Cooled CCDs for recording data from electron microscopes

A cooled-CCD camera based on a low-noise scientific grade device is described in this paper used for recording images in a 120 kV electron microscope. The primary use of the camera is for recording electron diffraction patterns from two-dimensionally ordered arrays of proteins at liquid-nitrogen temperatures leading to structure determination at atomic or near-atomic resolution. The traditional method for recording data in the microscope is with electron sensitive film but electronic detection methods offer the following advantages over film methods: the data is immediately available in a digital format which can be displayed on a monitor screen for visual inspection whereas a film record needs to be developed and digitised, a lengthy process taking at least several hours, prior to inspection; the dynamic range of CCD detectors is about two orders of magnitude greater with better linearity. The accuracy of measurements is also higher for CCDs, particularly for weak signals due to inherent fog levels in film. Some general requirements of electronic detectors for use in electron microscopy are discussed along with details of the CCD camera which uses tapered fibre optics to demagnify the phosphor image on to the CCD. A few important components of the camera are discussed in greater detail, e.g. choice of phosphor for optimum recording.