Comparison of optimal performance at 300keV of three direct electron detectors for use in low dose electron microscopy.

G Mcmullan,D Clare,A.R Faruqi,R Henderson

doi:10.1016/j.ultramic.2014.08.002

Abstract

Low dose electron imaging applications such as electron cryo-microscopy are now benefitting from the improved performance and flexibility of recently introduced electron imaging detectors in which electrons are directly incident on backthinned CMOS sensors. There are currently three commercially available detectors of this type: the Direct Electron DE-20, the FEI Falcon II and the Gatan K2 Summit. These have different characteristics and so it is important to compare their imaging properties carefully with a view to optimise how each is used. Results at 300 keV for both the modulation transfer function (MTF) and the detective quantum efficiency (DQE) are presented. Of these, the DQE is the most important in the study of radiation sensitive samples where detector performance is crucial. We find that all three detectors have a better DQE than film. The K2 Summit has the best DQE at low spatial frequencies but with increasing spatial frequency its DQE falls below that of the Falcon II.

Highlights

Electron microscope images were originally recorded on photographic film and more recently electronically using detectors based on phosphor/fibre-optic CCD technology
Detector performance is of particular importance in the study of radiation sensitive samples such as in electron cryo-microscopy, where the signal-to-noise ratio in images is inherently poor due to the limited number of electrons that can be used before radiation damage is too great
In this paper we present modulation transfer function (MTF) and detective quantum efficiency (DQE) measurements of the three currently available backthinned monolithic active pixel sensors (MAPS) detectors that offer improvements over photographic film in terms of DQE at 300 keV namely: the Direct Electron DE-201; the FEI Falcon II2; and the Gatan K2 Summit

Summary

Introduction

Electron microscope images were originally recorded on photographic film and more recently electronically using detectors based on phosphor/fibre-optic CCD technology. These work well for electron energies in the 80–120 keV range but at higher electron energies their performance drops. Higher electron energies are necessary with thicker samples and advantageous in looking at insulators, such as ice embedded biological samples, due to the reduced sensitivity to sample charging. The shorter wavelength results in improved electron optics and simpler interpretation of the resulting images. Detector performance is of particular importance in the study of radiation sensitive samples such as in electron cryo-microscopy (cryoEM), where the signal-to-noise ratio in images is inherently poor due to the limited number of electrons that can be used before radiation damage is too great. The amount of additional noise added by a detector is measured by its detective quantum efficiency (DQE) which is defined [1] as the square of the ratio of the output signal-to-noise, SNRo, to that of the input, SNRi, i.e., DQE 1⁄4 SNR2o=SNR2i : ð1Þ

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