31. January 2017

Electrons are the new photons

Hybrid photon-counting detectors have caused a major breakthrough in macromolecular crystallography. The new EIGER detectors for synchrotron applications feature a pixel size of 75 µm and continuous read-out at frame rates of up to 3000 Hz. In its first year in regular operation at MX beamlines, EIGER has already contributed to several outstanding publications.

Since EIGER was designed for noise-free single-event counting, Wasi Faruqi and Greg McMullan from the detector group at the MRC Laboratory for Molecular Biology in Cambridge (U.K.) wondered how it would perform with electrons instead of X-rays. Better performing detectors are considered a key to further improvements in the resolution of electron cryo-microscopy [1]. To find out how EIGER compares, we collaborated with Faruqi and McMullan to characterize an EIGER X 1M in a Philips CM200-FEG transmission electron microscope installed in the EM suite of the LMB. For all tests, the detector was operated at a frame rate of 2200 Hz and with its threshold tuned to between 5 and 50 keV. The microscope was set to electron energies between 40 and 200 keV.

Figure 1. Detective quantum efficiency of EIGER X 1M. The DQE of EIGER X 1M is shown for electron energies between 40 and 120 keV.

Figure 1. Detective quantum efficiency of EIGER X 1M. The DQE of EIGER X 1M is shown for electron energies between 40 and 120 keV.

 

Using the knife-edge method [2], we projected the shadow of a thin gold wire onto the surface of a specially calibrated EIGER X 1M detector. From the shadow images, McMullan determined the edge spread function and the modulation transfer function (MTF) of the detector. Processing the data in single-event mode and using noise binning [3], he then calculated the detective quantum efficiency (DQE, see figure) of EIGER. At energies of 80 and 100 keV, the DQE curves are very near the theoretical limit across all spatial frequencies. EIGER turns out to be the ideal counting detector for electrons with energies of up to 100 keV [4].

[1] W. Kühlbrandt. (2014). The resolution revolution. Science 343: 1443-4.

[2] A. Faruqi and R. Henderson. (2007). Electronic detectors for electron microscopy. Curr Opin Struct Biol 17: 549-55.

[3] R. Meyer and A. Kirkland (1998). The effects of electron and photon scattering on signal and noise transfer properties of scintillators in CCD cameras used for electron detection. Ultramicroscopy 75: 23-33

[4] Detective Quantum Efficiency of EIGER X 1M for Electron Microscopy. Manuscript in preparation.