Detectors x-ray

Detectors are used to convert X-ray flux into an electrical signal, which can then be digitized and stored. For imaging cabinet X-ray systems, the detectors usually consist of a folded linear array of scintillators optically coupled to photodiodes. Typically, 500-1000 such detector elements are present for single-energy imaging 

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Due to large improvements in computer technology in combination with new designs of area x-ray detector systems it is possible to extend the 2D-CT systems up to the third dimension. Therefor special algorithms and techniques for 3D-CT of the measured projection data and 3D visualisation and measurement of the results had to be developed. 

In this paper a new design for a high-energy 3D-CT scanner equipped with a linear accelerator as radiation source and an area high-energy x-ray detector is presented. This system is the extension of a 2D system which is installed at present time [3,4]. 

The setup as seen in Figure 1 mainly consists of a Varian Linatron 3000A linear accelerator (LINAC) as radiation source, a rotational stage for sample manipulation, and a two-dimensional high-energy x-ray detector array consisting of four amorphous silicon area detectors Heimann RIS 256. The source to detector distance is 3.7 m. 

Modem NDT film systems (with Pb screens) are very linear X-ray detectors. This is shown in fig.l for different NDT film systems and a X-ray tube at 160 kV. Note that for histoncal reasons the film response curve is often plotted as film density versus log (radiation dose), which hides this linear relationship. The film density is the difference between the measured optical film density and the fog density Db of the film base. 

In the field of radiation methods of control, development work was performed in order to create the X-ray detectors with a low content of silver. X-ray TV systems with improved performance for automatic interpretation of the X-ray TV images, portable radiometers and dosimeters, creation of portable equipment for radioscopy of the welded joints of pipelines, etc. 

By inserting a semiconductor x-ray detector into the analysis chamber, one can measure particle induced x-rays. The cross section for particle induced x-ray emission (PIXE) is much greater than that for Rutherford backscattering and PIXE is a fast and convenient method for measuring the identity of atomic species within... 

Figure Bl.24.1. Schematic diagram of the target chamber and detectors used in ion beam analysis. The backscattering detector is mounted close to the incident beam and the forward scattering detector is mounted so that, when the target is tilted, hydrogen recoils can be detected at angles of about 30° from the beam direction. The x-ray detector faces the sample and receives x-rays emitted from the sample.

X-rays are collected and analy2ed in ema in one of two ways. In wds, x-rays are dispersed by Bragg diffraction at a crystal and refocused onto a detector sitting on a Rowland circle. This arrangement is similar to the production of monochromati2ed x-rays for xps described above. In the other approach, edx, x-rays are all collected at the same time in a detector whose output scales with the energy of the x-ray (and hence, Z of the material which produces the x-ray.) Detectors used for ema today are almost exclusively Li-drifted Si soHd-state detectors. 

Jin X-ray detector. Two different types of detectors are commonly used. 

Future developments of this instrumentation include field emission electron sources at 200-300 kV that will allow better elemental detectability and better spatial resolution. Multiple X-ray detectors having large collection angles will also improve elemental detectability in X-ray microanalysis. The higher accelerating... 

Often, more detailed information is needed on the distribution of a constituent. The technique of X-ray area scanning, or dot mappings can provide a qualirative view of elemental distributions. As the beam is scanned in a raster pattern on the specimen, a cathode ray rube scanned in synchronism is used to display a full white dot whenever the X-ray detector (WDS or EDS) detects an X ray within a certain narrow energy range. The pattern of dots is recorded on film to produce the dot map. Dot maps are subject to the following limitations ... 

The energy resolution of an X-ray detector is experimentally defined by the full width at half maximum (FWHM) of the Mn-Ka line. The FWHM, in eV, can also be calculated by use of the relationship ... 

In SXAPS the X-ray photons emitted by the sample are detected, normally by letting them strike a photosensitive surface from which photoelectrons are collected, but also - with the advent of X-ray detectors of increased sensitivity - by direct detection. Above the X-ray emission threshold from a particular core level the excitation probability is a function of the densities of unoccupied electronic states. Because two electrons are involved, incident and the excited, the shape of the spectral structure is proportional to the self convolution of the unoccupied state densities. 

X-rays are detected by observing an effect of their interaction with matter. The name x-ray detector came into use when such observations were predominantly qualitative. Nowadays, the emphasis is on high precision and efficiency so that most modern observations are measurements either of intensity or of dosage (x-ray quanta absorbed during exposure time). X-ray detector as a name has survived this change in emphasis although it does not describe the quantitative function of these devices. 

The effects in question are often translated into electric currents, pulsed or continuous. For the convenient reading or recording of these currents, complex electronic circuitry (2.3) may be needed. Modern methods of measuring x-ray intensity are therefore primarily a concern of the experimental physicist. Nevertheless, the analytical chemist must know something about them because x-ray detectors are now among the tools of his trade. This chapter, which cannot hope to do justice to modern x-ray detection, will attempt to provide him with an acceptable minimum of knowledge. 

At present, the Geiger counter is the most popular x-ray detector in analytical chemistry. Although it is yielding ground to the proportional counter and the scintillation counter, it will be remembered for having greatly accelerated the use of x-ray emission spectrography in analytical chemistry. 

The top and the bottom x-ray detector each contain a multiplier phototube coated with phosphor. This tube compares the intensity of the x-ray beam entering the detector with that of the light from the reference standard, a discharge lamp. The reference beam is part of a circuit that maintains the x-ray source at constant intensity. The deviation wedge comes to rest when the intensities of the transmitted x-ray beams stand in a predetermined ratio. At this point, the unbalance in the servo system has been compensated, and the position of the deviation wedge consequently indicates the thickness of the strip. In 1955, this application was made fully automatic that is, the unbalance (or error signal) just mentioned was used to readjust tandem cold reduction mills of the United States Steel Corporation. Automatic control proved significantly more effective than manual control. 

Here, as in the thickness gaging of steel (3.2) and in the blending of tetraethyllead fluid and gasoline (3.15), one may look forward to automation. In the present instance, if the tin plating is too thick or too thin, an error signal from the x-ray detector could be fed back into the system to produce an automatic adjustment, perhaps by changing the plating current. 

Slices of the body are irradiated from one side. X-ray detectors quantify the remaining intensity of the X-rays after passing through the various tissues. The X-ray tube and the detectors are rotating around the body. The data are used to calculate images of the corresponding slice which reflect the absorption of X-rays in a great number of pixels of an individual slice. 

X-ray microtomography is a new development of great promise for reconstructing, displaying, and analyzing three-dimensional microstructures. Resolution of around 1 pm has been demonstrated with currently available synchrotron sources of x-rays, x-ray detectors, algorithms, and large-scale computers. The potential for microstructural research in composites, porous materials, and suspensions at this and finer scales appears to be tremendous. 

Figure 4.14. Schematic set up of an electron microscope in the transmission (TEM) and the scanning (SEM) mode. The SEM instrument also contains an X-ray detector for composition analysis.

The literature on XRF is abundant. Recent general reviews are refs [235,237] for sample preparation see ref. [247]. EDXRF was specifically dealt with in ref. [248] and an excellent X-ray detector overview is available [225]. Several recent XRF monographs have appeared [233,249,249a], also covering TXRF [250] and quantitative XRF [232,251]. 

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