The noise performance of electron-multiplying charge-coupled devices at X-ray energies

Electron-multiplying charge-coupled devices (EM-CCDs) are used in low-light-level (L3) applications for detecting optical, ultraviolet, and near-infrared photons (10-1100 nm). The on-chip gain process is able to increase the detectability of any signal collected by the device through the multiplication of the signal before the output node. Thus, the effective readout noise can be reduced to subelectron levels, allowing the detection of single photons. However, this gain process introduces an additional noise component due to the stochastic nature of the multiplication. In optical applications, this additional noise has been characterized. The broadening of the detected peak is described by the excess noise factor. This factor tends to a value of √2 at high gain (>100x). In X-ray applications, the situation is improved by the effect that Fano factor f has on the shot noise associated with X-ray photon detection ( f ≈ 0.12} at X-ray energies). In this paper, the effect of the detection of X-ray photons in silicon is assessed both analytically and through a Monte Carlo model of the gain amplification process. The excess noise on the signal is predicted (termed the modified Fano factor) for photon detection in an EM-CCD at X-ray energies. A hypothesis is made that the modified Fano factor should tend to 1.115 at high levels of gain (> 10x). In order to validate the predictions made, measurements were taken using an ⁵⁵Fe source with Mn k-alpha X-ray energy of 5898 eV. These measurements allowed the hypothesis to be verified.