[B._Beckhoff,_et_al.]_Handbook_of_Practical_X-Ray_(b-ok.org).pdf

Detector performance on ground and in orbit the best

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Detector Performance (on Ground and in Orbit) The best value for the readout noise of the on-chip electronics is 2e rms at 90 C for the most recent devices, whereas typical values scatter around 5e rms, e.g., for the XMM system. This includes all noise contributions described in (4.60). The charge transfer properties of the pn-CCDs on XMM are reasonably good, in the order of a several % signal loss from the last to the first pixel over a distance of 3cm charge transfer. As the charge transfer losses describe the position dependent energy resolution, it is one of the key parameters for the spectroscopic performance, especially after radiation damage may have occurred. Figure 4.55 shows a 55 Fe spectrum of a pn-CCD in a flat field measurement resulting in a typical energy resolution of 130 eV at an operating temperature of 120 C [68]. The XMM flight cam- era was operated at 90 C during calibration on ground with a resolution of about 145 eV (FWHM) over the entire area of 36 cm 2 . The main effect on the degradation of energy resolution was the reduction of the charge trans- fer efficiency at warmer temperatures. Leakage currents and on-chip JFET properties only played a minor role. The impact of the material properties of silicon and related impurities and their consequences for the operation of scientific grade X-ray pn-CCDs including the effects of radiation damage, are treated in detail in the references [79, 80]. The radiation damage accumulated over the expected life time of XMM is equivalent to a 10 MeV proton fluence of 4 – 5 × 10 8 p/cm 2 . Figures 4.52 and 4.53 show the results of the irradia- tion tests with 10 MeV protons: the expected decrease of energy resolution over the 10-year dose is from 146 eV to 164 eV at an operating temperature of 100 C. At the actual operating temperature of 90 C , the expected effect of trapping and detrapping at A-centers, generated by the radiation, is even more reduced. In a single photon counting mode the quantum efficiency was measured with respect to a calibrated solid state detector. Figure 4.54 shows measure- ments from the synchrotron radiation facilities in Berlin and Orsay. At 525 eV, a 5% dip can be seen due to the absorption at the oxygen edge in the SiO 2
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X-Ray Detectors and XRF Detection Channels 273 1.0 0.8 0.6 0.4 0.2 0.0 5000 5500 6000 6500 7000 Energy [eV] Fig. 4.52. Energy spectrum from an Fe 55 source after different 10 MeV proton flu- ences of 0 p/cm 2 (dotted line), 4.1 × 10 8 p/cm 2 (solid line), 6.1 × 10 8 p/cm 2 (dashed line), measured at the low (and after irradiation unfavorable) temperature of 142 K. The expected dose for the pn-CCD aboard XMM over a life time of 10 years is 5 × 10 8 10 MeV p/cm 2 layers. The same happens at the Si K edge at 1840 eV showing the fine struc- ture of a typical XAFS spectrum (see insert of Fig. 4.54). For all energies the quantum efficiency is nicely represented by a model using the photo absorp- tion coefficients from the atomic data tables. The quantum efficiency on the low energy side can be further improved with respect to the measurements shown in Fig. 4.54, by increasing the drift field at the p
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