Reduction in Radiation Damage at Low Temperature
Previous low-temperature measurements of radiation damage in organic materials have been based on the diffraction pattern, changes in total scattering power, background in the X-ray emission spectrum or the O - 10 eV region of electron energy-loss spectra. Here we observe instead the 200-600 eV region of the energy-loss spectrum, which contains information relating to the concentration of carbon, nitrogen and oxygen present in a thin sample. Specimens were irradiated by 80 keV electrons in a JEM IOOB transmission microscope fitted with a specially-constructed specimen holder which could be cooled to 77°K by circulation of liquid nitrogen. Cooled apertures above and below the specimen reduce heat radiation and possible condensation of residual gases from the surroundings. For the dose rates (<0.4 mA cm"2) and specimen thicknesses (< 50 nm) employed here, temperature rise due to the electron beam is expected to be negligible7.