The high field transport properties of polyethylene investigated by the electrostatic mirror technique

In the electrostatic mirror technique promoted by Le Gressus, an insulating sample is charged locally by a focused high energy beam from a SEM, then a low energy beam analyses the charged area. Scattered by the trapped charge, this probing beam is deviated back to the SEM chamber, so that the gun appears on the screen as a dark circle, since the secondaries generated inside are lost. While the stable dark spot radius is usually measured as a function of the probing energy to draw the "mirror characteristic", the present work concentrates on the spontaneous contraction of this spot, which is observed at constant probing energy whenever a large enough charge is trapped in a thin film lying on a grounded substrate. This is ascribed to the drift of the charge across the sample, in the strong field of its image in the substrate. To interpret quantitatively the spot contraction, the trapped charge is assumed homogeneous and hemispherical. This approximation allows a fair analytic derivation of the dark spot radius. The relative decay rate α of this radius is related to the velocity of the charge centroid in the image field, which can be calculated for the hemispherical model. Consequently, the measurement of α may yield an estimate of the charge mobility. By this technique, it was shown in particular that the high field transport properties of polyethylene depend quite sharply on the thermal history of the sample, and thereby on its micromorphology.