Electrical differentiation for energy-loss analysis

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Abstract
An experimental arrangement is described which was set up to measure the energy loss spectra of electrons which have passed through thin specimens in the electron microscope. It consists of a nondispersive filter lens which rejects electrons of less than a certain energy, determined by the voltage of the central electrode. By modulating this voltage with a small ac component and measuring the resulting modulation of the transmitted beam, the apparatus can be used as an energy analyser. In this paper, the process of `differentiation by modulation' is reviewed and the effects of using different waveforms and frequencies examined. Also discussed are high-voltage insulation and breakdown in vacuum, the effect of focusing the beam on the phosphor (which was used as the electron detector), and limitations upon possible sweep speeds. A novel feature of the system is a 2 kHz optical link used to supply a synchronizing signal to the high-voltage line. The energy resolution of the system comes close to the theoretical minimum of about 15 eV. At low accelerating voltages output noise is almost entirely shot noise, but above 60 kV microdischarges increase the noise level substantially. Noise from these two sources limits the usefulness of the system for measuring low currents.