The secondary electron emission yield of muscovite mica: Charging kinetics and current density effects

Using a dedicated scanning electron microscope, operating in the spot mode, the charging properties of muscovite mica have been studied in the energy range of 100-8000 eV. The intrinsic yield curve σ 0 ( E ) , representing the variation of the yield of the uncharged material with the energy E , has been established: the maximum value of the yield is 3.92 at E = 300 eV and the two crossovers corresponding to σ 0 ( E ) = 1 are, respectively, at energies E I < 100 eV and E II = 4850 eV . At a given energy and under a low current density J ≤ 100 nA / cm 2 , the yield varies with the electron fluence from its intrinsic value σ 0 up to the value corresponding to the self-regulated regime for which σ = 1 . This variation is independent of J . The fluence dependence of the yield σ ( D ) is due to the internal field produced by the accumulation of charges that blocks the emission when the charging is positive and enhances it when it is negative. At room temperature, the relaxation time of stored charges is estimated to be of the order of 250 s for holes and 150 s for electrons. Three current density effects have been observed when J ≥ 400 nA / cm 2 . (i) The variation of σ ( D ) with the fluence D depends on J . (ii) Negative charging is obtained at high current density in the energy range ( E I , E II ) where the material is normally positively charged at low current density. (iii) Electron exoemission (bursts of electrons) is produced at low energy when the net stored charge is positive. The interpretation of the current density effect on σ ( D ) is based on the high rate of charging, the effect relative to negative charging is due to the expansion of the electron distribution, while the exoemission effect is due to the collective relaxation process of electrons.