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 $6.9\;\unicode{x00B5}{\rm m}$, 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 ${{\rm LaB}_6}$ 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.

中文翻译：

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从热电子枪发射的电子束空间相干性的定量测量

在这项研究中，据我们所知，首次实现了对热电子发射枪发射的电子束空间相干性的定量测量。对于测量，使用特制的孔径在 200 kV 透射电子显微镜中的样品平面上形成约 10 nm 的有效直径，形成艾里图案。随后，从光束直径范围从 2.0 到$6.9\;\unicode{x00B5}{\rm m}$记录的艾里图案的拟合分析，成功地测量了与光束直径成比例的 7 到 19 nm 的空间相干长度。此外，考虑到在试样平面上测量的总束流的时间演变，显示了轴上电流密度与相干长度之间的关系，其中前者与后者的平方成反比。与先前研究中使用肖特基型场发射枪测量的结果相比，${{\rm LaB}_6}$热电子发射枪透射电子显微镜中的空间相干长度缩短了一到两个数量级在样品平面上形成了类似的光束强度。