Low energy neutrons from a $^{239}$PuBe isotopic neutron source inserted in moderating media

Abstract

Several neutron applications share a common problem: the neutron source design. In this work MCNP computer code has been used to design a moderated $^{239}$PuBe neutron source to produce low energy neutrons. The design involves the source located at the center of a spherical moderator. Moderator media studied were light water, heavy water and a heterogeneous combination of light water and heavy water. Similar moderating features were found between the 24.5 cm-radius container filled with heavy water (23.0-cm-thick) and that made with light water (3.5-cm-thick) plus heavy water (19.5-cm-thick). A $^{239}$PuBe neutron source inserted in this moderator produces, at 27 cm, a neutron fluence of 1.8 x 10$^{ - 4}$ n-cm$^{ - 2}$ per source neutron, with an average neutron energy of 0.34 MeV, where 47.8 % have an energy $ \le $ 0.4 eV. A further study of this moderator was carried out using a reflector medium made of graphite. Thus, 15-cm-thickness reflector improves the neutron field producing a neutron fluence of 1.1 x 10$^{ - 3}$ n-cm$^{ - 2}$ per source neutron, whose average neutron energy is 0.098 MeV, where 75.8 % have an energy $ \le $ 0.4 eV.