Imaging signals

Three important signals from the specimen are backscattered electrons, secondary electrons and x-rays x-rays are dealt with in Section 2.6. Back-scattered electrons are primary beam electrons which have been elastically scattered by nuclei in the sample and escape from the surface. The frac- 

Secondary electrons are emitted from the specimen with low energy, less than 50 eV, so they come from the top few nanometers of the material [29,36]. If the beam falls on a tilted surface or onto an edge, more secondaries will escape from the specimen (Fig. 2.5). If the beam falls into a 

Backscattered electrons travel in straight lines after leaving the specimen, so a detector placed to one side will give topographic contrast. The 

The detector in the SEM that is normally used for imaging is the Everhart-Thornley scintillator/photomultiplier (E-T detector). This is a low-noise, high-speed and efficient detector that detects a small fraction of the 

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Figure 15.3 Example of the use of a region of interest (ROI) on ToF SIMS images to obtain a more specific spectrum. On account of the selected area on the image, signals from fatty acids are emphasized on the selected spectrum (see colour Plate 3)...

Absolute Densities, Relative X-Ray Absorptions and Radiologic Image Signal to Noise Ratios of Several X-Ray Phosphors... 

The iron(III) tris-complex of Tiron (Fig. 13) has an R, value of 3.15 mM"1 s 1 at 60 MHz and 25 °C [163]. This value is higher than that reported for any other low-molecular weight iron(III) complex [10]. Injection of 0.15 mmol kg-1 into rats demonstrated maximum increases in the kidney and liver image signals of 137% and 73 %, respectively. No toxic effects were seen at this concentration, but all animals injected with 0.3 mmol kg1 of contrast agent died. 

None of these properties hold for the tip sample forces, the imaging signal of the AFM, and therefore substantial hurdles had to be overcome before atomic resolution by AFM became possible. 

With an AFM even insulating surfaces can be investigated with atomic resolution. The potential tip-sample energy Uts involves a z component of the tip-sample force Fts = —dVts/dz and a tip-sample spring constant kts = —dFts/dz. Depending on the mode of operation, the AFM uses Fts or some entity derived from l K as imaging signal. 

A 5-fold decrease in the image signal-to-noise ratio is assumed from 8 to 4 A. 

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