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The NNSA Experimental AreaExperimental measurements of neutron induced cross sections are performed at the Shielded Neutron Source Area (SNSA) at TUNL. A multi-layered wall composed of concrete, steel, iron, lead, and paraffin is used to separate the neutron-production cell from the target area, as shown in Figure 1.
Fig. 1. The TUNL Shielded Neutron Source Area (SNSA). A pulsed and collimated deuterium beam from the FN Tandem Van de Graaff accelerator is used to produce a monoenergetic neutron beam via the 2H(d,n)3He reaction. The neutron flux is about 8x10^7 n/(cm2.s) using a deuteron current of Id = 1 uA, calculated at a distance of 4.0 cm from the center of the deuterium gas cell (filled to 7.8 atm). The total neutron energy spread is dEn = 1.1 MeV at 6 MeV and 0.5 MeV at 18 MeV. The calculated energy dependence of the neutron flux is shown in Figs. 2 and 3. The repetition rate of the pulsed beam is 2.5 MHz with a pulse width of typically 2.5 ns.
Fig. 2. Calculated neutron flux from 2H(n,n)3He, for Id = 1 uA and a 3.0 cm long 2H gas cell filled to 7.8 atm, at a location 4.0 cm downstream from the center of the gas cell (at zero degrees).
Fig. 3. Same as Fig. 2, but 427.0 cm downstream from the center of the gas cell.
Figure 4a) shows the measured beam profile at the target position (215 cm from the center of the gas cell), while Figure 4b) shows the beam profile at the neutron beam monitor position (427 cm from the gas cell). The beam profile was measured in 3 mm steps in the horizontal and vertical direction from the beam center. It is clear that the shielded neutron source provides an excellently collimated beam. The beam drops off sharply at the edges, making it possible to position the detectors quite close to the target.
Fig. 4. Fig. 4. Horizontal and vertical neutron beam profiles: a) at the target position (215 cm from the center of the gas cell), b) at the neutron monitor position (427 cm from the gas cell). Geometrical corrections have been applied.
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