High-resolution detectors for soft X-ray spectroscopy

Abstract

Resonant Inelastic X-ray Scattering (RIXS) is a modern soft X-ray spectroscopy technique used to investigate the structure of and excitations in materials. It requires high resolution spectrometers and a brilliant, tunable, X-ray source and therefore is carried out at spectrometers such as SAXES at the Swiss Light Source Light, a synchrotron at the Paul Scherrer Institut. SAXES uses a grating to disperse X-rays scattered from a sample across a position sensitive detector, a Charge-Coupled Device (CCD). It has been recognised that the spatial resolution of the CCD currently limits the spectrometer resolution and therefore the investigations described in this thesis focus on developing and testing methods of improving the detector resolution. Whilst this thesis addresses improving the resolution of the detector at SAXES specifically, the methods and results are applicable to other applications requiring high spatial resolution soft X-ray detection. After an introduction, Chapters 2 and 3 describe the importance of RIXS, operation of SAXES and background of soft X-ray detection in CCDs. A comparison of models that describe charge spreading in a CCD is in Chapter 4, and the best model is implemented in a simulation package that generates populations of soft X-ray events. Chapter 5 explores the resolution improvements possible through applying centroid algorithms to simulated X-ray events, and Chapter 6 begins by describing experimental work undertaken to verify simulation results. Due to the limitations of applying centroiding algorithms to the current SAXES camera, a small-area Electron Multiplying CCD (EM-CCD) is experimentally tested (Chapter 7). Results with the EM-CCD proved positive, therefore in Chapter 8 the spatial resolution achievable with a large area EM-CCD is verified for a future SAXES camera upgrade. Due to the successful results presented in this thesis, negotiations to develop a new camera system for SAXES are underway, and interest from other RIXS beamlines in the community may lead to the work also being applied elsewhere. The detection of soft X-rays with high spatial resolution is applicable to other future CCD and EM-CCD instruments, such as astronomical X-ray observatories.