Beam and Specimen: Radiation Damage and Image Resolution

Energy loss of the electron beam in the specimen affects the electron microscopical resolution, owing to chromatic aberration, and causes radiation damage by ionisation, heating and direct ejection of atoms. Measurements recently made of the energy loss in carbonaceous specimens confirm that the effect on image resolution will be negligible in very high resolution microscopy. Additional assumptions are required in order to obtain the rate of ionisation and bond breakage. On the best evidence available, it is estimated that the order of 2 chemical events will be caused during the minimum exposure needed to image an atom. The rate of ejection of atoms from carbonaceous material can be calculated rather more accurately. About 1 atom in 2500–3000 would be ejected in such an exposure, which would probably not seriously degrade a macromolecule.