Detector geometry

The selection of an instrument is not just a question of comparing specification sheets. Furthermore, since vendors usually include legal wording in their brochures that allows them to change components (detector geometry, plumbing details, electronics, etc.) without notification, even two instruments of the same make, model, and nominal performance... 

Simulation spectra were generated using parameters that describe the ion beam, target and detector geometry, beam and detector resolution, and sample characteristics. The sample parameters, which include the number of layers and the areal density and atomic composition of each layer were then varied until the simulation conformed to the experimental data. The HIBS spectra were analysed using a modified version of the RBS analysis program. 

It is worthwhile at this point, therefore, to examine in more detail some of the electron scattering mechanisms which govern the various signals involved and to show how detector geometries may be optimized for maximum sensitivity. 

Figure 4 shows the coordinate systems associated with the example shown in Figure 3 The horizontal axis is x, and the vertical direction is y. The conveyor belt is perpendicular to the y axis and moves in a direction into the page. The disk rotation angle, 9, is measured counter-clockwise from the y-axis. This example has 501 detectors in a straight hne, which is defined as the 5 direction. The straight hnes running from the source to the detectors represent rays of radiation detected at each detector location. There are 501 such rays that the figure represents with 21 hnes. (The detector geometry is often modified to place individual detectors along an arc of a circle centered on the X-ray source.)...

The Heat of Adsorption Detector, devised by Claxton (16) in 1958 has been Investigated by a number of workers (17,18,19) but although once commercially available, has not been extensively employed as an LC detector. One reason for this is the curious and apparently unpredictable shape of the temperature-time curve that results from the detection of the usual Gaussian or Poisson concentration peak profile. The shape of the curve changes with detector geometry, the operating conditions of the chromatograph, the retention volume of the solute and for closely eluted peaks, it produces a complex curve that is extremely difficult to interpret. 

The typical resolution of a single detector is 20 keV but depends on the detector geometry, in particular, on the detector capacitance. Notice that SSB detectors have parallel electrodes separated by a thin dielectric the capacitance of such an object will increase with increasing area and with decreasing thickness. Thus, thin large-area devices will have the largest capacitance and thus the poorest resolution. 

B. Grass, D. Siepe, A. Neyer and R. Hergenroder, Comparison of different conductivity detector geometries on an isotachophoresis PMMA-mi-crochip, Fresenius J. Anal. Chem., 371 (2001) 228-233. 

A consistent protocol for the collection and analysis of thin-film EDS data requires an assessment of both instrument and specimen dependent parameters. Major parameters which should be considered for thin-film analyses include spurious X-rays, spectral artifacts, detector geometry, probe diameter, beam broadening, contamination, sample preparation artifacts, sample orientation and temperature and X-ray absorption. Many of these parameters are interdependant during an analysis and the prudent operator will evaluate as many as possible before routine use of an AEM. Further explanations of these parameters can be found in a number of publications [4,6.,9.,7] Only selected parameters are discussed below. 

A further example of the importance of cell and detector geometry is provided by the work of Clyne et al. [90—95] in measuring the lifetimes of Cl2 (B—X). Initial studies [90—93] were carried out in a fluorescence cell of 7.5 cm radius and at pressures < 1 mTorr. The measured lifetime in these studies was 85 pis. For a lifetime of this magnitude, the diameter of the fluorescence cell is sufficiently large for negligible errors to be encountered. However, the use of a powerful lens system to focus the... 

The electric field E(r) in the detector can be calculated from known quantities applied bias voltage, detector geometry, and resistivity of the bulk material. Once the... 

The detector geometries also result in different energy resolutions especially for lower y-ray energies. This is shown in Fig. 5.33. Typical absolute efficiency curves for various Ge detectors in the Marinelli-beaker configuration are shown in Fig. 5.34, while Fig. 5.35 shows typical absolute efficiency curves for various Ge detectors with 2.5 cm source to the end-cap spacing. 

The Lorentz and polarization corrections,often called Lp, are geometrical corrections made necessary by the nature of the X-ray experiment. The Lorentz factor takes into account the different lengths of time that the various Bragg reflections are in the diffracting position. This correction factor differs for each type of detector geometry. For example, the Lorentz correction for a standard four-circle diffractometer... 

A significant amount of literature regarding the antioxidant properties of flavonoids and other plant polyphenols is available. As the essence of redox chemistry involves electron transfer, it seems natural that electrochemical detection rivals spectrophotometric detection techniques for the compounds that are supposed to be antioxidants. With the improvements in electrochemical detector geometries and electronics over the last decade, coupled with a requirement for increased sensitivity, the use of electrochemical detectors offers significant additional advantages when combined with the traditional UV-VIS detection in the analysis of flavonoids and other plant polyphenols. ... 

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