Schematic view of the secondary electrons and induced current measurements. The secondary electron detector (diameter 15 cm) is positioned at a distance from the sample surface. The samples have a thickness of about 0.2 cm and a diameter of 1.6 cm.
Current curves recorded by HP 54600B oscilloscope coupled with Keithley 428 current amplifier during the first injection in polycrystalline alumina sample (grain diameter ) at 473 K. The currents, and , are measured after a short lag time. The irradiation conditions are: , , , and .
Evolution with time, at 473 K, of the net charge, , during the first pulse injection in polycrystalline alumina sample (grain diameter ). The quantity is derived from the currents of Fig. 2 via Eq. (1). The solid line represents the exponential fit of the data, as given by Eq. (5).
Current curves obtained during the second pulse injection performed after a pause time . Irradiation is carried out over the same area as in the first injection under identical experimental conditions (given in Fig. 2).
Illustration of the time evolution of the amounts of charges remaining in the irradiated volume during the three stages. The first and the third curves correspond to the currents and experimental conditions of Figs. 2 and 4. The net charge during the pause evolves from to , which remains in the irradiated volume after a pause of 900 s [as justified below by Eq. (9)].
Evolution with the pause time at 473 K of the amount of charges that is removed from the irradiated volume (in polycrystalline sample of grain size). The solid line is the exponential fit of the data [Eq. (8)]. The asymptotic value is attained after a pause time of only 300 s.
Fraction R expressing the degree of discharge (i.e., the fraction of charges removed from the irradiated volume) as a function of temperature for undoped and silver doped sapphire.
Semilogarithmic plot of the fraction R expressing the degree of discharge versus reciprocal temperature for silver doped sapphire (lines: linear fit of the data). Two distinct activation energies, 0.51 and 0.04 eV, are deduced.
Fraction R of charges removed from the irradiated volume as a function of temperature for polycrystalline alumina samples of different grain diameters d.
Semilogarithmic plot of the ratio R expressing the degree of discharge versus reciprocal temperature for polycrystalline alumina (solid line: linear fit of the data). For T below 573 K, discharging is characterized by the same activation energy (0.12 eV). Above 573 K, a second energy (0.26 eV) in small grains sample arises.
Composition, in parts per million, of the alumina materials. The composition of sapphire RSA was determined by x-rays fluorescence (Ref. 24) and that of alumina CRICERAM powders used for sintering of polycrystalline alumina was provided by the manufacturer.
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