Analysis of the Neutron Radiographic Image Quality and Beam Intensity Produced by a Compact Multichannel Collimator

We investigate the image quality and beam intensity of thermal neutron radiography after replacing a standard single-channel neutron collimator with a compact array of microcollimators. In this study, the MCNP6 Monte Carlo computer code was used to simulate a 2 × 2-cm-area isotropic thermal neutron source, which then was collimated by an array of micron-sized neutron collimators that measured 29.8 μm in diameter and with lengths that varied from 0.6 to 3 mm. These microcollimators were spaced 30 μm apart and assembled into a 2 × 2-cm array.

The image quality of the neutron beams produced by the resulting collimator arrays was assessed by imaging the edge of a very thin (~0.01-mm) gadolinium foil to obtain the image Modulation Transfer Function (MTF). The MCNP6 resulting flux map from each simulation then was converted into a grayscale .tiff image and the image’s resulting MTF obtained using the ImageJ computer program with the imaging beam geometric unsharpness, which is a limiting factor in the image resolution determined at the 10% value of the MTF curve.

In this study, we found that a 2 × 2× 0.298-cm microcollimator, corresponding to a length-to–hole diameter ratio of 100:1 and a collimator length of 2.98 mm produced a beam with a geometric unsharpness of 32 μm. Compared to a standard single-channel collimator with a 2 × 2-cm aperture, the single-channel collimator would need to be 660 cm long to produce an equivalent geometric sharpness. Yet because of its shorter length, the imaging beam intensity from our 2.98-mm-thick collimator array was approximately 50 times greater than that of an equivalent single-channel collimator.