The ejectiles leave behind a track of ion pairs
in the concentric electric field inside the drift region. The time the ejectiles
need to travel through the chamber is small compared to the drift time
of the electrons. After some amplification in the GEM, the electron current
is detected in the anode pads. The radial position of each point of the track
can be derived from the time difference of the pulses. Additionally, the
charge contained in each pulse is a measure for the energy loss of the particles.