Probe current determination in analytical TEM/STEM and its application to the characterization of large area EDS detectors.

A simple procedure, which enables accurate measurement of transmission electron microscopy (TEM)/STEM probe currents using an energy loss spectrometer drift tube is described. The currents obtained are compared with those measured on the fluorescent screen to enable the losses due to secondary and backscattered electrons to be determined. The current values obtained from the drift tube allow the correction of fluorescent screen current densities to yield true current. They also enable CCD conversion efficiencies to be obtained, which in turn allows images to be calibrated in terms of electron fluence. Using probes of known current in conjunction with a NiO reference specimen enables the X-ray detector solid angle to be determined. The NiO specimen also allows a wide range of other EDS detector parameters to be obtained, including the presence of ice and carbon contamination. A range of performance characteristics are reported for two large area EDS detector systems. Many of the measurements reported herein have been automated via the use of freely available scripts for DigitalMicrograph.