Comparison of methods of measuring the primary charge-cloud shape produced by an X-ray photon inside the CCD

We report here the comparison of two methods of directly measuring the charge-cloud shapes produced by X-ray photons inside a CCD. The measurements are performed using a mesh technique in which we can confine the X-ray interaction location with subpixel resolution. There are two methods: a DD (double differential) method employs all X-ray events and a CG (center of gravity) method employs only split events. The DD method reveals the mean charge-cloud shape generated in a relatively shallow region, while the CG method reveals one generated in a relatively deep region. We performed the measurement using Al–K X-rays and Mo–L X-rays. The charge-cloud sizes generated by these X-rays are 0.7∼1.7 μm (standard deviation). The charge-clouds clearly show asymmetric shape, elongated perpendicular to the charge transfer direction. This is probably due to the nonuniformity of the electric field inside the CCD.