CCD detectors.

The spatial resolution of CCD detectors together with their good energy resolution, efficiency and background rejection quality make them especially suitable for measurements with a crystal spectrometer. We will use a newly developed CCD detector system which is presently being set up for the measurement of the pion mass [86]. The detector comprises an array of 6 large area CCDs that were developed specifically for X-ray photon detection for the XMM EPIC X-ray astronomy mission[87]. Each CCD has an image area of $600\times 600$ pixels of $(40\mu m)^{2}$ . The CCDs are manufactured on high-resistivity epitaxial silicon for deep depletion and high quantum efficiency (plotted as a function of energy in Fig 7) and a resolution of $\sim$ 150eV at $6.4keV$ . The array is arranged as two adjacent columns of three devices, with a total read out time of about two minutes. Read out is performed one column at a time, with data from the three CCDs being digitized in parallel. Alternate columns will be read out continously with minimum integration time. This will provide the maximum X-ray event/background event ratio, with the minimum integration time being determined mainly by data handling capabilities.

**Figure 7:** The intrinsic efficiency of the CCD detector is shown as a function of energy.
$\resizebox* {0.7\textwidth}{!}{\includegraphics{ccdeff.eps}}$

The pixel structure not only provides good spatial resolution, but also allows post processing of the data to perform background rejection. Most of these unwanted events come from the pions stopped at a rate about $\sim$ $10^{8}/s$ in the region of the cyclotron trap only $\sim$ $2m$ away. The solid angle of the detector with respect to the target is $\sim$ $10^{-4}$ of $4\pi$ . Even with a well tuned shield the background event rate is still of the order of $100/s$ , while the expected pionic or muonic X-ray event rate is about $(2-3)\cdot 10^{-2}$ /s . The unwanted background mainly comprises charged particle and gamma ray events. Since these events deposit their energy over several pixels, and the low energy X-ray events are mainly contained within a single pixel, event topology can be used to discriminate between them. Event reconstruction combined with energy discrimination can also be used to further enhance the efficiency and contribute to the high background rejection, as illustrated in Fig 6.