Mesh experiment for back-illuminated CCDs in improvement of position resolution

Abstract

We have carried out a mesh experiment with back-illuminated (BI) CCDs. BI CCDs possess the same structure to those of front-illuminated (FI) CCDs. X-ray photons entering from the back surface of a CCD produce a primary charge cloud far from the electrodes. The cloud expands by diffusion process until it reaches the potential well just below the electrodes. Since the diffusion time of the charge cloud is longer than in the FI CCD, a larger charge cloud is expected. The mesh experiment enables us to specify the X-ray point of interaction with a subpixel resolution. We measured the charge cloud shape in the BI CCD. We found that there are two components of the charge cloud: a narrow component and a broad component. The size of the narrow component is 2.8– 5.7 μm and strongly depends on the attenuation length in Si of the incident X-rays. The shorter the attenuation length, the larger the charge cloud. This result is qualitatively consistent with a model of diffusion inside the CCD. On the other hand, the size of the broad component is independent of X-ray energy of ∼13 μm . We conclude that the narrow component is originated in the depletion region whereas the broad component is in the field-free region. Based on the charge cloud shape obtained, we determined the X-ray point of interaction within a pixel and found that the position accuracy of our method could be achieved up to 1.1±0.1 μm . Moreover, the fraction of events used to improve the position resolution can be increased by an order of magnitude. We can thus develop a high-throughput superfine X-ray resolution imager.