The depletion depth of high resistivity X-ray CCDs

We have developed several techniques which allow measurement of the depletion depth of X-ray CCDs used for imaging and spectroscopy in 0.2-10 keV energy band. These methods were developed as part of the calibration program for the AXAF CCD Imaging Spectrometer (ACIS). The depletion depth is a parameter that determines the high-energy detection limit of the CCDs. One technique is based on the analysis of long traces of high energy ionizing particles traveling through CCD nearly parallel to its surface. Measuring the ratio of the narrow portion of the trace to the broader diffused portion allows one to extract the depletion depth. Two other methods require illumination of the device with a monochromatic X-ray source (radioactive Fe⁵⁵, for instance). Analysis of the spectral distribution of the single- and multipixel events yields the depletion depth. By suitable choice of CCD operating voltages we were able to reach a depletion depth of 75 microns with the ACIS devices. We also present a simple analytical technique to calculate the depth of the depletion region in this essentially three dimensional structure