AbstractSolid-statep-njunction counters have been fabricated to measure fission rates of materials with different fission thresholds. The fission reactions are caused by neutrons varying in energy from thermal energies for U235to 1.5 MeV for Th232. The data gathered from these solid-state fission counters have been used to compare experimental with calculated fission rates in the AETR cores. The fission counter is assembled by placing an electroplated foil of a fissionable material near ap-njunction detector. An aluminum cap is placed over each detector and foil to form a neat, compact assembly. The resulting counter is small enough to fit inside a reactor with minimum distortion to the neutron spectrum. Fission counters employing Th232, U233, U234, U235, U236, U238, Np237, and Pu239as the principal fissionable material have been successfully fabricated. These solid-state fission counters have proved reliable instruments to measure neutron fluxes in high gamma-ray fields. True fission events are easily separated from other induced reactions in the counter.