Development of CCD and CMOS detector technology for X-ray spectroscopy

CCD technology has been developed in the past for direct X-ray detection and is quite mature, having been flown on X-ray astronomy missions such as XMM, Swift and Chandra. In these CCD X-ray detectors, what is referred to as 'Fano-limited' performance has been achieved, meaning that the intrinsic energy resolution of the sensor is limited by a combination of the system read noise and the Fano-limited shot noise in silicon, which is a basic physical limit. This enables for example a full width at half maximum (FWHM) resolution of ~130 eV at 6 keV, and has been used in the non-dispersive imagers returning images that include photon spectroscopic information for EPIC and Swift with high detection efficieny. The devices do have limitations, particularly the maximum count rate capability (restricted by the need to transfer the whole image through a small number of output nodes), and in charge transfer efficiency under space proton damage which degrades the intrinsic energy resolution. Beyond Space Science, these detectors have found use in a range of terrestrial applications ranging from synchrotron research and free electron lasers, to industrial X-ray spectroscopy and bio-medical imaging. In future, the requirements for X-ray astronomy might be seen to diverge into two main classes, non-dispersive imaging as in the case of XMM/EPIC or the wide field imager for IXO, where the focused flux could be very high, and in dispersive instruments such as XMM/RGS or the grating spectrometer (XGS) on IXO. Over the last year, the CEI has lead the study of the XGS on IXO for ESA and is well placed to build upon this work in the future and the mission concept for such large astronomy telescopes progresses through the ESA/NASA/JAXA systems. The current detector of choice for the dispersive-type instrument is currently still the CCD, whilst for high throughput applications (with large optics), the pixel array is more favoured. We currently have two STFC-funded PhD students, one studying X-ray CCDs towards XEUS (now IXO) (Tutt), and another studying e2v's new CMOS technology for space applications (Dryer), particularly concentrating on the impact of space radiation damage in these new sensors. During the course of the CMOS PhD however, the student has developed test techniques using monochromatic X-rays for calibration, and the initial results are promising, yielding ~250 eV resolution at room temperature (where conventional CCDs may need to be cooled to below -60 degrees C to achieve similar performance). The CCD technology, being more mature, provides a state of the art reference (low noise with good energy resoution, pixels well matched to the application, high detection efficiency, reasonable radiation hardness). The newer CMOS technology is much less mature, comprising higher noise, very small pixels, but with very high frame rates, the possibility of complex windowing readout modes and further in-pixel signal processing possibilities. In addition, future developments at e2v during the course of the studentship will see the development of higher efficiency, lower noise designs, which should become much more suitable for X-ray applications. In the course of this PhD, we would propose a candidate who studied and developed X-ray detection for future astronomy applications using both CCD and CMOS technologies. The student would use the readily available CCDs as a foundation for their work and understanding of the technology and instrument requirements, but would spend >60% of their time on the development of CMOS imager technology toward a truly useful imager capable of performing imaging spectroscopy on X-ray photons for science applications. We would anticipate that the work performed would be of benefit to other science areas such as synchrotron research, solar physics, and fusion research. The student could take advantage of a recent PhD-student programme in collaboration with PSI, Switzerland, for access to synchrtron beamlines.