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Anomalous Secondary Emission
In the March issue of Phys. Rev., Malter¹ described an interesting phenomenon, which he called “anomalous secondary emission.”
The experiment consisted in directing, at a specially prepared aluminum electrode serving as the cathode of secondary emission, an electron beam of ordinary electron-gun type, and measuring the current of secondary electrons in the circuit secondary-emission cathode—collector.
The secondary-emission cathode was an aluminum plate oxidized electrolytically in an aqueous solution of boric acid and borax, which was then placed in vacuum and, for 10 minutes, subjected to the action of cesium vapor (at 200°), after which the free cesium was oxidized by flushing the vessel with oxygen.
Such an electrode proved to possess very remarkable secondary-emission properties. First of all, it turned out that the secondary-emission coefficient \(\sigma\) (the ratio of the number of secondary electrons to the number of primary ones) was unusually large. In some cases it reached several thousand. Malter found that the current to the collector, over a wide range of collector voltages and over a wide range of velocities of the primary electrons, can be expressed by the formula:
\[ I_c=\alpha e^{\beta V_c} I_B^{\gamma}, \]
where \(I_c\) is the current to the collector, referred to unit cross-sectional area of the primary beam, \(V_c\) is the voltage on the collector, \(I_B\) is the current density in the primary beam, and \(\alpha\), \(\beta\), and \(\gamma\) are constants.
Another peculiarity of the secondary emission of such an electrode was that the secondary current did not cease immediately upon switching off the primary beam, but only gradually decreased to zero; moreover, sometimes the presence of current in the circuit secondary-emission cathode—collector could be detected even after 24 hours. An analogous delay was also observed upon switching on the primary beam, especially if the secondary-emission cathode had previously been subjected to electron bombardment at a negative voltage on the collector.
These properties of the secondary current led the author to the following conclusions. As a result of the described treatment of aluminum, a layer possessing a considerable secondary-emission coefficient forms on the surface of the insulating aluminum-oxide film. Owing to the secondary emission of this layer, the outer surface of the aluminum-oxide film becomes positively charged, whereby a large potential gradient is created near the aluminum surface, causing electrons to be torn out.
In the author’s opinion, the anomalous secondary emission described is a new effect. This is not quite so, for a phenomenon of the same character was described by Dember², who observed a considerable increase in photosensitivity and a shift of the work-function threshold toward longer waves with simultan-
under the simultaneous action of light and an electron beam (also on aluminum), and, as was shown by subsequent investigations, the effect occurred only in the case of oxidation of the metal surface.
N. Khlebnikov, Moscow
Literature
- L. Malther, Phys. Rev., 49, 478, 1936.
- A. Dember, Ann. Phys., 33, 529, 1925.