ABSTRACT
We briefly touched on the use of charge-coupled devices (CCDs) as x-ray detectors in Section 1.15, where we gave them as an example of a ‘charge delivery detector’—that is to say, that one polarity of charge (usually electrons) generated by an x-ray event is effectively stored until a predetermined time when it is transferred and deposited onto a ‘readout node’. Then, and only then, is it measured. In CCDs, the charge packet is transferred by a ‘clocking’ procedure from one pixel to the next, as described below, until it is eventually deposited on the readout node. There is no signal at the readout node from this charge packet while it is being transferred between pixels. CCDs were developed in the 1970s as imaging devices that could replace the light-sensitive film in cameras. Indeed, the digital camera is still by far their largest application and they are mass produced cheaply for this purpose. Their first use as a direct sensor for x-rays was reported in 1979 [1], and since then their main application has been in x-ray astronomy, for which purpose they have been reviewed [2,3]. Here, we will describe the basic structure and leave the reader to find more details in the aforementioned references. We will concentrate on their use as x-ray spectrometers and not as imaging devices, although it should be noted that a CCD device was recently chosen for the Mars rover Curiosity chemical analyzer [4] because of its capability of combined x-ray fluorescence (XRF) and x-ray diffraction (XRD) (imaging diffraction patterns).
