The effects of electron and photon scattering on signal and noise transfer properties of scintillators in CCD cameras used for electron detection

The detection properties of scintillators used in charge-coupled device cameras suitable for electron microscopy are examined with particular emphasis on the statistics of electron scattering and photon generation in the scintillator. We show that the root of the power spectrum of an evenly illuminated white noise image is in general not equal to the modulation transfer function (MTF) of the scintillator, as the former corresponds to the statistical properties of the detectable light intensity caused by single incident electrons while the latter corresponds to the statistical properties of the detected intensity caused by many electrons incident on the same point. A difference between these statistical properties leads to over-optimistic estimations of the MTF when the noise method is used and furthermore deteriorates the detection quantum efficiency (DQE) at high spatial frequencies. Monte Carlo simulations are used to calculate the true MTF, the expected outcome of a noise method measurement (MTFnoise) and the spatial frequency dependent DQE for various scintillator thicknesses and acceleration voltages.