Concrete enclosure for shielding a neutron source

Abstract

In the aim to design a shielding for a 0.185 TBq 239PuBe isotopic neutron source several Monte Carlo calculations were carried out using MCNP5 code. First, a point-like source was modeled in vacuum and the neutron spectrum and ambient dose equivalent were calculated at several distances ranging from 5cm up to 150cm, these calculations were repeated modeling a real source, including air, and a 1×1×1m3 enclosure with 5, 15, 20, 25, 30, 50 and 80cm-thick Portland type concrete walls. At all the points located inside the enclosure neutron spectra from 10−8 up to 0.5MeV were the same regardless the distance from the source showing the room-return effect in the enclosure, for energies larger than 0.5MeV neutron spectra are diminished as the distance increases. Outside the enclosure it was noticed that neutron spectra becomes “softer” as the concrete thickness increases due to reduction of mean neutron energy. With the ambient dose values the attenuation curve in terms of concrete thickness was calculated.