Background studies of the pn-CCD detector of CAST

The CERN Axion Solar Telescope (CAST) experiment searches for axions from the Sun converted into photons with energies up to around 10 keV via the inverse Primakoff effect in the high magnetic field of a superconducting Large Hadron Collider (LHC) prototype magnet. A backside illuminated pn-CCD detector in conjunction with an X-ray mirror optics is one of the three detectors used in CAST to register the expected photon signal. Since this signal is very rare a detailed study of the detector background has been undertaken with the aim to understand and further reduce the background level of the detector. The analysis is based on measured data taken during the data taking period of 2003 and 2004 of CAST and on Monte Carlo simulations of background with different origin. A background study performed for this detector show that the level of background (8.00±0.07)x10 counts cm s keV between 1 and 7 keV is dominated by the external gamma background due to natural activities at the experimental site, while radioactive impurities in the detector itself and cosmic neutrons contribute with a smaller fraction.