## 3 The High-Rate Sealed MRPC
### Cosmic Test  
The high-rate sealed MRPC prototype was tested in cosmic rays (i.e., with a flux as low as \(\sim\)1.5 Hz/m\({}^{2}\)) along with 2 CBM-MRPC2 counters for the Triggering and Readout Board (TRB3) [22] test stand [23], as shown in Figure 5. PADI Front-end Electronics [24] provides a conversion gain of 30 mV/fC, and the threshold is set to 300 mV during the test. For the investigation of the time resolution, the start-time (T0) is provided by one of the MRPC2 counters, and the other is served as the event selector. The three MRPCs are piled with the prototype under test on the top. This layout is for the X-ray test in the next  
Figure 4: The schematic view of the high-rate sealed MRPC prototype, which has minute change from [12]. The PMMA plates are 1 mm in thickness. The copper HV conductor is pasted on the graphite layer.  
part. Standard gas mixture (Freon/iC4H10/SF6 in a fraction of 90/5/5) is streamed to the counters with a 10 mL/min flow for the prototype and 60 mL/min for the gas box that places the two MRPC2 counters. The sealed prototype has been verified for its good gas tightness with HV applied in the atmosphere.  
Figure 6 shows the HV scan results, which indicate a good performance under the low flux cosmic rays. As described above, the test used three MRPC counters, so the efficiency is defined by the number of coincidences by three counters over that by two MRPC2s. To determine a coincidence, the time and space correlation of the detection between counters is examined. The cluster size is defined by the number of continuous fired strips in one detection. The prototype reaches the working condition at 5.4 kV, which corresponds to an electric field of 108 kV/cm in gas gaps. The plateau efficiency is 97%, with a 1.6 cluster size and a 100 ps flight-time resolution. Mentioned that the tested prototype and the T0 counter have the same geometry, if we reasonably expect their same timing precision, the time resolution of the prototype is about \(100/\sqrt{2}\ \sim 70\) ps. During the test, the dark current and the noise rate were kept in reasonable levels at around 20 nA and 1.5 Hz/cm\({}^{2}\), respectively.  
Figure 5: The TRB3 test stand. For the cosmic test, trigger is provided by the coincidence of two scintillators. In the X-ray test discussed afterward, the scintillators are replaced by an X-ray source, and the TDC operates in the free-running mode for the record of counting rate.  
Figure 6: Cosmic test results of the high-rate sealed MRPC prototype: (**a**) the efficiency curve and cluster size; (**b**) the resolution of the flight time between the prototype and the T0 counter, with fluctuation from the readout chain included.  
Moreover, a flow scan to the prototype has been carried out at the working point. The HV is set to 5.4 kV with different gas flows. Results of efficiency and flight-time resolution of the high-rate sealed MRPC are shown in Figure 7. There is no significant flow dependence within the efficiency uncertainty and a 5% measurement error of time resolution on the performance at low counting rate, probably because of the slow pollutant generation in cosmic ray tests.