Reduction of radiation damage in an electron microscope with a superconducting lens system

Abstract

OBSERVATION of biological specimens at high resolution by electron microscopy is limited by radiation damage. In general, structural details of non-periodical arrangements can only be identified for doses of radiation up to 10⁻³ C cm⁻², corresponding to 63 e nm⁻², which restricts the resolution to approximately 2 nm (refs 1,2). Many methods have been applied in an attempt to reduce radiation damage, including high voltage, high vacuum and specimen pretreatment. The most successful, however, was the reduction of the radiation dose, used for certain specimens with periodical structure, where the information could be obtained by computer-aided image enhancement from underexposed films³. In special cases, although it was possible to obtain information at high resolution, the specimen was partly destroyed⁴. The advantages of low temperatures were also investigated, but due to the lack of an adequate instrument, no agreement between the results could be obtained. Furthermore, little attention was paid to the significance of the dose rate at low temperatures. We now have in operation an electron microscope with a superconducting lens system in which the specimen is cooled to liquid helium temperature⁵. The most important features of this instrument are the high resolution, which allows the transfer of space frequencies corresponding to the inverse of 0.15 nm, the mechanical and electromagnetic stability (specimen drift 0.01 nm min⁻¹) and the ultra high vacuum around the specimen. These features are necessary for testing the effect of radiation damage. We report here experiments which demonstrate that radiation damage is considerably reduced at low temperatures in a dose rate-dependent manner.