High Speed Imaging and Spectroscopy with Low Energy X-Rays

Abstract

Counting, imaging, and spectroscopic measurements of X-rays at low energies used in synchrotron and Free Electron Laser (FEL) science (30 eV up to 2 keV) all require detectors with unique properties. As the penetration depth of low-energy X-rays in, for instance, silicon in the above energy range varies from 40 nm to $10\,\upmu \mathrm{m}$, special attention must be given to the properties of the radiation entrance window. And because the number of generated signal charges (electron-hole pairs) is low (approximately 27 signal charges for 100 eV and 540 for 2 keV), the detector systems must be operated with very low electronic noise. This is especially important if standard imaging and spectroscopy are to be performed simultaneously, at low-signal-level detection, in the presence of experimental and instrument background radiation. As the local photon intensities per unit area can be as high as 10⁵ X-rays/s/pixel, long-term stability, especially radiation hardness, is an important requirement. Given these requirements for readout frame rates below 1 kHz, charge-coupled devices (CCDs) have proven their usefulness in experiments at X-ray Free Electron Laser sources. Two types of CCDs will be described: MOSCCDs (Metal Oxide Semiconductor) and pnCCDs. The basic functional principles will be shown as well as the achieved performance figures, as demonstrated in real experiments. Next, the physical limitations of the measurement precision will be discussed. Finally, attention will be given to some options for future CCD architectures and operations and a trade-off between CCDs and CMOS active pixel sensors.