Moderator–collimator-shielding design for neutron radiography systems using 252Cf

This paper is concerned with the presentation of a study of the general design of an optimized neutron radiography system that utilizes 252Cf. Moderation, collimation and shielding aspects are considered. A Monte Carlo code, MCNP, was used to obtain a maximum and more homogeneous neutron flux in the collimator outlet next to the image plane, taking into account geometric characteristics and an adequate radiation shielding strategy that complies with the radiological protection rules. Among the various moderator materials investigated, the high density polyethylene proved to be the most efficient, with a thermalization factor of 56 cm2. Using a collimator design assembly it was possible to obtain a normalized thermal neutron flux, at the image plane, equals 6×10−6 n cm−2 s−1 at an effective collimator ratio of 7.5, or 3.2×10−7 n cm−2 s−1 at an effective collimator ratio of 50. The total dose equivalent rates were significantly reduced by the shielding optimization process.