  The tracking of radiation dose is a fundamental concept for the safe
operation of nuclear instruments, and this concept is a multi-task challenge in
the case of neutron sources since not only the neutron emissions from the main
radioactive source but also the neutron capture gamma-rays as well as the
gamma-rays from the inelastic neutron scattering make contribution to the total
dose received by the facility personnel. The present study is devoted to the
assessment of the equivalent dose rates on the surface of a Nuclear-Chicago NH3
neutron howitzer with a 5-Ci 239Pu-Be neutron source by means of Monte Carlo
simulations. The 5-Ci 239Pu-Be neutron source is mounted within a vertical
aluminum channel at the center of the neutron howitzer, and we use the axial
symmetry of the aluminum drum filled with an ordinary paraffin without any
potent neutron absorber such as boron or cadmium. We utilise the ICRP-21
flux-to-dose rate conversion factors and we track the neutron and photon
equivalent dose rates on the lateral and top surfaces including the outlets of
the vertical and horizontal irradiation channels by the aid of point detectors
in the MCNP5 code. From the results over a number of sub-regions on the
semi-cylindrical drum of the Nuclear-Chicago NH3 neutron howitzer, we show that
the outlets of both the vertical and horizontal irradiation channels deliver
approximately the same level of equivalent dose rates, and the maximum
equivalent dose rates for neutrons and photons on the lateral surface are
obtained at the spatial coordinates aligned with the location of the 5-Ci
239Pu-Be neutron source, whereas the periphery around the source channel on the
top surface of the neutron howitzer constitutes the highest level of equivalent
dose rates among the tracked locations.
