DETERMINATION OF THE INELASTIC MEAN FREE-PATH IN ICE BY EXAMINATION OF TILTED VESICLES AND AUTOMATED MOST PROBABLE LOSS IMAGING

Using electron microscopy, the thickness of ice-embedded vesicles is e stimated examining tilted and untilted views and assuming an ellipsoid al shape of the vesicles that appear to be circular in the untilted vi ew. Another thickness measure is obtained from the ratio of the unfilt ered and zero-loss-filtered image intensities of the vesicle. From the se two measure ments, the mean free path Lambda for inelastic scatteri ng of electrons in ice is calculated as 203 +/- 33 nm for 120 kV accel eration voltage. It is found that vesicles in thin ice films (less tha n or equal to 1.5 Lambda) significantly protrude out of the ice film. Due to surface tension the shape becomes an oblate ellipsoid. In holes covered with a thick ice film (greater than or equal to 3 Lambda) and strong thickness gradients, vesicles are predominantly found in regio ns where the ice thickness is appropriate for their size. Also, a way of imaging the most probable loss under low-dose conditions involving thickness measurement is proposed. Even at large ice thicknesses zero- loss filtering always gives better image contrast. Most probable loss imaging can only help where there is no intensity in the zero-loss ima ge, at very large thicknesses (Lambda>8).