Investigation of the feasibility of sensing transient velocity by means of gaseous ionization

Sensitivity Sⁱ_p is defined as the ratio of the fractional change of gas ionization current to the fractional change of gas pressure. A maximum obtainable sensitivity was calculated, Sⁱ_p]_max = -0.309 A/B V, where A, B are constants for a given gas within a particular range of field-to-pressure ratio. Experiments were conducted to investigate the sensitivity of air and krypton under different combinations of voltage, electrode separation and primary electron source strength. A curie of tritium placed on the central portion of either electrode produced a constant initial emission current and a high sensitivity. Sensitivity could be increased by using a stronger current source, higher voltage and a larger electrode separation. Observed values of Sⁱ_p for high voltage at larger separation are greater than those expected even when breakdown occurs at a (pd) quite larger than the one for Sⁱ_p]_max. The fractional pressure change, Δp/p, is independent of the initial pressure, p, and is greater for a heavier gas subject to the same acceleration, a fact which suggests the use of a heavier gas to obtain a higher fractional current change for the same sensitivity. Krypton behaves in a similar way as air in the system. Operating current level may be higher than 10⁻⁵ ampere if stronger source is used. There is a limit in the separation for a given source configuration. Further increase in Sⁱ_p and current level may be achieved through better design of the configuration and location of the current source between the electrodes. The fractional current change, Δi/i, depends more on Δp rather than on the initial operating pressure, p.

From the experiment performed, it appears that a gas system will perform satisfactorily as a transient velocity gauge.