Triple-GEM detectors for COMPASS

A detailed description of the construction method and test procedures of the COMPASS chambers is available: see Compass Chambers Production Guide


The T-GEM tracker chambers for the COMPASS experiment have three cascaded GEM foils, a two-dimensional projective readout on anode strips and a 3 mm thick sensitive volume; to reduce cost and material budget, the component are permanently assembled with epoxy.


The light assembly relies on honeycomb support plates, 3 mm thick, over which the various frames with stretched electrodes are glued in sequence. The readout pattern consists of two sets of perpendicular strips, 400 µm apart, separated by 50 µm polymer ridges; the width of the strips is optimized for equal charge sharing:


A completed chamber has ~ 1000 cm2 active area, with 768 strips analogue readout on each projection; a central cut in the honeycomb plates reduces the material in the beam region:

Detection in the central region, traversed by the primary beam, can be inhibited on control with a voltage applied to an independently powered disk, 5 cm in diameter (the beam killer). Full efficiency is restored for calibration and alignment low intensity runs.
To reduce the energy in case of discharges, the GEM foils are divided in 12 sectors, having individual external current-limiting resistors.

Beam killer, 5 cm Ø
Sector separation, 200 µm wide

The GEM foils are glued on thin fiberglass frames; to maintain the gap uniformity, thin ribs, cut in the same raw material, serve as spacers. All along the assembly, the HV rigidity of all sectors in the GEM foils is tested in a dry gas box:

For a full description of the design, construction and test of the COMPASS T-GEM see C. Altumbas et al, Nucl. Instr. and Meth. A490(2002)177, and the COMPASS TGEM production guide.


The charge collected on anode strips is amplified and recorded making use of an analogue-to digital readout based on the APV25 chip, originally developed for the needs of Silicon Microstrip detectors (M. J. French et al, Nucl. Instr. and Meth. A446(2001)359). The chip includes 128 channels each with a low-noise fast amplifier, a switched analogue memory and a serial readout and digitization of three selected analogue memory cells. A discrete diode-capacitor input circuit has been added for protection.

Schematics of the APV25 amplifier-analogue memory circuit The 3-chip (384 channels) COMPASS readout card. The upper section is the input diode protection circuitry.


Twenty-two fully equipped T-GEM detectors are installed in the COMPASS experiment at CERN, and have been taking data in the physics runs with high intensity muon and hadron beams. They consistently provide very good efficiency and space position accuracy. For more data, see B. Ketzer et al, Nucl. Instr. and Meth. A535(2004)314.

Single coordinate detection efficiency and signal/noise vs operating voltage. Single coordinate position accuracy for minimum ionizing tracks


Detected tracks density in running conditions, peaking at 104mm-2s-1 Uniformity of efficiency (>98%) over the active area. The effect of the beam killer and of the spacers are visible.