Effects of Gas Mixture, Electrode Spacing, Gas Pressure, and Applied Voltage on the Gamma Performance of Fission Counters

Electron drift velocity, a function of gas composition, electric field, and pressure, represents the most important design parameter for optimization of fission-counter collection time. References in the literature provide a significant source of information on drift velocities for various gases, but the information does not extend to sufficiently high electric field/pressure (E/p) ratios for use with the high-sensitivity fission counters under consideration. The data obtained for this investigation and reported here extend the drift-velocity data for argon-nitrogen mixtures to E/p regions useful for present design considerations, and at the same time compare detector performance in a high gamma field (10⁶ R/h) with the various gas mixtures employed. Six combinations of argon-nitrogen ranging from 1 to 15% nitrogen were included in the tests. Although several other gas mixtures, such as argon-methane and argon-CO₂, provide faster drift velocities, only argon-nitrogen has proven stable at the high neutron exposure levels anticipated for the Fast Flux Test Facility (10¹⁸ n/cm²). Performance comparisons show that for 800-Vdc operation the neutron counting sensitivity for Ar - 10% N₂ exceeds that for Ar - 1% N₂ (the fill gas most commonly used) by over a factor of 2. Corresponding collection times decreased from 160 nsec for the Ar - 1% N₂ mixture to 80 nsec for the Ar - 10% N₂ combination. For specific applications, it may be required to limit the voltage to <800 Vdc. The curves provide information to permit selection of the best gas mixture for a given bias voltage requirement.