Reactivity effects of various materials of interest to epithermal and fast reactors have been measured in a series of neutron spectra with median energies of neutrons causing fission ranging from 422 keV to 8.5 eV. An oscillator technique capable of sensitivities of 10−8 Δk was used. In addition, a power-history or ‘reactivity-vs-time’ method has been developed for fast but less sensitive reactivity measurements. It has been used to obtain radial reactivity traverses and rod calibrations and for other routine reactivity measurements.