Installation of the first of the big wheels of the ATLAS muon spectrometer, a thin gap chamber (TGC) wheel — The muon spectrometer will include four big moving wheels at each end, each measuring 25 metres in diameter. Of the eight wheels in total, six will be composed of thin gap chambers for the muon trigger system and the other two will consist of monitored drift tubes (MDTs) to measure the position of the muons (Image: CERN)

Identifies and measures the momenta of muons

The outer layer of the ATLAS experiment is made of muon detectors. They identify and measure the momenta of muons – particles similar to electrons but 200 times heavier, which allows them to cross the thick layers of the ATLAS Calorimeters.

Five different detector technologies are used: Thin Gap Chambers, Resistive Plate Chambers, Monitored Drift Tubes, Small-Strip Thin-Gap Chambers and Micromegas.

Precision Detectors

The precision detectors of the Muon Spectrometer are able to determine the position of a muon, to an accuracy of less than a 10th of a millimetre!

Monitored Drift Tube (MDTs) detectors are composed of 3 cm wide aluminium tubes filled with a gas mixture. Muons pass through the tubes, knocking electrons out of the gas. These then drift to a wire at the tube’s centre to induce a signal. Over 380,000 aluminium tubes are stacked up in several layers in order to precisely trace the trajectory of each muon.

Fast-Response Detectors

ATLAS uses fast-response detectors to quickly select collision events that are potentially interesting for physics analysis. They make this decision within 2.5 μs (400,000th of a second).

The Resistive Plate Chambers (RPCs) surround the central region of the ATLAS experiment. They consist of pairs of parallel plastic plates at an electric potential difference, separated by a gas volume. Thin Gap Chambers (TGCs) are found at the ends of the ATLAS experiment and consist of parallel 30 μm wires in a gas mixture. Both chambers detect muons when they ionise the gas mixture and generate a signal.

Micromegas and Small-Strip Thin-Gap Chambers (sTGCs) are two additional detector technologies specially designed for high-intensity LHC collisions. These detectors can track muons in high-density areas on either side of the experiment close to the LHC beam pipe, both quickly and with high precision.

The combined data from fast-response detectors gives a coarse measurement of a muon’s momentum, allowing ATLAS to choose whether to keep or discard a collision event.