  The new beam profile measurement for the Antiproton Decelerator (AD) at CERN
is based on a single Gas Electron Multiplier (GEM) with a 2D readout structure.
This detector is very light (~0.4% X0), and measures horizontal and vertical
profiles directly in one plane. This overcomes the problems previously
encountered with multi-wire proportional chambers for the same purpose, where
beam interactions with the detector severely affect the obtained profiles. A
prototype was installed and successfully tested in late 2010, with another five
detectors now installed in the ASACUSA and AEGIS beam lines. This paper will
provide a detailed description of the detector and discuss the results
obtained.
