Low energy neutrons from a 239PuBe isotopic neutron source inserted in moderating media
Héctor René Vega-Carrilloa, b, c/* and Celia Torres-Muhech†
Unidades Académicas de Estudios Nuclearesa, Ingeniería Eléctricab y Matemáticasc, Universidad Autónoma de Zacatecas Apdo. Post. 336, 98000 Zacatecas, Zac., México.
Recibido el 23 de octubre de 2001. ]]> Aceptado el 13 de mayo de 2002.
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 239PuBe 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 239PuBe 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 ≤ 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 ≤ 0.4 eV.
Keywords: Isotopic neutron source; moderation; Monte Carlo method.
Resumen
Las aplicaciones de los neutrones comparten un mismo problema: el diseño de la fuente. En este trabajo el código MCNP se utilizó para disear una fuente de neutrones moderada, a base de una fuente isotópica de 239PuBe, con el propósito de producir neutrones de baja energía. En el diseño, la fuente se colocó en el centro de un moderador esférico. Los medios moderadores estudiados fueron agua ligera, agua pesada y una combinación heterogénea de agua ligera con agua pesada. Características similares en la moderación se observaron para el contenedor de 24.5 cm de radio lleno con agua pesada (23.0 cm de espesor) y con contenedor lleno heterogeneamente con agua ligera (3.5 cm de espesor) y con agua pesada (19.5 cm de espesor). Una fuente de 239PuBe en este último moderador produce a 27 cm una fluencia de 1.8 x 10-4n-cm-2 por cada neutrón emitido por la fuente, con una energía promedio de 0.34 MeV, donde el 47.8% tiene una energía ≤ 0.4 eV. Un estudio posterior sobre el moderador heterogéneo se realizó para determinar el efecto de utilizar un medio reflector de grafito. Así un reflector de 15 cm de espesor mejora el campo de neutrones de baja energía ya que se obtienen 1.1 x 10-3n-cm-2 por cada neutrón emitido por la fuente, la energía promedio es de 0.098 MeV, donde el 75.8 % tiene una energía ≤ 0.4 eV.
Descriptores: 239PuBe; moderación; Monte Carlo; neutrones.
]]> PACS: 29.25.Dz; 28.41.My; 87.53.Vb
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Acknowledgment
This work was supported by CONACyT (México), contract 31288U.
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]]>* Corresponding author, e-mail: rvega@cantera.reduaz.mx.
† Celia Torres-Muhech is a graduate student at the Centro Regional de Estudios Nucleares - UAZ.
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