From Northwestern University Center for Atom-Probe Tomography
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NUCAPT's (decommisioned) 1DAP

The first atom probes employed a small aperture to limit the analysis to individual atoms passed through this aperture, or probe hole. The physical dimension of the probe hole is typically 2 to 3 mm in diameter, but its projected size is less than 5 nm; nevertheless, in this atom probe, the position information of the atoms is not recorded

Since atoms always evaporate from the surface, however, the spacial resolution in the depth direction is a mono-atomic layer. A unique feature of the atom probe compared with other analytical instrument is its extremely high spatical resolution and the equal detection efficiency for light elements.

Because of the nature of the information obtained from this type of atom probe, the conventional time-of flight atom probe is now called the one-dimensional atom probe (1DAP) to differentiate it from the recently developed three-dimensional atom probe (3DAP).