MIXE was a large-area hard-X-ray proportional counter.
It featured a mechanical collimator that only allowed X-rays from a
narrow region of the sky to reach the detector where the X-rays photoionize
the Xe fill gas (see diagrams below). The photoelectron released in this
process drifts through the chamber, driven by an applied electric field, onto
a microstrip detector where its position and energy are recorded.
Along the way, the photoelectron causes further ionization in the chamber
resulting in a small electron cloud, thus amplifying the electrical signal
at the microstrip detector.
MIXE flew in several campaigns in the 1990's. It has since been supplanted by MSFC's
hard-X-ray focusing optics program.
Click on images below for enlarged views