Improving the spatial resolution of soft X-ray detection using an Electron-Multiplying Charge-Coupled Device

Abstract

The Super Advanced X-ray Emission Spectrometer (SAXES) isan instrument at the Swiss Light Source designed for Resonant InelasticX-ray Scattering with an energy resolution (E/ΔE) better than 12000at 930 eV. Improvements to the instrument have been predicted that couldallow the energy resolution to be improved by a factor of two. To achievethis, the spatial resolution of the detector (currently a Charge-CoupledDevice, CCD) over which the energy spectrum is dispersed would have to beimproved to better than 5 μm.X-ray photons with energies between a few hundred to a few thousand electronvolts primarily interact within the field-free region of back-illuminatedCCDs, where each photon forms an electron cloud that diffuses isotropicallybefore reaching the depleted region close to the electrodes. Each photon'selectron cloud is likely to be detected as an event with signal split acrossmultiple pixels. Analysing these split events using centroiding techniquesallows the photon's interaction position to be determined to a sub-pixellevel.PolLux is a soft X-ray microspectroscopy endstation at the Swiss LightSource that can focus 200 eV to 1200 eV X-rays to a spot size of approximately20 nm. Previous studies using data taken with a linear scan across thecentre of a pixel in 3 μm steps predicted an improved resolution byapplying centroiding techniques and using an Electron-Multiplying CCD(EM-CCD). In this study, a full 2D map of the centroiding accuracy in thepixel is presented, formed by rastering in two dimensions across the imageplane in single micron steps. The improved spatial resolution fromcentroiding events in the EM-CCD in all areas of the pixel over the standardCCD is attributed to the improved signal to noise ratio provided by themultiplication register even at high pixel readout speeds (tens of MHz).