5.1 Specialized Trigger Detector

The Resistive Plate Chamber is a cheap charged particle detector often used for triggering purposes over large areas, because of its fast response time and its robustness. Due to the large amount of charge necessary for the detection, the standard RPC is quite slow to recover after an hit. This peculiarity makes problematic its use in the LHC enviroment where particle rate, including radiation background, is very large. The RPCs should show to be able to stand in CMS a target value of ~1kHz/cm2 without a significant efficiency loss.

The final output of the research and development of the past years [5] was the decision to use a double gap RPC (sketched in Figure 10), to overcome the problem of the spacers between bakelite plates while minimizing the readout channels, andthe decision to operate it in avalanche mode, using C2H2F4/C4H10 (90/10), to reduce the amount of charge and therefore stand a larger rate.

Figure 10: Sketch of a double gap RPC.

The results of a recent test done using a radioactive source to simulate a radiation background uniformly illuminating the chamber are shown in Figure 11: we see that the efficiency loss at the nominal voltage of HV = 12.5kV is negligible up to a minimum ionizing particles rate of 750Hz/cm2.

We can state that the RPC is a suitable detector for the LHC muon detection environment.

Figure 11: RPC Efficiency for different radiation background: the quoted rate is the m.i.p. rate equivalent to a given dose.

Figure 12a shows that the measured RPC time resolution is very good and Figure 12b shows that the chamber remains quiet after the hit detection, since the strip multiplicity in a very large gate time after the detected hit is small.