Quantum efficiency of the pn-CCD for XMM and ABRIXAS

M. Popp, H. Soltau, R. Hartmann, L. Strüder

At the HLL Pasing the pn-CCD focal plane detectors for the X-ray satellite missions XMM and ABRIXAS were developed [1]. In order to have a better understanding of the system performance as a whole, this PhD work is devoted to build a model of the response function of the whole detector system under influence of its relevant parameter variations.

There are three main aspects of the output signal of the pn-CCD: the temporal response, which is mainly given by the readout scheme, the spectral response (the CCD is operated in single photon counting mode), including the quantum efficiency and the spatial response (how the image generated by the telescope is represented by signals in the pixels). These aspects are treated as modules.

In order to determine the quantum efficiency of the pn-CCD, calibration measurements were performed by R. Hartmann in collaboration with the Physikalisch Technische Bundesanstalt at the BESSY synchrotron facility in Berlin. Using a SX-700 Monochromator, spectra were taken oparating the pn-CCD in full frame single photon counting mode. They are compared to reference measurements taken directly before and after the CCD measurement under identical setup using a calibrated photodiode in DC mode. The number of photons seen by the CCD is compared via the energy-current calibration curve of the diode, which is provided by the PTB. The results are compared to earlier DC-mode quantum efficiency measurements performed by R. Hartmann [2].

Different types of entrance windows and substrate materials were tested, such as different implantation profiles of the (p-implanted) entrance window and < 100 > against < 111 > - material. Despite the different peak shape due to differences in charge collection efficiency, all of the different CCDs have a photon detection efficiency of more than 90 % over a wide energy band from 400 eV to 10 keV due to the very shallow p-implant. The influence of the Oxygen -K absorption edge on the detection efficiency can be clearly seen. Analysis is ongoing on this subject.

[1] H. Soltau et al., Performance of the pn-CCD X-Ray Detector System Designed for the XMM Satellite Mission, NIM A, Vol.377, 340-346, 1996
[2] R. Hartmann et al., Low energy response of silicon pn-junction detector, NIM A, Vol.377, 191-196, 1996

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