Investigation of gadolinium thickness effect on electron absorption and image quality in radiography film

Gadolinium converters are commonly used in thermal neutron radiography because gadolinium has one of the highest thermal neutron capture cross sections among all elements. It is typically placed in direct contact with the imaging medium, such as X‑ray film or a digital detector, so that emitted electrons create a latent image corresponding to the neutron attenuation pattern. Thin Gd (5-10 µm) layer results in higher resolution but lower sensitivity. Thick Gd (more than 40 µm) layer results in higher sensitivity, but poorer spatial resolution and possible blurring. This study investigates the effect of gadolinium converter thickness, varied from 25 μm (commercial ones) to 100 μm, on electron emission and image quality in neutron radiography films using MCNPX Monte Carlo simulations. The computational investigations were performed using monochromatic and polychromatic neutron beam of the Tehran Research Reactor. The (n, γ) reaction rate, electron spectra, and energy deposition within the emulsion layer were analyzed to discuss the converter performance. Results show that increasing the converter thickness from 25 μm to 100 μm enhances the reaction rate and total electron production, which can reduce imaging time. However, thicker converters also increase electron scattering and self-shielding, negatively impacting image sharpness and contrast. The findings showed monochromatic neutron beam provides better resolutions than polychromatic beam in the case of both 25 μm and 100 μm thicknesses. Also 100 μm converter disturbs image quality for both monochromatic and polychromatic beams. These findings offer valuable insights for optimizing gadolinium converters in neutron radiography applications.