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This publication describes a technique for the suppression of accidental noise in very low intensity Scanning Electron Microscopes (SEM).
English (United States)
This text was extracted from a PDF file.
This is the abbreviated version, containing approximately
86% of the total text.
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Coincidence Technique in Scanning Electron Microscopes
This publication describes a technique for the suppression of accidental noise
in very low intensity Scanning Electron Microscopes (SEM).
The standard SEM uses the analogue signal of a photomultiplier (PM) tube to
modulate a video signal. At very low elec beam intensities, this method may
introduce photomultiplier noise (dark current) into the signal.
The time coincidence technique can be used to suppress this noise. This
can be done as a modification of the Everhart- Thornley detector as shown in the
drawing. The basis of the method is that the signal from the PM tube is fed to a
discriminator circuit which puts out a logic pulse if a threshold i crossed. Two
pulses, from independent PM tubes, are tested for coincidence and form a signal
pulse if they arrive simultaneously. A noise pulse in only one PM tube does not
become a signal pulse. The number of coincidence pulses per unit time is
proportional to the rate of detected electrons, and may be converted to an analog
signal for video viewing.
The amount of noise suppression depends on the widths of the discriminator
outputs and the noise rates: Accidental coincidences = 2t RARB where t is the
discriminator pulse width, and RA and RB are the noise rates of the PM tubes.
For example, if RA = RB = 105/sec, and t = 10 ns, then the accidental rate is
200/sec, which is .2% of the single PM noise rate.
The technique could be used in a variety of ways. One could us...