Abstract
An accurate and convenient method has been devised for measuring spot size, spherical aberration and current density in electron probe instruments of all kinds. In this method the normal specimen is replaced by a fine mesh grid whose magnified shadow is projected on to a fluorescent screen placed some distance away. From this shadow image the spherical aberration coefficient of the lens system may be deduced. By measuring the current density distribution in the image plane by means of a Faraday cage, the actual current density distribution in the probe may be accurately deduced. Based on this method, an improved telefocus tetrode gun for high intensity microbeam illumination has been developed for the purposes of a fine focus x-ray unit. It is shown experimentally that a microbeam illumination system is most useful for special x-ray applications made possible by the high power concentration.
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