The thermal-neutron flux produced by a 252Cf spontaneous fission neutron source was computed for various spherical assemblies with moderators of iron, aluminum, carbon, D2O, beryllium, and H2O and reflectors of H2O and carbon. Consideration of spherical assemblies with a 252Cf source located at the center and at various distances from the center lead to a configuration, adaptable to neutron radiography, that has a small thermalization factor and a flat thermal-neutron flux.