## 2 Simulation and Observations Related to Gas Pollution  
Gas pollution is formed mostly by the ionization and avalanche of the working gas molecules [13]. For MRPCs, the working gas usually contains Freon (C\({}_{2}\)H\({}_{2}\)F\({}_{4}\)), i-C\({}_{4}\)H\({}_{10}\), and SF\({}_{6}\). Freon as the main component dissociates into F\({}^{-}\) and other fragments after ionization. The glass plates can be corroded by the F\({}^{-}\); hence, the noise rate increases and the fragments may attenuate the electronegative feature of the working gas [14]. The i-C\({}_{4}\)H\({}_{10}\) serves as the photon absorber and its consumption may affect the avalanche development as well. A flow field simulation to the MRPC gas volume based on the ANSYS Fluent software has been executed by former work [15; 16]. It treats the gas molecules in the volume, ignoring its specific components and fraction, as the combination of \"working\" molecules and \"pollutant\" molecules. With one avalanche, about 10\({}^{7}\) \"working\" molecules are tagged as \"pollutant\" so that the gas behavior with flux can be simplified to the uniform production of the pollutant molecules in the gas gap. On the other hand, the diffusion of the gas for both types is simulated in strict Computational Fluid Dynamics (CFD), which provides the basic rationality to the gas model. In this thought, two models have been built with the same active area of 33 \(\times\) 27 cm\({}^{2}\), defined by the size of resistive electrodes, but with different sizes of gas box volume: 41 \(\times\) 37 \(\times\) 10 cm\({}^{3}\) which represents the common practice of placing the MRPC into a gas-streaming box, and 34 \(\times\) 28 \(\times\) 1.2 cm\({}^{3}\) as a model of a well-enclosed structure. Correspondingly, the net gas volumes excluding the detector solid are 14.4 L and 414 mL. For both geometries, the working gas flows from the inlet on the one side of the volume to the outlet on the opposite side. The pollution can be quantified by the maximum pollutant concentration \(n_{\text{pollutant}}/n_{\text{total}}\) at steady state in the active area. Figure 1 shows the rate and flow scan results, where the pollutant concentration rises linearly with the flux rate and declines with higher flow. In comparison with the box model, the enclosed structure with lower gas volume has less pollution when provided with the same counting rate and gas flow, and there is no obvious lower limit of pollutant concentration such as the box model when increasing the gas flow at high rate. The limit might be explained by the mechanism of the gas exchange: with a larger gas box, the pollutant is exchanged by diffusion where the concentration gradient limits the exchange rate; with a reduced gas volume, the direct flow through the gas gaps may be realized and the exchange is promoted with a higher gas flow. In experimental operation, the negative behavior of MRPC that may come from gas pollution has been observed and examined. At STAR, the Muon Telescope Detector (MTD) and TOF are both constructed with MRPCs [17], but the former has a larger active area, thus is equipped with a larger gas box. In Run-12 with a rate range in 20-100 Hz/cm\({}^{2}\), the MTD had an increasing noise rate which failed to recover in 1-2 days with the beam off. Meanwhile, the TOF MRPCs were controlled within a reasonable noise level of less than 2 Hz/cm\({}^{2}\), as shown in Figure 2a. The noise problem of the MTD detectors was solved after blocking the extra space in the gas boxes (see Figure 2b). It can be noticed that the MTD-26 detector shows a relatively lower noise in the MTDs, which might come from the high-quality production. Despite the peculiarity, an irreversible rise in Figure 2a and significant improvement in Figure 2b can still be observed in MTD-26. A valid explanation of the results might be explained by the gas pollution effect: the noise increased with pollutants accumulated and, when the beam was off, pollution was relieved, but the gas exchange was still slow before the gas volume was reduced.