Neutronic Analysis of a Nuclear-Chicago NH3 Neutron Howitzer

TL;DR

This study uses Monte Carlo simulations to analyze the neutron flux distribution in a Nuclear-Chicago NH3 neutron howitzer, revealing the presence of thermal and fast neutrons suitable for neutron activation analysis.

Contribution

First neutronic analysis of the NH3 neutron howitzer using MCNP5, focusing on neutron flux distribution in horizontal irradiation channels with detailed geometry modeling.

Findings

Significant thermal neutron flux in the plexiglass bar.

Presence of fast neutrons suitable for activation analysis.

Axial variation of neutron flux along the channels.

Abstract

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.