The Resistive Plate Counters

The basic elements of ARGO-YBJ are single-gap RPCs made of bakelite (volume resistivity >5 10¹¹ohm cm) with a 128 X 282 cm² area. The RPCs read-out is performed by means of Al strips 6.7 cm wide and 62 cm long at the edge of which the front-end electronics is connected.

The FAST-OR of 8 strips defines a logic unit called PAD: 10 pads (56 by 62.5 cm²) cover each chamber (18480 in total). The FAST-OR signal from each pad is sent, via coaxial cable, to dedicated modules that generate the trigger and the STOP signals to the TDCs. Each channel of the TDC measures, with a time resolution of about 1 ns, the arrival times (up to 8 hits per channel) of the particles hitting the pads. The trigger logic allows to select events basing on the pad multiplicity.

The pads are the basic element ("pixel") which defines the time pattern of the shower. The pick-up strips define the space pattern inside the pad.

The RPCs are operated in streamer mode with a gas mixture of Argon (15 %), Isobutane (10 %) and Tetrafluoroethane C₂H₂F₄ ( 75 %), at ~ 7400 V, ~ 500 V above the plateau knee.