Electronic detectors for electron microscopy

Abstract

Electron microscopy (EM) is an important tool for high-resolution structure determination in applications ranging from condensed matter to biology. Electronic detectors are now used in most applications in EM as they offer convenience and immediate feedback that is not possible with film or image plates. The earliest forms of electronic detector used routinely in transmission electron microscopy (TEM) were charge coupled devices (CCDs) and for many applications these remain perfectly adequate. There are however applications, such as the study of radiation-sensitive biological samples, where film is still used and improved detectors would be of great value. The emphasis in this review is therefore on detectors for use in such applications. Two of the most promising candidates for improved detection are: monolithic active pixel sensors (MAPS) and hybrid pixel detectors (of which Medipix2 was chosen for this study). From the studies described in this review, a back-thinned MAPS detector appears well suited to replace film in for the study of radiation-sensitive samples at 300 keV, while Medipix2 is suited to use at lower energies and especially in situations with very low count rates. The performance of a detector depends on the energy of electrons to be recorded, which in turn is dependent on the application it is being used for; results are described for a wide range of electron energies ranging from 40 to 300 keV. The basic properties of detectors are discussed in terms of their modulation transfer function (MTF) and detective quantum efficiency (DQE) as a function of spatial frequency.

A Medipix quantum area detector allows rotation electron diffraction data collection from submicrometre three-dimensional protein crystals

“…In this fashion its noise is almost exclusively determined by the counting statistics of the electrons. This gives a significant improvement over conventional CCD cameras in electron microscopes (Faruqi & McMullan, 2011;Georgieva et al, 2011).…”

Protein structure determination by electron diffraction using a single three-dimensional nanocrystal

“…For 200 and 300 keV electrons the potential scattering distances are approximately 225 and 450 mm, respectively (McMullan et al, 2007(McMullan et al, , 2009Faruqi & McMullan, 2011). For the prototype, we used a 300 mm sensitive silicon layer.…”

“…However, because of the perpendicular impact of a 300 keV incident electron on the detector, on average the first pixel and the last pixel of its track receive the highest deposited dose. This means that at the energy threshold used for each pixel ($60 keV), the electron is counted once (70%) or twice (30%) (McMullan et al, 2007(McMullan et al, , 2009Faruqi & McMullan, 2011). This means that the Bragg spot is spread out over a larger area.…”

“…Since electron diffraction data can have spikes of high intensity at low resolution and in Bragg peaks, monolithic detectors are currently not used for measuring electron diffraction data. However, in hybrid pixel detectors the high-energy electrons are stopped by the silicon sensor layer that is bump-bonded to the counting and integration electronics (McMullan et al, 2007(McMullan et al, , 2009Faruqi & McMullan, 2011). The integrating electronics of CMOS detectors can be shielded by a phosphor, at the expense of an increased point spread.…”

The properties of SIRT, TVM, and DART for 3D imaging of tubular domains in nanocomposite thin-films and sections

“…Finally, the filtered projection is recorded by the TEM electron detector, i.e., a charge-coupled device (CCD) camera. The recording process introduces shot noise and CCD noise [48][49][50]. The intensity of the final projection is calculated by adding both shot noise and CCD noise to the filtered projection.…”