  Time Projection Chambers (TPCs) with charge readout via micro pattern gaseous
detectors can provide detailed measurements of charge density distributions. We
here report on measurements of alpha particle tracks, using a TPC where the
drift charge is amplified with Gas Electron Multipliers and detected with a
pixel ASIC. We find that by measuring the 3-D topology of drift charge and
fitting for its transverse diffusion, we obtain the absolute position of tracks
in the drift direction. For example, we obtain a precision of 1~cm for
1~cm-long alpha track segments. To our knowledge this is the first
demonstration of such a measurement in a gas TPC. This technique has several
attractive features: it does not require knowledge of the initial specific
ionization, is robust against bias from diffuse charge below detection
threshold, and is also robust against high charge densities that saturate the
detector response.

Instrumentation and Detectors