AbstractThere is a continuing interest in the use of fixed neutron absorbers (poisons) for criticality control, since their use would permit safely handling larger quantities of nuclear materials with reduced probability of criticality. The effectiveness of such absorbers as neutron poisons depends on self-shielding effects, which in turn are determined by the magnitude of the absorption cross sections and their variation with energy, the thickness of material, and the neutron energy spectrum. Criticality experiments were performed to obtain data on the reactivity worths of several thicknesses of the following materials in two different neutron energy spectra:BoralCadmiumType 304-L stainless steel containing 1.6 wt% boronType 304-L stainless steel containing 1.1 wt% boronType 304-L stainless steelUranium depleted to 0.2 wt%235ULead.The measurement data reported are limited to a single region of a given absorber material in each critical assembly. Combinations of absorber materials or multiregions were not investigated; however, material thicknesses were varied from 0 to∼60 mm. The data are presented as sets of clean, well-defined, poisoned critical assemblies that can be used to check calculational techniques and cross-section data in two different neutron energy spectra. The materials are listed above in the order of their measured relative worth as fixed poisons in either neutron energy spectrum.