Electron multiplication in bistable storage tube targets

The operation of direct-view bistable storage tubes is generally explained in terms of secondary emission at the target. The maximum stored-writing speed is determined by the current density of the primary beam, the target capacity, and the secondary emission properties of the target. Since these quantities are known or can be measured, the maximum stored-writing speed can be estimated. In MgO = doped phosphor targets used in certain bistable storage tubes, stored-writing speeds were observed which were too fast to be explained by simple secondary emission. Storage speeds increased with MgO content and target porosity. These observations led to speculation on the possibility of charge-multiplication occurring within the target because of a field-dependent secondary-electron multiplication process. This paper presents the results of a series of experiments undertaken to detect and measure charge-multiplication in certain storage targets. The method involved pulsing the writing gun and viewing gun of a storage tube in sequence and measuring the resultant charge-transients in the target circuit. Target-gain was determined by comparing the total primary charge with the charge delivered by the flood gun in restoring the surface potential. Charge-multiplication was observed in certain targets using this technique, and maximum gain factors in the range 10 to 100 were measured in various samples. Target-gain was dependent primarily on the voltage across the storage layer and increased rapidly as this voltage was raised above the first crossover potential of the target material. The gain was relatively independent of primary-beam voltage over the range 0.5 to 3 kV.