that using 7=1.7, the then accepted value, does not yield good quantitative agreement with experiment and suggested that better agreement would be obtained with a smaller value of 7. In a further paper 13 (appar-13 H. Messel, Proc. Phys. Soc. (London) A64, 726 (1951). ently overlooked by Treiman), Messel presented results adopting "the upper limit 7=1.1." Instead of the agreement between Messel's theory (with 7=1.7) and experiment being spurious, the facts are, as stated in the literature, that with 7=1.7 the theory is not in good quantitative agreement but with 7= 1.1 they are.In recent experimental investigations, it was found that secondary emission ratios as high as 10,000 to 1 could be attained utilizing field dependent secondary emission from magnesium oxide. Early tests showed the mechanism causing the high gains to be fundamentally different from the more standard secondary emission phenomenon.The hypothesis was made that the mechanism of field dependent secondary emission was a process similar to that of the "Townsend avalanche" occurring in gas discharges. As the surface of the dielectric film was bombarded with primary electrons, the high resistivity of the material in combination with the secondary emission current caused the surface to charge to the potential of the collector grid, producing a high field within the dielectric. Electrons released within the material could then gain enough energy to liberate additional electrons, and an avalanche type process resulted.Experiments were conducted to test this hypothesis and each proved to be consistent with the above theory. The main content of these experiments can be summarized in the following statements:(1) The high yields appeared to be independent of the base material. This indicated that the surface or volume effects were most important, implying that a Fowler type field emission from the base metal was not a significant factor.(2) In studying the secondary current as a function of field, the gas discharge equations were found to be correct. These equations predicted a straight line plot of the In ln5 vs 1/E, and in addition, gave a close estimate of the magnitude of the secondary emission ratio.(3) By means of retarding potential measurements, the energies and mean free paths of the emitted secondary electrons were determined. These data were in good agreement with the results in item (2).(4) The rise time for surface charging was determined by using square wave variations of bombarding currents, and was found to be consistent with the original hypothesis.

No abstract

Thin films of magnesium oxide, under the influence of an electric field, have been observed to emit extremely large secondary emission currents when bombarded with electrons. This current has been found to be exponentially dependent upon the electric field in the oxide film, and under certain conditions, to persist for many hours after the bombarding current has been cut off. Experiments have shown that the enhanced emission is due to the fact that the surface acquires a positive charge during bombardment, thereby creating an intense field in the thin film. Electrons released in the film by internal ionization are then accelerated by the field and cause further ionization, so that eventually an electron avalanche ensues. It has been found that there are two components of the enhanced electron emission. The first component is a true fieldenhanced secondary emission effect similar to that described in previous work. The second component is a self-sustained electron emission. This self-sustained electron emission is produced by the same type of avalanche effect, except that the internal ionization of the dielectric is initiated by electrons produced from within the material rather than by electrons bombarding the material from an external source.