Abstract Detective Quantum Efficiency (DQE) is proposed as being a particularly useful way of specifying the efficiency of electron detectors for the Scanning Transmission Electron Microscope (STEM). An expression for the DQE of a STEM detector consisting of a scintillator and photomultiplier coupled by an optical system is derived by considering the statistical fluctuations in the number of particles at each stage in the detection chain. Hence, the DQE is found to depend essentially on the mean number of photo-electrons from the cathode of the photomultiplier per incident primary electron. Optical systems designed to maximise this number for detectors in use in our own STEM are discussed. We describe a simple method of deducing this number, and hence the DQE, based on a comparison of theoretical and experimental pulse height distributions. Measurements for a theoretically interesting detector of unusual design are presented, the DQE of which was found to be approaching 100% when used with typical primary beam energies. Finally, we discuss the implications of our results for practical detector design.
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