  We describe possible applications of pixelized micro-well detectors (PMWDs)
as three-dimensional charged particle trackers in advanced gamma-ray telescope
concepts. A micro-well detector consists of an array of individual
micro-patterned gas proportional counters opposite a planar drift electrode.
When combined with pixelized thin film transistor (TFT) array readouts, large
gas volumes may be imaged with very good spatial and energy resolution at
reasonable cost. The third dimension is determined by timing the drift of the
ionization electrons. The primary advantage of this technique is the very low
scattering that the charged particles experience in a gas tracking volume, and
the very accurate determination of the initial particle momenta that is thus
achieved. We consider two applications of PMWDs to gamma-ray astronomy: 1) A
tracker for an Advanced Compton Telescope (ACT) in which the recoil electron
from the initial Compton scatter may be accurately tracked, greatly reducing
the telescope's point spread function and increasing its polarization
sensitivity; and 2) an Advanced Pair Telescope (APT) whose angular resolution
is limited primarily by the nuclear recoil and which achieves useful
polarization sensitivity near 100 MeV. We have performed Geant4 simulations of
both these concepts to estimate their angular resolution and sensitivity for
reasonable mission designs.
