4.1.1 Design

The neutron source of D-T neutron tube can outgo the neutrons with average energy of 14.1 MeV and the neutron energy range is relatively narrow, so it can be approximately viewed as monoenergetic neutron source. To obtain 14.1 MeV quasi monoenergetic neutrons, a tapered alignment hole should be designed on the shielding structure and 14.1 MeV monoenergetic neutrons can be directly obtained without any shielding. In consideration of the size of shielding structure, the scheme of W (35 cm) + (CH)_n (10 cm) + B₄C (0.l cm) + Pb (15 cm) is taken as the example to design the alignment structure. The tapered alignment hole has an axial symmetry structure which guarantees that the tapered alignment projection can completely cover the monitor at the location (1m from the source center), with a taper angle of 25°, and the alignment structure is displayed in Fig. 2.

But due to scattering effects of various materials, a lot of scattering neutrons will generate. The primary scattering neutrons include two parts. The first part includes the scattering neutrons created by interaction between the innermost shielding materials and 14.1 MeV neutrons; the other part contains the scattering neutrons produced by interaction of alignment hole inner surface and neutrons as well as the neutrons leaked from the alignment hole inner surface in the transportation process of shielding layer.

In addition, through simulation of variation of scattering neutrons with material thickness, W shows good scattering effect on 14.1 MeV neutrons. To reduce as many as impacts of scattering neutrons, a kind of materials with weak scattering effect on 14.1 MeV neutrons is added to the innermost shielding layer. Based on the simulation result, at the innermost shielding layer, a layer of boracic polyethylene with a thickness of 10 cm is added to reduce generation of scattering neutrons; on the tapered alignment hole inner surface, the optimal thickness of boracic polyethylene is 25 cm . In this case, except reducing the scattering effect, the boracic polyethylene can reduce the leaking neutrons to enter the alignment hole; if boracic polyethylene filtration layer is added to the alignment hole, due to serious scattering factors, it has worse result than the case where boracic polyethylene is not added. Thus it is only necessary to add boracic polyethylene to the innermost shielding layer and alignment structure inner surface to obtain 14.1 MeV monoenergetic neutrons.

4.1.2 Verification

A shielding hall in Xi’an is adopted as the background to simulate the filtration effects of different design schemes. The average neutrons and γ energy spectrums within the measurement zone with radium of 50cm and interval of 1m at a distance of 1~9m from the source are recorded. It is assumed that the yield of neutron tube is 5.0×10⁸ s^-1 (the yield of neutron tube in this paper is not less than 5.0 × 10⁸ s^-1). The shielding filtration scheme is used to obtain 14.1 MeV neutrons, and the neutrons and γ energy spectrums are displayed in Fig. 5 (a) and (b).

Fig.5Full-screen View

Energy spectra of neutron (a) and γ (b) for obtaining 14.1 MeV neutrons

Figure 6 shows that this design scheme is able to obtain 14.1 MeV neutrons with good monochromaticit. With increased radial distance, reducing of neutrons and γ relative fluence will be fast. The neutron energy spectral patterns at different locations are similar basically. The neutron monoenergetic peak is 14.1 MeV while γ ray has some influence.

Fig.6Full-screen View

Energy spectra of neutron (a) and γ (b) for obtaining 0.18 MeV neutrons

4.2.1 Design

Tungsten (W) has good shielding effect on 14 MeV fast neutron and can generate a large amount of intermediate and low-energy neutrons after being slowed down; B₄C has good shielding performance on the thermal neutrons, and when B₄C has a thickness of 0.2cm, it can absorb all thermal neutrons. To obtain monoenergetic neutrons with energy of about 1 MeV, W material layer can be combined with 0.2cm B₄C to filter and absorb 14 MeV fast neutrons. Through simulation of MCNP software, the result shows that with the increase of W layer thickness, high-energy 14 MeV neutrons are gradually reduced and intermediate and low energy neutrons are gradually increasing. When the thickness of W layer is 30cm, the ratio of intermediate and low energy neutrons and high-energy neutrons is maximized and optimal monoenergetic neutrons can be obtained with energy peak of 0.18 MeV.

4.2.2 Verification

The shielding filtration scheme is used to obtain 0.18 MeV neutrons, and the neutrons and γ energy spectrums are displayed in Fig. 6 (a) and (b). This figure suggests that this design scheme can obtain 0.18MeV neutrons with good monochromaticity. With increased radial distance, reducing of neutrons and relative γ fluence will be fast. The neutron energy spectral patterns at different locations are similar basically. The neutron monoenergetic peak is 0.18 MeV while γ ray has some influence.

4.3.1 Design

Polyethylene has good shielding performance for the intermediate and low energy neutrons below 1 MeV, while has bad shielding performance for thermal neutrons. It is suggested to combine W and polyethylene as the shielding materials to obtain thermal neutrons.

The result of MCNP software simulation shows that with increased thickness of polyethylene, outgoing neutrons gradually reduce, but the neutron energy spectrums resolution gradually decreases. When the thickness of polyethylene is 4cm and W is 30cm, we can obtain the thermal neutron energy spectrums with best monochromaticity.

4.3.2 Verification

The shielding filtration scheme is used to obtain the thermal neutron structure, and the neutrons and γ energy spectrums are displayed in Fig. 7 (a) and (b). According to the figures, we can find that this design scheme can obtain thermal neutrons with good monochromaticity. With increased radial distance, reducing of neutrons and elative γ fluence will be fast. The neutron energy spectral patterns at different locations are similar basically. The neutron energy decreases below eV order while γ ray has some influence.

Fig.7Full-screen View

Energy spectra of neutron (a) and γ (b) for obtaining thermal neutrons