# Multigap Resistive Plate Chambers for Time of Flight Applications
## 2 Description of MRPC Detector  
In the 1980 s, [PERSON] [31] built the first prototype of the RPC, which can be considered to substitute resistive electrodes for the metallic electrodes of the PPC detector. The use of resistive electrodes is a major improvement, which allows the detector to operate continuously (no need for pulsed HV (High Voltage) for discharge quenching) and restricts discharges to a local area (the rest remains active). Unlike the early gas detectors, the high voltage electrodes of RPC are separated from the signal readout electrodes. The carbon film attached to the resistive electrode is used as the high voltage electrode and the conductive material as the readout electrode. The choice of resistive electrode materials is crucial. On one hand, it is not supposed to affect the generation of induced signals due to the movement of charge, which requires the resistive plate to have relatively high resistance. Thus, the discharge durations (-10 ns) are far less than the characteristic time constant (\(\tau=\mathrm{RC}\approx 10\) ms). Macroscopically, the resistive electrodes are called \"transparent\" to the induced signals. On the other hand, the counting rate of the detector is related to the time that the resistive electrode needs to recover and charge up again, which depends on the resistivity of the electrode. Excessive resistivity will increase the detector's dead time and decrease the detector's counting rate capability. Thus, RPC detectors generally adopt glass or Bakelite as the resistive material, whose volume resistivity is usually within the range of \(10^{9}\)-\(10^{13}\)\(\Omega\) cm. For trigger applications, typical RPCs can operate at around 95% efficiency with a time resolution of sub-ns and at a counting rate of hundreds of Hz/cm\({}^{2}\). Considering one primary electron created somewhere in one gas gap, [PERSON] [32] derived a nice formula to estimate the intrinsic time resolution for one-gap RPC:  
\[\sigma(t)=\frac{1.28}{(\alpha-\eta)v} \tag{5}\]  
where \(v\) is the electron drift velocity, \(\alpha\) the Townsend coefficient (=1/lambda, lambda is the mean distance between ionizing collisions), and \(\eta\) the attachment coefficient. They are mainly determined by the gas mixture and working electric field. To improve the time resolution for timing measurements, the effective approach is to decrease the gap width as much as possible so that the electric field can be greatly increased under the avalanche working mode. Hence, the effective Townsend coefficient \((\alpha-\eta)\) is much higher, and the electron drift velocity is much faster. To maintain the efficiency and improve the performance of normal RPCs, more gas gaps are introduced to increase the total gap thickness of the detector and the number of primary electrons. Consequently, the multigap RPC [33] was developed by [PERSON]. As shown in Figure 1, several equally spaced resistive plates are placed so that small gaps can be created. Commonly nylon fishing lines are used to confirm a uniform gap width. Thus, the avalanche is limited to a small space, and the electric field can be increased, which leads to a much better time resolution. The high voltage is just applied to the graphite layer coated on the outer resistive plates of each stack, while the intermediate resistive plates are allowed to float electrically. Moreover, to separate the high voltage electrodes from the readout electrodes, a thin insulating layer of Mylar is used and glued on the printed circuit board (PCB) copper layer. The honeycomb boards are attached to the top and bottom to support the whole detector. The core part of an MRPC is the working gas in the gaps. Usually, the MRPC detector works in avalanche mode. To suppress the growth of the avalanche in the high electric field, some electronegative gases are added to increase the attachment coefficient.