1

    ModeXR can be used for practical tasks in X-ray processing, in particular, related with the equalization of X-ray dose distribution by non-uniform scanning using a special shape of scan magnet current or with cover materials. In addition ModeXR allows the calculation of optimal target thickness for a two sided irradiation, determination of locations Dmin, Dmax and DUR in volume of target irradiated by X-rays.

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