The direct measurement of the signal charge behavior in pnCCDs with subpixel resolution

In the MPI-Halbleiterlabor (MPI-HLL) we are developing fully depleted pn-type Charge Coupled Devices (pnCCD) with a thick radiation-sensitive region. Since 1997, the mesh experiment has been established as an unique technique to determine an X-ray interaction position with subpixel resolution. In order to investigate the signal charge behavior in pnCCDs, we performed the mesh experiment using various characteristic X-ray lines. Cu-L (0.9 keV), Al-K (1.5 keV) and Ti-K (4.5 keV) were used and the device operated under various conditions, e.g. register voltages. We obtained the restored pixel images for various X-ray event patterns. The photon entrance window of the back illuminated pnCCD shows highly uniform detection efficiency within a pixel. We measured the charge cloud size generated by an X-ray photon inside the CCD. The measured charge cloud sizes are around 6 μm (rms) depending on the X-ray energy. The operation with different register conditions revealed asymmetries and regions with a weaker electric field around pixel boundaries.