Applications of cryogenics in electron microscopy

Results of earlier work with superconducting lenses (objective lens excited with 27,220 ampere-turns) indicated some unique advantages inherent in systematic application of this approach in high resolution electron microscopy. Operating in the persistent current mode made possible resolutions of 10 to 20 A during longer exposure times at lower beam intensities, thus reducing specimen radiation damage. With the first prototype of a cryoelectron microscope system (i.e., large-scale, 20 W capacity, Collins closed-cycle superfluid helium refrigerator with 36-ft transfer lines and novel heat exchanger integrated with a modified 200 kV electron microscope), significantly reduced radiation damage, contamination and thermal noise are available in prolonged vibration-free examination of specimens at 1.8 to 4.20K. Simultaneously, the system provides high penetration power, ultra-high vacuums, decreased spherical and chromatic lens aberrations, and enhanced image contrast. Specially-adapted vacuum-tight microchambers have been used in studies of ice-crystal structure, wet membranes and new types of organometallic superconducting compounds. Consistent resolutions of 8 to 16 A are achieved, compared with corresponding low temperature resolutions of about 50 A one year ago. Descriptions of these instrumental developments are given together with results of their applications and implications in specific research areas, particularly membrane ultrastructure, cryobiology, microelectronics and general superconducting work. (U.S.)