Designers of electron microscopes started putting liquid helium into their plans with the hopes of improving instrumental resolution. Liquid helium can be used to cool specimen stages within conventional lenses, but it can also be used to make some materials superconducting. Once started, current flows continuously in a superconducting coil because it faces negligible electrical resistance. Superconducting lenses were introduced in electron microscopy to take advantage of the high magnetic field strengths that could be generated as well as the stability of the persistent current in the superconducting coils. As time passed, it became clear that electronic regulation of power supplies could produce more than adequate current regulation to supercede the need for the persistent current maintained in a superconducting lens. Meanwhile, the realization grew that low temperatures might give another kind of advantage to electron microscopy of radiation-sensitive materials. The very low temperature might help protect sensitive specimens from damage.Several methods are available for detecting damage to specimens in electron microscopes. The loss of mass from specimens is one sign of radiation damage. A mass loss measurement with a liquid helium cooled stage seemed to show a significant reduction in mass loss; a different measurement that included a superconducting lens also indicated reduced damage. Another indicator of damage is the rate of fading of diffraction patterns of ordered specimens, and in this case it has been established that liquid helium temperature causes a moderate reduction of damage.
High Resolution Cryo-Electron Microscopy of Virus Capsids at Liquid-Helium Temperature
