The radiation entrance window of pn junction detectors

The performance of high-resolution, fully depleted silicon detectors at low photon energies has been found to be dominated by incomplete charge collection due to the pn junction entrance window. Two alternative experimental methods to characterize this behaviour have been investigated by using monochromatized synchrotron radiation at the radiometry laboratory of the Physikalisch-Technische Bundesanstalt with the electron storage ring BESSY: the measurement of the de photocurrent relative to a calibrated diode and the measurement of the energy-dispersive response function in the single photon counting mode. Both methods are discussed and compared with a theoretical model including a microscopic description of the photoelectric absorption and a charge-collection probability function.