Influence of the Thickness and Mutual Arrangement of Layers in an Iron-Polyethylene Composition on the Energy Distribution of Fast Neutrons
Abstract Generated abstract
This study examines how layer thickness and ordering in two-layer iron and polyethylene barriers affect the energy spectrum and integral flux of transmitted fast neutrons. Using a Monte Carlo solution of the neutron transport equation for a 3 MeV mono-directional source, the authors calculated spectra over 3.0 to 0.1 MeV for configurations with a fixed 10 cm first layer and variable second-layer thicknesses of 1, 2, 5, and 10 cm. The results indicate that placing polyethylene behind iron hardens the transmitted spectrum as the polyethylene layer increases, whereas placing iron behind polyethylene softens it, with the relative integral flux depending on second-layer thickness. The calculations suggest that such composites can be used for selective sensitivity to light-layer thickness, especially when detecting transmitted neutrons in the 0.1 to 1.2 MeV range.
Full Text
EFFECT OF THE THICKNESS AND RELATIVE ARRANGEMENT OF LAYERS IN AN IRON–POLYETHYLENE COMPOSITE ON THE ENERGY DISTRIBUTION OF FAST NEUTRONS
G. Sh. Lekarskii, Yu. Ya. Katsman
(Presented by the scientific seminar of the Research Institute of Electron Introscopy)
In this work, spectra were calculated for radiation that has passed through a two-layer barrier consisting of iron and polyethylene with densities \(\rho = 7.8\ \text{g/cm}^3\) and \(\rho = 0.93\ \text{g/cm}^3\), respectively. A point mono-directional \((\Theta_0 = 0^\circ)\) neutron source with an energy of 3 MeV was used. The thickness of the first layer of the barrier was unchanged and was equal to 10 cm; the thickness of the second layer was 1, 2, 5, and 10 cm. To study the influence of the alternation of layers, two variants were considered: iron—polyethylene and polyethylene—iron.
The neutron transport equation was solved by the Monte Carlo method on a BESM-4 computer. Information was output in the energy interval 3.0–0.1 MeV. The program provided output of the fast-neutron flux that had passed through the two-layer barrier, integral over angles and over the plane and differential in energy. The problem was solved in the transport approximation. The values of the interaction cross sections and the matrices of inelastic neutron scattering were taken from [1]. Inelastic scattering of neutrons was assumed to be isotropic in the laboratory coordinate system; neutron absorption was taken into account by means of statistical weights. For each thickness of the second layer, \(10^4\) trajectories were calculated. The results obtained, normalized to one incident neutron, are given in Table 1. Figure 1 shows the spectra of fast neutrons for different thicknesses of the second layer for the iron—polyethylene composition, with the iron thickness unchanged and equal to 10 cm. Figure 2 gives analogous results for the polyethylene—iron composition with the polyethylene thickness unchanged at 10 cm. The presented results of the energy distribution are normalized to the same neutron flux in the energy interval 3.0–2.57 MeV. Figure 3 illustrates the dependence of the integral flux on the thickness of the second layer.
Analysis of the results obtained shows that, when the light material is placed behind the heavy one (relative to the source), with the thickness of the first layer fixed, the spectrum of the transmitted neutrons becomes harder as the thickness of the light layer increases. When the heavy material is placed behind the light one, the opposite picture is observed, that is, a softening of the spectrum with increasing thickness of the second layer. The integral flux beyond the two-layer barrier, at small thicknesses of the second layer, is smaller in the case of the polyethylene—iron composition; however, at larger thicknesses of the second layer the opposite relation is observed.
The result obtained indicates the possibility of controlling two-layer compositions of heavy and light material by means of fast-
Table 1
Energy distribution of the fast-neutron flux behind a two-layer barrier from a plane unidirectional source with energy 3 MeV and intensity 1 neutron/cm²·s
| Interval, MeV | iron—polyethylene, thickness of 2nd layer 10 | iron—polyethylene, thickness of 2nd layer 5 | iron—polyethylene, thickness of 2nd layer 2 | iron—polyethylene, thickness of 2nd layer 1 | polyethylene—iron, thickness of 2nd layer 10 | polyethylene—iron, thickness of 2nd layer 5 | polyethylene—iron, thickness of 2nd layer 2 | polyethylene—iron, thickness of 2nd layer 1 |
|---|---|---|---|---|---|---|---|---|
| 3,0—2,57 | 0,162 | 0,309 | 0,433 | 0,506 | 0,179 | 0,4 | 0,669 | 0,772 |
| 2,57—2,0 | 0,0223 | 0,0393 | 0,0464 | 0,0510 | 0,0269 | 0,0435 | 0,0651 | 0,0565 |
| 2,0—1,56 | 0,0235 | 0,0793 | 0,0563 | 0,0615 | 0,0433 | 0,0622 | 0,0659 | 0,0699 |
| 1,56—1,21 | 0,0242 | 0,0483 | 0,0792 | 0,102 | 0,0668 | 0,0682 | 0,0692 | 0,0654 |
| 1,21—0,95 | 0,0303 | 0,0695 | 0,134 | 0,176 | 0,126 | 0,115 | 0,0985 | 0,0735 |
| 0,95—0,74 | 0,0302 | 0,0826 | 0,181 | 0,270 | 0,134 | 0,131 | 0,0867 | 0,0888 |
| 0,74—0,57 | 0,0373 | 0,106 | 0,271 | 0,349 | 0,232 | 0,182 | 0,112 | 0,0786 |
| 0,57—0,45 | 0,0297 | 0,111 | 0,238 | 0,323 | 0,180 | 0,222 | 0,127 | 0,0999 |
| 0,45—0,35 | 0,0259 | 0,104 | 0,219 | 0,253 | 0,169 | 0,123 | 0,0806 | 0,0827 |
| 0,35—0,27 | 0,0282 | 0,105 | 0,243 | 0,376 | 0,273 | 0,188 | 0,0670 | 0,0661 |
| 0,27—0,21 | 0,0225 | 0,104 | 0,258 | 0,369 | 0,307 | 0,130 | 0,0933 | 0,0847 |
| 0,21—0,16 | 0,0380 | 0,118 | 0,207 | 0,267 | 0,147 | 0,0901 | 0,0988 | 0,0607 |
| 0,16—0,13 | 0,0211 | 0,0572 | 0,211 | 0,286 | 0,153 | 0,118 | 0,0890 | 0,0841 |
| 0,13—0,1 | 0,0326 | 0,0831 | 0,254 | 0,341 | 0,246 | 0,222 | 0,0915 | 0,225 |
Fig. 1. Energy distribution of the neutron flux that has passed through a two-layer iron—polyethylene barrier. Iron thickness 10 cm. Polyethylene thickness “—” 1 cm, “—△—” 2 cm, “—○—” 5 cm, “—×—” 10 cm.
Fig. 2. Energy distribution of the neutron flux that has passed through a two-layer polyethylene—iron barrier. Polyethylene thickness 10 cm. Iron thickness “—” 1 cm, “—△—” 2 cm, “—○—” 5 cm, “—×—” 10 cm.
Fig. 3. Dependence of the integral neutron flux on the thickness of the second layer with the thickness of the first layer unchanged, equal to 10 cm. X — iron — polyethylene, O — polyethylene — iron.
fast neutrons, with selective sensitivity to a change in the thickness of the light layer. It also follows from the results obtained that, in order to obtain maximum selective sensitivity to a change in the thickness of the light material behind heavy metals, the range of registration of the transmitted radiation should be limited from above by an energy of 1.2 MeV, since it is precisely in the energy range 0.1–1.2 MeV that the maximum influence of the thickness of the light layer on the transmitted fast-neutron flux is observed.
REFERENCES
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