Quantitative measurement of spatial coherence ofelectron beams emitted from a thermionic electron gun

In this study, quantitative measurement of the spatial coherence of electron beams emitted from a thermionic-emission gun was achieved for the first time, to our knowledge. For the measurement, Airy patterns were formed using a specially fabricated aperture with an effective diameter of approximately 10 nm at the specimen plane in a 200 kV transmission electron microscope. Subsequently, from the fitting analysis of the Airy patterns recorded with beam diameters ranging from 2.0 to 6.9µm, spatial coherence lengths from 7 to 19 nm were successfully measured in proportion to the beam diameters. In addition, considering time evolution of the total beam current measured at the specimen plane, the relation between the on-axis current density and the coherence length is shown, where the former is inversely proportional to the square of the latter. Compared with the result using a Schottky-type field emission gun measured in a previous study, the spatial coherence length in the LaB6 thermionic-emission gun transmission electron microscope is one to two orders of magnitude shorter when a similar beam intensity is formed at the specimen plane.