Multigap RPC time resolution to 511 keV annihilation photons
  The time resolution of Multigap Resistive Plate Counters (MRPCs) to $511$ keV gamma rays has been investigated using a $^{22}$Na source and four detectors. The MRPCs time resolution has been derived from the Time-of-Flight information, measured from pairs of space correlated triggered events. A GEANT4 simulation has been performed to analyze possible setup contributions and to support experimental results. A time resolution (FWHM) of $376$ ps and $312$ ps has been measured for a single MRPC with four $250$ $\mu$m gas gaps by considering respectively one and two independent pairs of detectors. 
G. Belli, M. Gabusi, G. Musitelli, R. Nard\`o, S.P. Ratti,   A.Tamborini, P. Vitulo
physics.ins-det physics.med-ph