For pt.I see NSSMI Conf. Record, Nov., 1991, NM, USA. Results showing the relative importance of several processes which are responsible for the efficiency and speed of a scintillator are presented. Arguments for concentrated materials are presented and illustrated in the case of Ce-pentaphosphate, (CeP/sub 5/O/sub 14/). The low density of this material does not make it a viable scintillator, but its remarkable simplicity and relatively easy technology make it perfect to study basic physical mechanisms of scintillation in concentrated materials. Based on studies of a series of Ce/sub x/La/sub 1-x/P/sub 5/O/sub 14/ crystals, the authors formulate general conclusions relating to the relative importance of the process starting with direct ionization of the emitting centers (Ce/sup 3+/) versus long range gathering of electron-hole pairs. These conclusions provide guidance for optimizing the speed and efficiency of future concentrated scintillators. Some peculiarities of the Ce-pentaphosphate luminescence are reported and interpreted. >