HOW BIG ARE CHARGE CLOUDS INSIDE THE CHARGE-COUPLED-DEVICE PRODUCED BY X-RAY PHOTONS

We report here the charge cloud shape produced by an X-ray photon insi de the charge-coupled device (CCD) as well as a method to measure it. The measurement is carried out by using a multi-pitch mesh which enabl es us to specify the interaction position of X-rays with subpixel reso lution not only for single events but also for split events. Split eve nts are generated when the X-ray interaction position is close to the pixel boundary. The width of this area depends on the apparent charge size. Finally we measured the signal output from the pixel according t o the interaction position of X-rays. By differentiating this function , we obtain, in detail, the charge cloud shape which can be well repre sented by an asymmetric Gaussian function. The charge cloud size for A l-K X-rays is 0.7 x 1.4 mu m(2) while that for Mo-L X-rays is 0.8 x 1. 4 mu m(2). The size of the photoelectron in Si produced by these X-ray s is about 0.04 mu m. Taking into account the mean absorption length f or these X-rays in Si, diffusion process in the depletion region canno t explain the charge cloud size. The asymmetry of the charge cloud pro bably arises from the asymmetry of the electric field in the CCD.