An Experimental Study of the Relative U²³⁵ Fission Activation as a Function of Energy in Slightly Enriched Uranium-Water Lattices

Measurements of the relative U²³⁵ fission rates as a function of energy have been made for the TRX facility, a slightly enriched uranium, light water moderated critical assembly. The parameter directly measured is the ratio of the activity of a bare U²³⁵ foil to that of a similar foil enclosed in a box of absorbing material, which was either cadmium, boron, or gadolinium. The energy dependence associated with these ratios was obtained by the introduction of “effective cutoff energies” for the absorbing shields. A comparison is made with calculated values, based upon a simplified model for the neutron energy spectrum present, and the agreement is considered adequate.