Stabilising a Peels spectrometer

Abstract

The resolution of a parallel recording electron energy loss spectrometer (PEELS) depends critically on the stabilities of the power supplies to the microscope column and the spectrometer over the period of the exposure. They may be the limiting factor in a dedicated scanning transmission electron microscope (STEM) with a field-emission electron gun (FEG), and improving them so that they are sufficiently stable can be a difficult task.An alternative approach is to compensate for any drift by deflecting the electron image of the spectrum leaving the spectrometer. The amount of deflection is that required to keep the zero-loss peak at a fixed position relative to the recording system. Such a stabiliser is being developed in the Microstructural Physics (MP) Group at the Cavendish Laboratory and a progress report is given here.The MP STEM is a VG Microscopes HB501 fitted with an Isaacson electron spectrometer. The normal accelerating potential is 100 kV and the FEG has an electron energy spread of about 0.25 eV. The spectrometer gives a dispersion of 1.8 pm per eV at the slit and from the evidence of very short exposures its resolution is adequate to match the FEG energy-spread in the absence of drift. A PEELS system with 3 quadrupoles and a YAG + CCD detector has been added, but the slit has been retained.