Characterizing Charge Diffusion in CCDs with X-Rays

We demonstrate the effectiveness of two techniques for using X-rays to evaluate the amount of charge diffusion in charge-coupled devices (CCDs). We quantify the degree of charge diffusion with two parameters: σ_d, the standard deviation for a Gaussian diffusion model, and Q, a ratio of the point-spread function (PSF) peak to its wings. The parameters σ_d and Q are determined by fitting a model to a pixel energy histogram and by summing the PSF of all X-ray events, respectively. Using seven test devices, we investigate the precision of these two techniques and demonstrate that they produce compatible results. The histogram-fitting method is sensitive to the structure of the electric field within these devices, in addition to the inherent charge diffusion properties. The Q-ratio is a very simple parameter to measure, and it provides an easily accessible method for quickly evaluating a CCD's diffusion length.