Space charge limited avalanche growth in multigap resistive plate chambers

The ALICE TOF array will be built using the Multigap Resistive Plate Chamber(MRPC) configured as a double stack. Each stack contains 5 gas gaps with width of 250 mu m. There has been an intense R&D effort to optimise this new detector to withstand the problems connected with the high level of radiation at the LHC. One clear outcome of the R&D is that the growth of the gas avalanche is strongly affected by space charge. The effect of the space charge is a decrease in the rate of change in gain with electric field; this allows more stable operation of this detector. We have measured the gain as a function of the electric field and also measured the ratio of the fast charge to the total charge produced in the gas gap. It is well established that RPCs built with 250 mu m gas gap have a much superior performance than 2 mm gaps; we discuss and compare the performance of 250 mu m gap MRPCs with 2 mm gap RPCs to show the importance of space-charge limitation of avalanche growth.