Townsend Ionization Coefficients in Cs-Ag-O Photo-Tubes Filled with Argon

The attempt is made to account for the amplification of photo-currents at low current densities in argon-filled Cs-Ag-O tubes on the basis of Townsend's theory assuming secondary electron emission by positive ions. Special sealed-off tubes were used having parallel plates at various separations and a constant gas pressure. Values of the Townsend coefficients $\alpha$ and $\gamma$ were determined at each value of field by simultaneous solution of the Townsend equations for two plate separations. The $\frac{\alpha }{p_0} \mathrm{vs}.\mathrm{} \frac{E}{p_0}$ plot has a maximum at $\frac{E}{p_0}\cong 700$ volts/cm · mm. The $\gamma$-values for the two types of cathodes used are sensibly constant at low fields. Measured values of the striking potentials of the glow discharge are in quantitative agreement with the positive ion mechanism assumed. The photosensitivity of the compound caesium cathodes was found to increase with increase of bombarding ion current.