An Automated System for Energy-Loss Microanalysis

Abstract

Figure 1 shows a system which we are using for storage and display of electron energy-loss spectra, based on a Tracor Northern TN-1710 multichannel analyser. For serial acquisition, scan coils of the magnetic spectrometer1 are driven directly from the ramp output of the multichannel analyser. The electron detector consists of a transmission phosphor followed by a photo-multiplier tube, whose analogue output is fed via a 5 MHz voltage-to-frequency converter (Teledyne model 4707) into a TN-1710-26 plug-in scaling module. To allow adequate dynamic range (>108) and avoid overloading the photomultiplier tube, PMT gain is changed by a factor G ˜ 500 during acquisition. This is done automatically by incorporating into the line from the power supply (Venus model LR-2N) a series of Zener diodes which are short-circuited by a reed relay (driven from a high-gain amplifier) when the scan voltage exceeds a preset threshold (fig. 2). The gain change is accomplished in a shorter time (a few ms) than was possible by voltage-programming the power supply, which makes the factor G easier to measure accurately.