Application of the Mesh Experiment for the Back-Illuminated Charge-Coupled Device: I. Experiment and the Charge Cloud Shape

Abstract

We have employed a mesh experiment for back-illuminated (BI) charge-coupled devices (CCDs). BI CCDs possess the same structure as front-illuminated (FI) CCDs. Since X-ray photons enter from the back surface of the CCD, a primary charge cloud is formed far from the electrodes. The primary charge cloud expands through a diffusion process until it reaches the potential well that is just below the electrodes. Therefore, the diffusion time for the charge cloud produced is longer than that in the FI CCD, yielding a larger charge cloud size than expected. The mesh experiment enables us to specify the X-ray point of interaction with subpixel resolution. We measured a charge cloud shape produced in the BI CCD. We found that there are two components of the charge cloud shape having different sizes: a narrow component and a broad component. The size of the narrow component is 2.8–5.7 µm in units of standard deviation and strongly depends on the attenuation length of incident X-rays in Si. The shorter the attenuation length of the X-rays, the larger the charge cloud. This result is qualitatively consistent with a diffusion model inside the CCD. On the other hand, the size of the broad component is roughly constant at ≃13 µm and does not depend on X-ray energies. Judging from the design value of the CCD and the fraction of each component, we conclude that the narrow component has its origin in the depletion region whereas the broad component originates in the field-free region.