A theoretical analysis concerning fission counter design and operation shows that an optimum ratio of cable impedance times detector capacitance over detector collection time exists for fission counters. Tests were performed to show the effects of gamma fields on fission chambers having different electrode spacings and various argon-nitrogen fill gas mixtures and pressures. Based on these tests, the FFTF in-vessel fission counters will have 0-080-in. plate spacing and be filled with 1.75 atm of Ar—7% N2. The electrode coating thickness will be 2 mg/cm2 of U3O8 (93% 235U enriched).