A simplified method for determining the mass-to-charge ratio of single pulse-field evaporated atoms in a time-of-flight (TOF), atom-probe, field-ion microscpoe (FIM) is described. The method is rather insensitive to the value of the pulse factor (α). This is achieved physically by making the pulse voltage a constant small fraction (f) of the steady-state imaging voltage, where f is much less than unity and is as small as possible to maximize the mass resolution. Examples of the W+3 and W+4 spectra of specimens pulse-field evaporated at ∼25 K in a background vacuum of 6×10-10 Torr are presented. The W+3 spectrum exhibited very clearly the five naturally occurring isotopes (180W, 182W, 183W, 184W, and 186W) with isotopic abundances that are in good agreement with the handbook values. The value of α chosen has only a very minor effect on the exact shape of the W+3 and W+4 spectra. The ratio of W+4 events to the total number of events (W+3 and W+4) is 0.17 for the (551) plane.
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