Radiation damage in l-valine at liquid helium temperature

Abstract

One of the most fundamental limitations to resolution in the electron microscopy of organic materials is the structural damage caused by the incident radiation. While the primary inelastic interactions between the electrons and the specimen molecules cannot be stopped, it has been hoped that the use of very low temperatures would ‘freeze’ molecular fragments in place to allow extended microscopy of minimally damaged structures. Knapek and Dubochet¹ have reported that the rate of damage is much less (30–300 times) when diffraction patterns of organic materials are observed in an electron microscope with a superconducting objective lens, in which the specimen is held at 4.2 K. Previous workers had not seen such an effect when using liquid helium cooled stages, and it was suggested¹ that this was mostly due to insufficiently low temperature. A microscope with a superconducting objective lens, which was constructed at Oak Ridge National Laboratory², has been installed recently at Duke University. We selected l-valine as a test material because it has been so extensively examined in the past. Knapek and Dubochet¹ reported that l-valine crystals at 4.2 K could tolerate an electron dose 67 times greater than at room temperature and Dietrich et al.³, using the same microscope, reported a tolerance 87 times greater; we report here that it is only 4 to 6 times greater. We have tested effects of temperature and of the specimen substrate, both suggested as important components in the limitation of radiation damage, and we find no further improvement. The results of Knapek and Dubochet and Dietrich et al. therefore remain unconfirmed, and the mechanism and extent of protection from radiation-induced damage at low temperatures must remain under study.