Abstract

VEGA-CARRILLO, H.R et al. Monte Carlo study to measure the neutron flux using the prompt gamma-rays in hydrogen. Rev. mex. fis. [online]. 2007, vol.53, suppl.3, pp.8-11. ISSN 0035-001X.

A Monte Carlo study was realized to use a gamma-ray spectrometer with a 3" Øx 3" NaI(Tl) detector, with a moderator sphere, to measure the neutron fluence rate. Materials with a large amount of hydrogen are able to slowdown and thermalize neutrons; once thermalized there is a probability of neutron capture in moderator nuclei. When the neutron is captured by hydrogen a 2.22 MeV prompt gamma-ray is emitted. The photons pulse-height spectrum shows a photopeak around 2.22 MeV whose net area is proportional to total neutron fluence rate. The characteristics of this system were determined by a Monte Carlo study using the MCNP 4C code; a model of the NaI(Tl) was utilized, the study includes water and polyethylene as moderators with 3, 5, and 10 inches-diameter spheres that were bombarded with neutrons emitted by a polyenergetic ²³⁹PuBe. With the best sphere the study was extended to determine the response to monoenergetic neutron sources. Calculations were carried out to obtain realistic gamma-rays pulse height spectra, where the 2.22 MeV is observed. In polyethylene spheres the 4.43 MeV photon is also shown due to ¹²C(n, n' γ)¹²C reaction.

Keywords : Monte Carlo; neutrons; moderators; prompt gamma-rays.

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