Electron Detectors

Abstract

High-energy electrons can be detected by a direct measurement of the electron current or by converting the electron energy into optical, electrical, or chemical signals, which then can be analyzed and visualized. A simple setup, called the Faraday cage, is used to measure the beam current. The cage is made by covering a hole drilled into a metal stub with a small diaphragm. Electrons passing through the diaphragm produce an electric current, while the small diaphragm prevents the escape of backscattered and secondary electrons generated inside the hole. The typical beam current employed in electron imaging or diffraction is of the order I = 10 −13 −10 −10 A; thus, the measurement requires a low-current-level electrom-eter, which is commercially available. This setup is used mostly for instrument calibration. Unlike X-ray diffraction which was largely developed using point detectors, TEM from the beginning was based on area detectors, and more recently array detectors, for image and diffraction pattern recording. Point detectors are used exclusively in SEM and STEM. Electron detectors are constructed based on the principles of electron energy conversion and signal processing. The detectors are made to meet various electron detection needs in the format of point (or serial), area or array (1D or 2D) detectors. In what follows, we will first introduce the scintil-lator–photomultiplier detectors and then discuss electron area and array detectors.