The Anti-Coincidence Counters (ACC)

The Anti-Coincidence Counters (ACC), shown in Figure 1, surround the inner tracker inside the magnet bore. Their purpose is to detect events with unwanted particles that enter or leave the inner tracker volume transversely. The ACC consists of sixteen curved scintillator panels of 0.8 m length, instrumented with wavelength-shifting fibers to collect the light. To maintain the hermiticity of the ACC cylinder, the counters have a tongue and a groove along the vertical edges such that particles crossing this area are detected simultaneously by two panels. Long duration tests of the counters show they have an efficiency close to 0.999 99.