Direct measurement of the charge cloud size in SIS CCDs

Abstract

We report here the method to measure the primary charge cloud size generated by X-ray photons in the CCD. If the incident X ray is photo-absorbed near the pixel boundary, the primary charge cloud splits into 2–4 pixels. We calculated the center of gravity of the split event weighted by the split charge. We found that the center of gravity is well-correlated with the X-ray interaction position. We analyzed the charge cloud size assuming either the Gaussian shape or the rectangular shape. From the analysis of the data using the model for the charge cloud shape, we measured a charge cloud size of 1∼2μm for X rays of Y L (1.9 keV), Ag L (3.0 keV) and Ti K (4.5keV). We also notice that we can determine the X-ray interaction position of the split event with an accuracy better than 1/10 of the pixel size.