The SC is composed by 25 aluminium plates (1) spaced by 10 mm enclosed in a gas tight box filled with pure He operated in flux mode. The aluminium plates are connected to a high voltage (HV) power supply (2), which supplies a fixed voltage of 5600V. The external trigger system is based on two (250x420 mm²) plastic scintillators (3) readout by two photomultipliers tubes (PMTs) (4) through light guides (5). When a charged particle, for instance a secondary cosmic ray, enters the scintillator it produces light that is guided to the PMT and there it is converted into an electrical signal. Since there are two scintillators, one above and another bellow the SC, both with areas similar to the aluminium plates inside the SC (active area), a time coincidence of signals from both scintillators indicates with high probability the passage of a cosmic ray through the active volume of the SC.

The signals generated by the PMTs are sent to an electronic board (6), where they are selected in amplitude by simple leading edge discriminators and their time coincidence is verified. When a coincidence occurs, the spark gap (SG)(7), a gaseous HV switch, connects the odd aluminium plates to ground and thus the even plates, kept at 5600V , immediately discharge into the grounded ones sparks produced at the “weak points”, the points where the through-going cosmic ray ionized the gas.