Enhanced spatial resolution of commercial soft X-ray CCD detectors by single-photon centroid reconstruction

In many back-illuminated Charge Coupled Device (CCD) detectors the electrons liberated by the absorption of one X-ray photon quickly spread and generate charge-spots larger than the pixel dimensions. Such detectors are considered here. In the soft X-ray range this phenomenon drastically limits the effective spatial resolution to approximately 25 mu m, irrespective of the pixel's lateral size. For very low flux the charge-cloud centroid determination can be used, on each individual spot, to estimate the actual photon impact position with sub-pixel precision. The readout noise and speed, together with the charge and spatial undersampling, are the main factors limiting the accuracy of this procedure in commercial devices. We have experimentally measured a position uncertainty better than 7 mu m and 10 mu m for 13.5 mu m and 20.0 mu m pixel sizes respectively, around 1 keV photon energy using centroiding algorithms. This study was motivated by the need of high resolution detectors in resonant inelastic (soft) X-ray scattering (RIXS).