The assembly consists of two parts: a main composite box including the readout printed circuit board and HV feedthrough ang the gas inlet , and a roof (not shown) with the drift plane and gas outlet. GEM electrodes, mounted on individual frames, can be piled up over the readout plane using insulating pins and spacers:

A raw GEM foil; the active area is 100x100 mm2, with a 4 mm wide metal strip around edges and two contact leads on opposite sides; the kapton foil outer dimensions are about 150x150 mm2. Holes in the foil can be used for positioning during framing.

Schematic of a thin (0.5 mm thick) fibreglass frame used to mount GEMs. To avoid warping, two frames are glued symmetricaly on each side of the GEM foil, pre-stretched and cut to size after gluing.

A framed GEM ready for mounting within the box.

GEM foils are HV tested before and after framing (requred less than 5 nA at 500 V in dry air). All tests and manipulations should be done in clean room conditions (class 1000 minimum). Any contact with the active area should be avoided, including loading with hard surfaces.