Backward fluorescence of optically thick Cs vapor: Experiment and Monte-Carlo simulations

•The measurement of backward fluorescence of an alkali vapor.•The development of Monte-Carlo simulations to analyze experimental spectra.•The role of resonant collisions and radiation trapping on the backward fluorescence spectra are highlighted by the simulations.•The interpretation of sub-Doppler signals as lowering of fluorescence quenching for excited atom flying parallel to the surface.