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Figure 4. Radial distribution of the neutron beam profile and the neutron background at the
sample position. The values correspond to the neutron fluence through concentric cylinders:
the first 20 cylinders with radial increments of 1 cm and the last 7 as indicated in the figure.
neutron fluence (n/cm 2 /MCNPX n with θ=0.22341)
10 -6
10 -7
10 -8
10 -9
10 -10
10 -11
beam of 2 cm radius
10 -5 5 10 15 20 25
background
Beam pipe (19.844 - 20 cm)
Air (20 - 40 cm)
Air (40 - 60 cm)
Air (60 - 80 cm)
Air (80 - 100 cm)
Air (100 - 165 cm)
Air (165 cm - wall )
10 -12
region number
The first important shielding element is the pipe itself. Several tens of metres after the Pb target,
the incidence angle of the neutrons hitting the pipe is small and the effective length of material seen by
the neutrons is large. In this way, a large fraction of the neutrons are deviated from its initial trajectory
and produce background in the upstream half of the tunnel. Due to the telescopic structure of the pipe,
the places at the diameter reductions (80 cm to 60 cm and 60 cm to 40 cm) should be reinforced, since
the neutrons traverse the pipe almost perpendicularly. At these positions, the pipe was covered after
the reduction with 1-m thick external iron cylinders.
The second important shielding element is the first collimator (SSC) starting at 135.54 m. Such a
strong scattering centre had to be shielded by a 3 m concrete wall. After the SSC, the neutron beam
divergence is small and the neutrons do not hit the pipe before the second collimator (BSC). The 2.5 m
long BSC diffuses and moderates very efficiently the whole neutron spectrum. However, it is a strong
scattering centre that has to be efficiently shielded by a 3.2-m thick wall, placed 1 m
beam-downstream. Figure 3 shows the top view of the area in scale, where the BSC is located, as has
been designed for the MCNPX simulations. The 2.5-m long BSC is observed on the left-hand side of
the 3.2-m concrete wall. Also visible is the chicane, a necessary escape path in case of emergency. The
impact of this shielding opening on the background at the sample position was studied with
Monte Carlo simulations.
The neutron beam profile and background at the sample position is shown in Figure 4. All the
values correspond to the neutron fluence of concentric cylinders: the first 20 values with radial
increments of 1 cm and the last 7 as indicated in the figure. The first two bins correspond to the main
neutron beam, which has a radius of 2 cm. The remaining bins correspond to the background level at
the different places. It can be seen that the neutron background is on average at the level of
2·10 -12 n/cm 2 per neutron emitted from the Pb target with a θ angle smaller than 0.223º. Such a value,
when compared to the beam fluence of 2·10 -5 , leads to a background to signal ratio of 10 -7 . This
background level is of the same order than the background of scattered neutrons produced at 15 cm
from a 10 mg 235 U sample placed in-beam. A similar result was found for the γ background produced
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