Thickness effect in radiation damage: real or artefact

Damage caused by an electron beam in a crystalline organic substance, as observed by the decay of its diffraction pattern, apparently decreased with increasing specimen thickness. The dose needed to destroy the pattern increased with thickness at given voltage. Such a thickness dependence of 'lethal' dose is difficult to explain on any simple theory of radiation damage. It is suggested that the effect is in part a consequence of the experimental method. If so, the same limitation would apply to any criterion of damage rate in which measurement of the end-point relies upon a certain minimum electron signal. (author)