Investigation and comparison of neutron image quality using monochromatic and polychromatic beams of Tehran Research Reactor by Monte Carlo simulation

Neutron radiography (NR) is a non-destructive testing (NDT) method which uses a neutron beam for materials investigation in the research and industrial application. In this work, the quality of the neutron radiography system based on thermal monochromatic and polychromatic beam tubes of the Tehran Research Reactor (TRR) has been studied and compared using Monte Carlo simulation. MCNPX is a Monte Carlo radiation transport code with a built-in radiography feature that has been used The parameters of neutron images have been evaluated by several standard test samples qualitatively and quantitatively. The image analysis of standard samples shows that the thermal neutron content of the monochromatic beam is about 29.8% more than a polychromatic beam. Moreover, the simulation results have been validated by measured quality parameters of the polychromatic neutron beam at the E channel of TRR which are in good agreement. The results show that the monochromatic neutron beam with a wavelength of 2.9 Å can have a lower neutron intensity by 36% on average for iron, brass, copper, and polyethylene. However, there was no significant difference between polychromatic beam and neutron beam at a wavelength of 1.75 Å for these materials. Furthermore, a polychromatic beam provides a better performance to show different percentages of uranium enrichment of fuel rods in the images than the monochromatic beam. So that, the maximum difference is about 90.09% between transmitted neutron intensities of rods with enrichment of 12 and 20% by a polychromatic beam.