  The design of a detector based on MICROMEGAS (MICRO MEsh GAseous Structure)
technology is presented. Our detector is characterized by a large active area
of 398(\times)281 mm(^{2}), a pad read-out with 20(\times)22 mm(^{2})
segmentation, and an uniform amplification gap obtained by insulating spacers
(100 (\mu)m high and 200 (\mu)m in diameter). The performances of several
prototypes have been evaluated under irradiation with secondary beams of 2
GeV/c momentum charged pions and electrons. We consider such a detector as the
basic element for a pre-shower detector to equip the PHOton Spectrometer (PHOS)
of the ALICE experiment. Its assets are modularity, small amount of material,
robustness and low cost.
