Unraveling Fano noise and partial charge collection effect in X-ray spectra below 1 keV

Fano and readout noise along with the partial charge collection (PCC) effect, are responsible for degrading the energy spectra in CCD measurements, especially at low energies. In this work, the X-ray produced by the fluorescence of F (677 eV) and Al (1486 eV) were recorded using a Skipper-CCD, which allowed to reduce the readout noise to 0.2e-. Based on an analytical description of the shape of photopeaks derived from the convolution of the PCC effect and Fano noise, we were able to accurately describe the energy spectra. This description accounts for lower energies tails,disentangling and quantifying the contributions of Fano noise and PCC effect. Thus, the Fano factor, electron-hole pair creation energy, and the PCC-region of the sensor were determined below 1 keV. Additionally, the skewness of photopeaks expected at low energies was also observed and experimentally determined for the first time.