  Neutron howitzers are the irradiation instruments, generally used for the
chemical analysis in the research laboratories, where the neutron sources are
encapsulated within a moderating medium and the target materials are irradiated
at the channels inserted through the corresponding neutron sources. In this
study, we employ Monte Carlo simulations by using the MCNP5 code in order to
perform the neutronic analysis of a Nuclear-Chicago NH3 neutron howitzer with a
5-Ci 239Pu-Be neutron source. This neutron howitzer is a cylindrical drum that
is covered by aluminum at the level of outermost layer, and the cylindrical
body is installed with paraffin for the neutron moderation. The drum consists
of two vertical and two horizontal irradiation channels and a vertical source
channel in which the 5-Ci 239Pu-Be neutron source is located and moved upward
or downward by the aid of a plexiglass rod. We focus our study on the two
horizontal irradiation channels, separated by an angle of 120o, which are
installed with a cylindrical plexiglass bar. We construct our geometry by
respecting the dimensional properties of the NH3 howitzer components and we
define a biased cylindrical source of 1.02 in.x 4.425 in. by taking into
account the neutron energy spectrum and source strength of the 5-Ci 239Pu-Be
neutron source. We determine the neutron flux within the plexiglass bar for
various energy bins in the energy range between 0 and 10.5 MeV, and also, we
investigate the axial variation of the neutron flux along the plexiglass region
in the horizontal irradiation channels. Our simulation results show that a
significant number of thermal neutrons as well as a non-negligible population
of fast neutrons propagate through the plexiglass bar, thereby demonstrating
the opportunity to product from a wide range of energy spectrum for the neutron
activation analysis.
