Scintillation SE detector for variable pressure scanning electron microscopes

We present results obtained with a new scintillation detector of secondary electrons for the variable pressure scanning electron microscope. A detector design is based on the positioning of a single crystal scintillator within a scintillator chamber separated from the specimen chamber by two apertures. This solution enables us to decrease the pressure to several Pa in the scintillator chamber while the pressure in the specimen chamber reaches values of about 1000 Pa (7.5 Torr). Due to decreased pressure, we can apply a potential of the order of several kV to the scintillator, which is necessary for the detection of secondary electrons. Simultaneously, the two apertures at appropriate potentials of the order of several hundreds of volts create an electrostatic lens that allows electrons to pass from the specimen chamber to the scintillator chamber. Results indicate a promising utilization of this detector for a wide range of specimen observations.     

A design for a simple electronic exposure meter for use with an electron microscope

A circuit has been developed to enable a simple yet sensitive exposure meter to be made for any electron microscope. The circuit is based around an inexpensive operational amplifier and will indicate a current range 10 ^?4?10 ^?11 A which corresponds from maximum illumination to a level of fluorescent screen illumination which is not detectable by eye. The exposure meter measures the charge on the fluorescent screen which is correlated to exposure time for photography.     

Automated analysis of submicron particles by computer-controlled scanning electron microscopy

Automated analysis of submicron particles by computer-controlled scanning electron microscopy is generally possible. The minimum diameter of the detectable particles is dependent on the mean atomic number of the particles and the operating parameters of the scanning microscope. The main limitation with regard to particle size is set by the quality of the particle detection system, which generally is the backscatter electron detector. The accuracy of the results of the x-ray analyses is very often strongly affected by specimen damage, omnipresent especially for environmental particles even at low electron energies and probe currents. With the exception for light elements, the detection limit is approximately 1 wt%. Device-related limitations to automated analysis may be specimen drift and an unreliable autofocus function.     

A very low energy compact electron beam ion trap for spectroscopic research in Shanghai

In this paper, a new compact low energy electron beam ion trap, SH-PermEBIT, is reported. This electron beam ion trap (EBIT) can operate in the electron energy range of 60-5000 eV, with a current density of up to 100 A/cm(2). The low energy limit of this machine sets the record among the reported works so far. The magnetic field in the central drift tube region of this EBIT is around 0.5 T, produced by permanent magnets and soft iron. The design of this EBIT allows adjustment of the electron gun's axial position in the fringe field of the central magnetic field. This turned out to be very important for optimizing the magnetic field in the region of the electron gun and particularly important for low electron beam energy operation, since the magnetic field strength is not tunable with permanent magnets. In this work, transmission of the electron beam as well as the upper limit of the electron beam width under several conditions are measured. Spectral results from test operation of this EBIT at the electron energies of 60, 315, 2800, and 4100 eV are also reported.     

Electrical Parameters in Drift Tubes for Ion Mobility Spectrometry

ABSTRACT

Electrical parameters in a drift tube for ion mobility spectrometry (IMS) were evaluated and optimized in resolution and signal intensity for a mixture of three aromatic hydrocarbons. Electric fields which influenced ion currents and peak shapes most were those between the aperture grid and detector plate and between neighboring wires in the ion shutter where optimum electric fields were 3000 and 300 V/cm, respectively. Electric fields on other drift tube components exhibited minor influences on IMS performance suggesting that electrical parameters do not need to be rigidly established throughout most regions of an IMS drift tube. Ion behavior in the IMS drift tube was consistent with models of potential contours for a conducting plane with circular hole.     

The quantitative analysis of thin specimens

Results are reported concerning the use of an energy dispersive X-ray detector to carry out the analysis of thin foils in the electron microscope. The combination of a thin specimen and the extreme stability of the energy dispersive X-ray detector enables the experimental determination of a calibration curve of X-ray production—detection efficiency vs characteristic X-ray energy. Quantitative analysis can be carried out using the calibration curve without reference to standards at the time of analysis.     

Calibration of an electron back-scattering pattern set-up

The determination of lattice orientations from electron back-scattering patterns (EBSPs) in a scanning electron microscope (SEM) requires accurate knowledge of the position of the pattern centre and the source point to screen distance. This paper outlines a new procedure that enables the determination of these parameters for any given set-up of the EBSP/SEM system. The calibration procedure simply requires the positions and indices of at least four poles in a pattern obtained from an arbitrary specimen, and eliminates the need for standard specimens or special attachments to the EBSP/SEM system. The pattern centre is shown to be located with a precision of approximately 0·5° and the source point to screen distance can be determined with a relative precision of approximately 0·5%.     

A routine correction for electron beam intensity variation during quantitative EDS microanalysis using continuum radiation

A detailed investigation of the relationship between the mean atomic number and the intensity of high energy continuum radiation has verified earlier studies in defining a linear relationship. Corrections for matrix effects on the continuum intensity are found to be unnecessary within the precision limits of the data. Incorporation of this relationship into a quantitative energy-dispersive spectrometer (EDS) microanalysis program allows internal correction for electron beam intensity variations between standard and unknown specimen analysis. It therefore allows quantitative analysis without normalisation on systems lacking any form of electron beam current measurement. Within limits, the need for reproducibility of electron beam intensity and counting time is eliminated.     

Is Scanning Electron Microscopy/Energy Dispersive X‐ray Spectrometry (SEM/EDS) Quantitative?

Scanning electron microscopy/energy dispersive X‐ray spectrometry (SEM/EDS) is a widely applied elemental microanalysis method capable of identifying and quantifying all elements in the periodic table except H, He, and Li. By following the “k‐ratio” (unknown/standard) measurement protocol development for electron‐excited wavelength dispersive spectrometry (WDS), SEM/EDS can achieve accuracy and precision equivalent to WDS and at substantially lower electron dose, even when severe X‐ray peak overlaps occur, provided sufficient counts are recorded. Achieving this level of performance is now much more practical with the advent of the high‐throughput silicon drift detector energy dispersive X‐ray spectrometer (SDD‐EDS). However, three measurement issues continue to diminish the impact of SEM/EDS: (1) In the qualitative analysis (i.e., element identification) that must precede quantitative analysis, at least some current and many legacy software systems are vulnerable to occasional misidentification of major constituent peaks, with the frequency of misidentifications rising significantly for minor and trace constituents. (2) The use of standardless analysis, which is subject to much broader systematic errors, leads to quantitative results that, while useful, do not have sufficient accuracy to solve critical problems, e.g. determining the formula of a compound. (3) EDS spectrometers have such a large volume of acceptance that apparently credible spectra can be obtained from specimens with complex topography that introduce uncontrolled geometric factors that modify X‐ray generation and propagation, resulting in very large systematic errors, often a factor of ten or more. SCANNING 35: 141‐168, 2013. ¹ Published 2012 Wiley Periodicals, Inc.     

Nanosecond response 'gasket-type' magnetic loop current monitor for relativistic electron beam current measurements

A fast response magnetic loop current monitor has been developed to measure relativistic electron beam return currents. The monitor has a rise time of about a nanosecond and a high degree of symmetry with moderate sensitivity, variable from about 1 to 10 V/kA. This simple monitor, with a thickness of 0.254 mm or less, is thin enough to be placed between segments of return current path in the diode or drift tube regions, is insensitive to flashover, beam and plasma bombardment, and radiation effects, and measures net current, thus offering some advantages over conventional magnetic probes, since the main components are outside of the vacuum region. Design criteria, an equivalent circuit analysis, and typical calibration waveforms are presented. Experimental current measurements for a pinched electron beam diode configuration using both conventional magnetic probes and 'gasket-type'current monitors with the FX-75 relativistic electron beam accelerator are presented.     

Parallel recording of electron energy loss spectra

Two silicon photo diode array devices were tested as parallel recording detectors for electron energy loss spectrometry (EELS). The direct bombardment of a Reticon photodiode array detector with high energy electrons (80 keV) causes an irreversible increase in diode dark current. The dark current saturates the detector amplifier after a dose of 10^?6 C/diode making it unsuitable for EELS. A scintillator coupled SIT vidicon is sensitive enough to count two high energy electrons with a spatial resolution of 100 μm, corresponding to 5 eV energy resolution with the electron optical system described. The large pixel-to-pixel gain variation inherent in the scintillator and vidicon can be reduced by averaging the spectrum over a large area of the target perpendicular to the dispersion direction. The L-edge of calcium for a 4 × 10^?3 weight fraction concentration biological specimen is observable in a 40 s parallel recorded spectrum. The minimum detectable concentration of calcium is estimated tobe ten times better for EELS than EDS X-ray analysis.     

A system for acquiring simultaneous electron energy-loss and X-ray spectrum-images

A compositional imaging system based on simultaneous scanning electron energy‐loss spectroscopy (EELS) and energy‐dispersive X‐ray spectroscopy (EDS) was developed. This system utilizes the combined power of EELS and EDS for quantitative compositional imaging at nanometre resolution. The system is particularly suitable for, but not limited to, biological research, as it simultaneously provides sensitive maps of an element such as Ca or P from EELS and of many other elements from EDS. Degradation of resolution by specimen drift is prevented by correcting for drift during data acquisition, using image cross‐correlation. Several advanced features are implemented for real‐time and/or off‐line quantitative analysis, and the performance of the system is illustrated with practical applications to compositional imaging of cardiac muscle.     

An atmospheric scanning electron microscope (ASEM)

A new detection configuration for the Scanning Electron Microscope (SEM) has been devised, which allows the imaging of the surfaces of a specimen in the open room, i.e., at atmospheric pressure. Such a device gives rise to a new microscope: the Atmospheric Scanning Electron Microscope (ASEM). In this configuration, a backscattered electron detector is placed between the pressure limiting aperture and the electron column. The electron beam passes through the final aperture, reaches the sample in the open room and the backscattered electrons passing through the same final aperture reach the detector. This principle has been tested and the result reported.     

Better visualization inside the environmental scanning electron microscope through the infrared chamberscope coupled with a mirror

Clearances are tight inside the specimen chamber of the environmental scanning electron microscope (ESEM), and it is difficult to see the relative positions of detectors and specimens through the viewport. For example, the relative placement of the energy-dispersive spectrometer (EDS) and the specimen is critical for attaining reasonable x-ray efficiency while protecting the detector window from damage. An infrared chamberscope and mirror attachment were added to improve the visibility inside the chamber.     

A method of measuring acoustic absorption coefficient of a material specimen using a dynamic microphone

This paper reports the development of a method for measuring the absorption coefficient of a material specimen mounted at one end of a planar wave tube using a dynamic microphone at the other end. In the proposed method, the dynamic microphone mounted is used as an actuator (loudspeaker) to generate sound waves and simultaneously performs as a probe to sense acoustic impedance at the same point. For the electro-mechanical acoustical system formed by the dynamic microphone and the tube, a “transduction matrix” is introduced to relate the input electrical variables (voltage and current) and the output acoustical variables (pressure and particle velocity). Once the matrix is calibrated, probing the input voltage and current to the dynamic microphone alone allows quantitative evaluation of the acoustic impedance of material specimen, from which absorption coefficient of the material is calculated. Measurements of fully-reflected end, anechoic end and a porous material specimen are carried out and compared to the results obtained by the conventional transfer function method. It is found that the results match well with each other in a frequency range depending on the length of the tube.     

A high-speed area detector for novel imaging techniques in a scanning transmission electron microscope

A scanning transmission electron microscope (STEM) produces a convergent beam electron diffraction pattern at each position of a raster scan with a focused electron beam, but recording this information poses major challenges for gathering and storing such large data sets in a timely manner and with sufficient dynamic range. To investigate the crystalline structure of materials, a 16×16 analog pixel array detector (PAD) is used to replace the traditional detectors and retain the diffraction information at every STEM raster position. The PAD, unlike a charge-coupled device (CCD) or photomultiplier tube (PMT), directly images 120–200 keV electrons with relatively little radiation damage, exhibits no afterglow and limits crosstalk between adjacent pixels. Traditional STEM imaging modes can still be performed by the PAD with a 1.1 kHz frame rate, which allows post-acquisition control over imaging conditions and enables novel imaging techniques based on the retained crystalline information. Techniques for rapid, semi-automatic crystal grain segmentation with sub-nanometer resolution are described using cross-correlation, sub-region integration, and other post-processing methods.     

A confocal laser microscope scanner for digital recording of optical serial sections

A confocal laser microscope scanner developed at our institute is described. Since an ordinary microscope is used, it is easy to view the specimen prior to scanning. Confocal imaging is obtained by laser spot illumination, and by focusing the reflected or fluorescent light from the specimen onto a pinhole aperture in front of the detector (a photomultiplier tube). Two rotating mirrors are used to scan the laser beam in a raster pattern. The scanner is controlled by a microprocessor which coordinates scanning, data display, and data transfer to a host computer equipped with an array processor. Digital images with up to 1024 × 1024 pixels and 256 grey levels can be recorded. The optical sectioning property of confocal scanning is used to record thin (～ 1 μm) sections of a specimen without the need for mechanical sectioning. By using computer-control to adjust the focus of the microscope, a stack of consecutive sections can be automatically recorded. A computer is then used to display the 3-D structure of the specimen. It is also possible to obtain quantitative information, both geometric and photometric. In addition to confocal laser scanning, it is easy to perform non-confocal laser scanning, or to use conventional microscopic illumination techniques for (non-confocal) scanning. The design has proved reliable and stable, requiring very few adjustments and realignments. Results obtained with this scanner are reported, and some limitations of the technique are discussed.     

A low cost image and data acquisition system for use with the Vickers M85 scanning microdensitometer

A simple and inexpensive interface has been constructed between the Vickers M85 microdensitometer and a BBC model B microcomputer. The interface incorporates three sensitivity ranges and enables the production of pseudocolour images of the specimen using the two-dimensional scanning mode of the M85. The operator can select a 160times256 pixel image with eight colours or a 320times256 display using only four colours. Each colour represents a defined range of transmittance which is software controlled. The image histogram can be displayed and the interval between colours redefined so as to enable contrast stretching. Intervals between colours can be either linear or logarithmic and the images thus obtained can be stored on disc or videotape, or a hard copy can be obtained using a screen dump routine. Two-dimensional absorption images can thus be obtained at any single wavelength from 400 to 700 nm at normal magnifications of the light microscope. In addition, the system can be used to acquire, store and process data from one-dimensional scans to obtain quantitative information about variations in optical density within the specimen, so considerably increasing the usefulness of the instrument. Although obviously limited in its capabilities, the system produces images of very high quality and one-dimensional data of high sensitivity. The interface can be constructed for less than £40. A small modification to one of the M85 circuit boards is necessary to obtain maximum resolution.     

Simulation of transmission electron microscope images of biological specimens

We present a new approach to simulate electron cryo‐microscope images of biological specimens. The framework for simulation consists of two parts; the first is a phantom generator that generates a model of a specimen suitable for simulation, the second is a transmission electron microscope simulator. The phantom generator calculates the scattering potential of an atomic structure in aqueous buffer and allows the user to define the distribution of molecules in the simulated image. The simulator includes a well defined electron–specimen interaction model based on the scalar Schrödinger equation, the contrast transfer function for optics, and a noise model that includes shot noise as well as detector noise including detector blurring. To enable optimal performance, the simulation framework also includes a calibration protocol for setting simulation parameters. To test the accuracy of the new framework for simulation, we compare simulated images to experimental images recorded of the Tobacco Mosaic Virus (TMV) in vitreous ice. The simulated and experimental images show good agreement with respect to contrast variations depending on dose and defocus. Furthermore, random fluctuations present in experimental and simulated images exhibit similar statistical properties. The simulator has been designed to provide a platform for development of new instrumentation and image processing procedures in single particle electron microscopy, two‐dimensional crystallography and electron tomography with well documented protocols and an open source code into which new improvements and extensions are easily incorporated.