For the two magnet spectrometers we plan to use a large area GEM based hodoscope in the focal plane of each spectrometer. Because of the high current MESA will provide, the decision was made to use GEMs instead of drift chambers for their higher rate capabilities and better resolution. Moreover they should be much easier to maintain.
The active area of the detector system will be 1.20 x 0.30 m2 in each spectrometer. To reach the proposed angular and momentum resolution each of the four detectors must have a spatial resolution better than 50 microns wherefor each detector will be read out by approximately 10k channels. MESA offers a very high beam current and to perform in a high rate environment we will need an ADC based readout for each of those channels with a sampling rate of the order of 10s of Mhz. To be able to really exploit the MESA beam we aim to achieve triggering rates of the order of 10s of kHz, which demands a fully integrated electronic system scalable to the foreseen number of channels like the CERN developed SRS. Concerning the low energy MESA is operated at, the material budget of the whole setup has to be kept as small as possible to avoid multiple scattering. We plan to use GEM foils with thinner copper coatings than the CERN standard ones. In addition we have to use a thin kapton based readout foil as well, which has to be developed and tested in the next years.
At the moment we are operating a first triple-GEM protoype with an active area of 10x10 cm2 in a coincidence setup with two scintillators to determine the pulse shape of the signals, efficiency and energy resolution. We developed the necessary stretching and framing methods and are working on a setup to frame 30x30 cm2 foils in our lab. An integrated ADC based readout developed at INFN was set up shortly ahead and is now tested intensively.