I. Dietrich

Improvements in electron microscopy by application of superconductivity

Resolution tests on amorphous carbon foils were carried out in an electron microscope with a superconducting system containing 4 lenses including a shielding lens at 200 kV beam voltage. Due to the mechanical and electrical stability of the system and the absence of contamination of the specimen the highest space frequencies transferred at vertically incident beam were 6 nm-1 corresponding to a resolution of 0.17 nm, a value which approaches the theoretical resolving power of the electron optical system. It should also be feasible to apply such a lens system for microprobe analysis without strongly reducing the theoretical resolution limit, if the construction of the shielding lens is slightly changed.

Superconducting lens design

Abstract A survey of various types of superconducting lenses is given. Cryogenic problems, particularly material problems, are discussed which must be solved in order to construct lens systems applicable for electron microscopy. The results obtained up to now with the few superconducting lens systems in operation prove a better resolution than is achievable with conventional lenses. The specimen has a temperature of approximately 4 K since it is completely surrounded by walls directly cooled by liquid helium, and the stability of the superconducting lens system permits long exposure times of the micrographs which make it possible to minimize the heating of the object by the beam. For this reason due to cryoprotection one can take advantage of a drastic reduction of radiation damage by the beam.

Electron radiation damage of a thin protein crystal at 4 K

Thin crystals of crotoxin complex embedded in glucose were used to investigate the radiation damage effect at 4 K in an electron microscope equipped with a superconducting lens. It was found that the high resolution (

Radiation damage due to knock-on processes on carbon foils cooled to liquid helium temperature

Radiation damage on a holey carbon foil was investigated in an electron microscope with a superconducting lens system, where the temperature of the specimen and its environment initially was 4 K. Due to an electron dose of 2 X 10(4) As/cm2 the diameter of a hole increased 5 nm. Rough calculations show that this increase can be ascribed to knock-on processes. Estimates of the rise in specimen temperature during the irradiation are given.

Electron microscopical results on cryoprotection of organic materials obtained with cold stages

Abstract Electron microscopy work on radiation damage performed with cold stages as described in the literature is reported. The gain in tolerable exposure dose due to cooling is of the order of five. There are indications that this gain will increase with decreasing temperature. The advantages of cold stages are discussed.

The interpretation of radiation damage measurements with electron diffraction of organic materials at very low temperatures

Abstract At very low object temperatures organic specimens suffer not only radiation damage but, to a higher extent, charging and heating during electron irradiation due to very low electrical and thermal conductivities of organic materials. The fading of electron diffraction patterns is seriously influenced by the superposition of both effects. For this reason an exact determination of the progress in radiation damage is nearly impossible by direct observations of the diffraction patterns, particularly for obtaining the cryoprotection factors of organics. Charging and subsequently heating cause intrinsic motions of the irradiated areas which also seem to lead to a destruction of the crystallinity. With a suitable preparation of organic specimens, charging and heating effects at very low temperatures could be avoided, and it could be demonstrated that the true cryoprotection was relatively good.

Superconducting lenses in electron microscopy

Abstract Various types of superconducting electron microscope lenses and lens-systems have been designed and tested in the last ten years. A resolution of 0.4 nm was achieved at several laboratories. Future potentialities for the application of superconducting lenses including microwave lenses, for example, in high voltage electron microscopy are discussed.

Reduction of Radiation Damage by Imaging with a Superconducting Lens System

Radiation damage is enemy number one for electron microscopical investigations of organic specimens at high resolution. Many different methods have been developed to obtain more information with a resolution better than 1 nm in spite of this handicap. In this paper we restrict ourselves to the application of low specimen-temperatures for fighting beam damage.

High resolution cryo electron microscopy of specifically stained specimens

Abstract At very low temperatures organic specimens with attached metal atoms can be irradiated with a relatively high electron fluence without losing the characteristic features of their architecture. This is demonstrated on three examples: a metal-organic molecule, a membrane-like biological layer, and a three-dimensional polymeric copper complex.

Superconducting Lenses for Steric Structure Determination of Organic Material in the Electron Microscope

Abstract The atomic structure of radiation sensitive organic material has been determined with a superconducting lens system for electron microscopes where the specimen is kept at a temperature of 4 K. Due to the high electromagnetic and mechanical stability of the instrument and due to the cryoprotection of organic specimens at low temperatures the “superconducting” electron microscope has been found to be very effective for this purpose.