The use of CCDs and EM-CCDs for future soft X-ray Spectrometers

CCDs have been used on several successful X-ray space missions including high resolution soft X-ray spectrometers, such as the Reflection Grating Spectrometer on XMM-Newton(1) and the LETG and HETG on Chandra(2). These instruments had a resolving power of (E/Delta E)similar to 300; however, with new technology this can be improved allowing resolution to the thermal limit. In the soft X-ray band (200 eV to 10 keV) a resolution of around 3000 is required resolve all of the possible absorption and emission features. Through the development of instruments for the OP-XGS on IXO3 and the WHIMEx explorer mission(4) it has been shown that an instrument capable of this resolution on a spacecraft is possible. CCDs are the ideal detector for use in detection of X-rays at this energy as they provide positional information allowing a high level of spatial resolution and their inherent energy resolution allowing diffracted orders to be separated. This paper will investigate the use of CCDs and possible use of EM-CCDs in soft X-ray spectroscopy. The multiplication of signal in the charge domain can increase the detectability of low energy photons, improving the Signal-to-Noise Ratio. Multiplication gain has been shown to degrade the resolution of a device as described by the Modified Fano Factor(5), so this has to be taken into account in instrument design when overlapping spectral orders are needed to achieved the necessary resolution. The use of optical filters on the CCDs and their effect on quantum efficiency at soft X-ray energies is discussed together with possible improvements to existing technology.