Stage cooling rates and specimen-transfer facilities of the Duke University cryomicroscope

Abstract

A liquid helium cold stage offers unique advantages for biological electron microscopy, including a five-fold reduction in radiation damage, higher ultimate specimen resolution and greater stability for frozen hydrated specimens. Ultra-high vacuum and reduced surface diffusion also reduce specimen contamination to negligible levels. To make efficient use of these advantages in biological studies, however, the microscope must be able to handle a variety of specimens while quickly achieving low temperature.The cryomicroscope that is currently in operation at Duke University (Fig. 1) was originally designed and built at Oak Ridge National Laboratory as a high-resolution electron microscope with a superconducting objective lens. The well-shielded cryostat for the objective lens assures a low specimen temperature. There are two thermal shields surrounding the liquid helium vessel, each cooled by the venting cold helium gas. The inner shield is at its nominal value, 20°K, at the end of the helium transfer when gas is actively venting from the system; later during routine operation, the inner shield temperature is about 30°K.