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Further investigations of our retarding potential measurements reported in (1) showed that the energy of emitted electrons can achieve values up to 80 eV. Earlier attempts of several authors for the interpretation of the exo-electron emission (2 to 6) cannot explain these high energies. Therefore a new model shall be proposed in this paper.If insulating materials are bombarded with electrons of an energy at which the > 1, the surface will be charged posi-secondary electron emission coefficient tively. Because of the interaction of the primary electrons with the solid they only penetrate to a certain depth. In the region of maximal penetration depth of the primaw electrons their energy is so small that they will be captured by traps. A negative space charge region is built up. Because of the high-resistive material no neutralization of charges will take place. The generated electric field between the negative space charge region and the positive surface layer effects an equilibrium state at which in equidistant time intervals the same number of traps are emptied and occupied. In the region between negative space charge and positive surface no traps are occupied, because electrons just being trapped are released immediately by subsequent electrons into the conduction band, from where they are removed by the internal electric field. Electrons which are thermally released into the conduction band can leave the solid, if they are able to overcome the energy barrier at the surface and compensate the energy loss caused by scattering processes along the way to the surface. The average energy loss of an electron per scattering process amounts to 0.03 eV, the mean free path to 5x10 cm (7). If the strength of the electric field exceeds 10 V/cm the energy loss of a scattered electron is smaller than the energy gain along the mean free path. -7 5We assume that the emission process only takes place if insulating surface layers are present. Then the emission process takes the following course (see

The P?+ spectra of Gd3GasOn Cwchralski-grown crystals, investigated by absorption, sibselective excitation and emission and fluorescence decays, shows a complex s v U a . Besides the speeba assigned to P?+ in dodecahedral c sites (Dz local symmetry), dt in other sites is also significant. An attempt to assign the most intense lines to svUctural defects is made. Three of the most intense satellite lines are assigned to a P?t(e) ion p d e d by a nearby non-stoichiomemc defect (Gd3+ in octahedral D sites) whose concentration is estimated to be about 7%. The spectra for perturbed Pr3+ centres show additional lines mnnected with a reduction in local symmeuy. Two other lines have been assigned to d C ( c ) -d + ( c ) pairs.The emission decays are almost exponential with similar lifetimes, except one that presents a very strong quenching. This line has been assigned to smngly coupled P?+(c&dt(c) nearest-neighbour pairs. The influence of pair quenching on the relative quanhlm efficiency of ' Po emission is also estimated. This paper reports the data concerning the P$+ multisite structure in Gd3Ga5OI2 (COG), Czochralski-grown crystals. High-resolution transmission, integral excitation spectra, siteselective excitation and emission spectra and lifetime measurements have been performed

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