100.

Thermionic emission patterns from the single-crystal LaB//6 left bracket 100 right bracket , left bracket 110 right bracket , left bracket 111 right bracket , and left bracket 210 right bracket tips have been observed under the Schottky condition. The observed emission patterns are interpreted on the basis of a model involving an anisotropic work function of LaB//6; the regions around the left bracket 111 right bracket crystal orientation have a high-work function, while those around the left bracket 210 right bracket crystal orientation have a low work function. This model also explains qualitatively the order of the total emission currents from those tips. The observed patterns predominantly consist of the emission from the conical face rather than the top of the tips under this experimental condition. Changes in the emission pattern and cathode surface in operation at 10** minus **4 Torr are also discussed.