  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, as required by the low energy of the
antiprotons, 5.3 MeV. This overcomes the problems previously encountered with
multi-wire proportional chambers (MWPC) 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. We will provide a
detailed description of the detector and discuss the results obtained.
  The success of these detectors in the AD makes GEM-based detectors likely
candidates for upgrade of the beam profile monitors in all experimental areas
at CERN. The various types of MWPC currently in use are aging and becoming
increasingly difficult to maintain.
