Silicon Photodiodes for Absolute Soft X-ray Radiometry

Absolute spectroscopic photocurrent calibration of detectors in the photon energy range of 50-6000 eV is performed at beamlines U3c and X8a of the National Synchrotron Light Source at Brookhaven National Laboratory. These beamlines are specifically designed to provide high flux over a wide energy range, with particular attention paid to harmonic purity of the monochromatic beam. Examples of optics which enhance the beam purity include transmission foils and grazingincidence mirrors. The AXUV-100G silicon photodiode available from IRD is used as a reference detector. Its relatively simple design, and the availability of x-ray optical data for silicon and silicon dioxide, permit application of the “self-calibration” method of estimating absolute response, typically to 2% accuracy or better. Characteristics of typical such diodes will be described, including spectroscopic responsivity and models, lot matching, electrical characteristics, visible and infrared light responsivity, and soft x-ray photon-induced damage.