Improvement of position resolution of CCDs: mesh experiment for small-pixel and back-illuminated CCDs

Abstract

We have employed the mesh experiment for the front-illuminated (FI)CCD having small pixel size of 8μm and for the back-illuminated(BI) CCD having pixel size of 24μm. BI CCDs possess the same structure as the FI CCDs. Since X-ray photons enter from the back surface of the CCD, the primary charge cloud is formed far from the electrodes. The primary charge cloud expands through diffusion process until they reach the potential well which is just below the electrodes. Therefore, the diffusion time for the charge cloud produced by X-rays is longer than those in the FI CCD, resulting the larger charge cloud shape to be expected. The mesh experiment enables us to specify the X-ray point of interaction with a subpixel resolution. We then have measured a charge cloud shape produced in the FI CCD as well as the BI CCD. We found that there are two components of the charge cloud shape having different size: a narrow component and a broad component for both CCDs. The size of narrow component obtained with the FI CCD is 0.6-1.4μm in unit of a standard deviation which is consistent with the previous experiments with FI CCD whole pixel size is 24μm. For the BI CCD, the size of the narrow component is 2.8-5.7μm and strongly depends on the attenuation length in Si of incident X-rays. The shorter the attenuation length of X-rays is, the larger the charge cloud becomes. 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 of ≈ 13μm 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 is originated in the depletion region whereas the broad component is in the field-free region. Taking into account the charge cloud shape obtained, we calculated the X-ray point of interaction for all X-ray events. We estimated the uncertainty of the position resolution to compare it with the location of the mesh hole. We then obtained the position resolution of 1 μm for both CCDs which is similar value of the previous results whereas the fraction of split pixel event becomes roughly three times and an order of magnitude larger than previous results. We can thus develop the X-ray spectroscopic detector having a micron order position resolution with a high throughput.