Liberation of Electrons by Positive-Ion Impact on the Cathode of a Pulsed Townsend Discharge Tube

The parallel-plate Townsend discharge triggered by a flash of ultraviolet light on the cathode gives a current to the anode which depends strongly on the value of $\gamma$, the number of new electrons liberated from the cathode per positive-ion impact. An expression is derived relating $\gamma$ to the first Townsend coefficient for ionization by electrons $\alpha$ and the anode-cathode separation $X$, which provides an easy experimental procedure for arriving at $\gamma$. Measurements are then given for $\gamma$ as a function of $\frac{E}{p}$, the ratio of field strength to pressure, for Ne, A, and Kr on a freshly cleaned Mo cathode. The values of $\gamma$ are essentially independent of $\frac{E}{p}$ above 175 volts/(cm×mm Hg) with values of 0.20 for ${\mathrm{Ne}}^{+}$, 0.083 for ${\mathrm{A}}^{+}$, and 0.053 for ${\mathrm{Kr}}^{+}$. At lower values of $\frac{E}{p}$, $\gamma$ decreases toward zero, but this decline is probably related to the back diffusion of electrons to the cathode and does not represent a decrease in the basic $\gamma$ process. With ${\mathrm{A}}^{+}$ ions and a nickel cathode coated with activated BaC${\mathrm{O}}_3$, the value of $\gamma$ also leveled off at higher $\frac{E}{p}$ at a $\gamma$ value related to the degree of activation of the BaC${\mathrm{O}}_3$. Significantly, the value of $\gamma$ for this surface did not tend toward zero at zero $\frac{E}{p}$. This may be attributed to a different distribution of electron velocities for the ejected electrons from the coated surface than for the pure surface.