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

TL;DR

This study analyzes the combined effects of Fano noise and partial charge collection on low-energy X-ray spectra in CCDs, achieving precise characterization and quantification of these effects using a Skipper-CCD with ultra-low readout noise.

Contribution

It provides a new analytical model for disentangling Fano noise and PCC effects in low-energy X-ray spectra, with experimental validation and novel observations of spectral skewness.

Findings

Quantified Fano factor and electron-hole pair creation energy.

Estimated PCC-region of the sensor.

First experimental observation of photopeak skewness at low energies.

Abstract

Fano noise, readout noise, and the partial charge collection (PCC) effect collectively contribute to the degradation of energy spectra in Charge Coupled Devices (CCD) measurements, especially at low energies. In this work, the X-ray produced by the fluorescence of fluorine (677 eV) and aluminum (1486 eV) were recorded using a Skipper-CCD, which enabled the reading noise to be reduced to 0.2 e-. Based on an analytical description of photopeak shapes resulting from the convolution of the PCC effect and Fano noise, we achieved a precise characterization of the energy spectra. This description enabled us to disentangle and quantify the contributions from both Fano noise and the PCC effect. As a result, we determined the Fano factor and the electron-hole pair creation energy. Additionally, we estimated the PCC-region of the sensor and, for the first time, experimentally observed the expected skewness of photopeaks at low energies.