Relative calibration of photodiodes in the soft‐x‐ray spectral range

A method of obtaining a relative calibration of Si photodiodes for the spectral range of soft x rays (1–30 keV) is presented. A simple mathematical model of the p‐n diode is adopted which allows the response to be described in terms of a small set of parameters. The diffusion length as well as the thickness of a dead layer below the front surface of the diodes are obtained from measurements of angular dependences of the photoinduced current. It is shown that a precise characterization of the diode response and an accurate relative calibration can be obtained using this method. However, it was found that the presence of a dead layer a few tenths of a micrometer thick can pose severe restrictions on the use of planar diode arrays in x‐ray tomography systems where uniformity of response is crucial. The method has been applied to the diode arrays equipping the x‐ray tomography system built for the TCV tokamak, a magnetic fusion research device.