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 mathe matical 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 len gth as well as the thickness of a dead layer below the front surface o f 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 obta ined using this method. However, it was found that the presence of a d ead layer a few tenths of a micrometer thick can pose severe restricti ons on the use of planar diode arrays in x-ray tomography systems wher e uniformity of response is crucial. The method has been applied to th e diode arrays equipping the x-ray tomography system built for the TCV tokamak, a magnetic fusion research device. (C) 1995 American Institu te of Physics.