Two-dimensional detector

Therefore it is reasonable to prepare already the data acquisition for a three dimensional evaluation in cone-beam-technique by means of two-dimensional detectors. The system is already prepared to integrate a second detector- system for this purpose. An array of up to four flat panel detectors is foreseen. The detector- elements are based on amorphous silicon. Because of the high photon energy and the high dose rates special attention was necessary to protect the read-out electronics. Details of the detector arrangement and the software for reconstruction, visualisation and comparison between the CT results and CAD data are part of a separate paper during this conference [2]. 

The potential of LA-based techniques for depth profiling of coated and multilayer samples have been exemplified in recent publications. The depth profiling of the zinc-coated steels by LIBS has been demonstrated [4.242]. An XeCl excimer laser with 28 ns pulse duration and variable pulse energy was used for ablation. The emission of the laser plume was monitored by use of a Czerny-Turner grating spectrometer with a CCD two-dimensional detector. The dependence of the intensities of the Zn and Fe lines on the number of laser shots applied to the same spot was measured and the depth profile of Zn coating was constructed by using the estimated ablation rate per laser shot. To obtain the true Zn-Fe profile the measured intensities of both analytes were normalized to the sum of the line intensities. The LIBS profile thus obtained correlated very well with the GD-OES profile of the same sample. Both profiles are shown in Fig. 4.40. The ablation rate of approximately 8 nm shot ... 

Fig. 1. Schematic diagram of the multimass ion imaging detection system. (1) Pulsed nozzle (2) skimmers (3) molecular beam (4) photolysis laser beam (5) VUV laser beam, which is perpendicular to the plane of this figure (6) ion extraction plate floated on V0 with pulsed voltage variable from 3000 to 4600 V (7) ion extraction plate with voltage Va (8) outer concentric cylindrical electrode (9) inner concentric cylindrical electrode (10) simulation ion trajectory of m/e = 16 (11) simulation ion trajectory of rri/e = 14 (12) simulation ion trajectory of m/e = 12 (13) 30 (im diameter tungsten wire (14) 8 x 10cm metal mesh with voltage V0] (15) sstack multichannel plates and phosphor screen. In the two-dimensional detector, the V-axis is the mass axis, and V-axis (perpendicular to the plane of this figure) is the velocity axis (16) CCD camera.

The fragment recoil velocity resolution depends on the divergence of the molecular beam, molecular beam velocity distribution in the direction of the molecular beam axis, and the distance of fragments expanded in the velocity axis of the two-dimensional detector. If the divergence of the molecular beam is small and the fragment recoil velocity is much larger than the velocity difference of parent molecules, the recoil velocity resolution can be simply expressed as AV/V = s/L, where L is the length of expansion of... 

As modern one- or two-dimensional detectors are used, every pixel of the detector is enforcedly receiving the same exposure (time). Only by means of an old-fashioned zero-dimensional detector the scattering curve can be scanned in such a manner that every pixel receives the same number of counts with the consequence that the statistical noise is constant at least in a linear plot of the SAXS curve. The cost of this procedure is a recording time of one day per scattering curve. 

Unlike mapping, the detector used for infrared chemical imaging is a two-dimensional detector array. The infrared focal-plane array (FPA) detector, the long-wavelength analog of a typical digital camera, was... 

Having optimised the efficiency of a chromatographic separation the quality of the chromatography can be controlled by applying certain system suitability tests. One of these is the calculation of theoretical plates for a column and there are two other main parameters for assessing performance peak symmetry and the resolution between critical pairs of peaks. A third performance test, the peak purity parameter, can be applied where two-dimensional detectors such as diode or coulometric array or mass spectrometry detectors are being used. The reproducibility of peak retention times is also an important parameter for controlling performance. 

Picture of a CCD deteetor (top) and sehematie diagram of the two-dimensional detector array with respect to wavelength and order (bottom) (reproduced with permission from the Perkin Elmer Corporation). 

The use of the Z-technology [1 -3] results in a very compact detector unit. The detector unit is made by connecting a two-dimensional detector array to an end of a module formed by a layered structure. The layered structure comprises means for reading-out information from the detector array. 

A two-dimensional detector array is shown above, in which the technique described above is used to form detector elements 36 each associated with one electronic circuit 37. A metallization 38 provides a common connection 35. In this case the HgCdTe layer 9 and the CdTe buffer layer 8 are not removed at the region corresponding to the metallization 38. 

Figure 1 gives a schematic drawing of the basic setup used in the GISAXS experiments. The two-dimensional detector is only recording the intensity reflected above the sample surface. The direct beam is not recorded with the detector to avoid detector saturation as several orders of magnitude in intensity separate the incoming intensity from the reflected one. In addition the specularly reflected peak (condition af = at) is shielded with a beam stop to... 

So far only one alkyne-substituted (387) and two vinylic cluster derivatives (186, 388) have been analyzed by neutron diffraction. Hopefully, the development of high-energy neutron sources and two-dimensional detectors over the next few years will see an advance in this area of structural chemistry. 

In addition to this information, however, the diffraction pattern also provides information on the quality of the crystal lattice and the thermal motion of the atoms in the unit cell. Figure 3 shows examples of the diffraction patterns that would be observed under various circumstances for the lattice shown in Fig. 2. The examples assume diffraction from the (010) and (100) planes of the lattice that are, respectively, planes parallel to the jcz-plane (Fig. 2A) and yz-plane (not shown). A diffraction pattern for a crystalline sample is recorded by rotating a crystal in the x-ray beam to record systematically the reflections from the various lattice planes. To see the diffracted intensities for the example under consideration, the crystal would be mounted with its z-axis parallel to the x-ray beam and then rotated about the jr-axis to obtain reflections from the (010) planes with spacings of d(0k0) and about the y-axis to obtain reflections from the (100) planes with spacings of d h00). All of the reflections are recorded on a single frame of a two-dimensional detector or a single piece of photographic film. 

Oriented polymers will no longer show a symmetric scattering pattern. For such systems a two dimensional detector is necessary. A commercial data aquisition system (Westinghouse), based on a Vidicon tube, is shown in Fig. 32. The video signal... 

The various read-out techniques, as described before, for linear PSD s can be applied for two-dimensional systems too. One should however bear in mind that, when complete diffraction patterns have to be measured, the conting rate capability plays an even more important role. A linear detector is exposed to only a fraction of the total flux of a pattern, as compared to a two-dimensional detector, which is intended to measure the entire area at once. 

In many applications full two-dimensional information is necessary, and thus fast read-out systems have to be provided. The various techniques, which have been discussed before, have been applied with area detectors. The delay-line read-out has been used with two-dimensional detectors in synchrotron radiation laboratories. At the DORIS-ring in Hamburg an area detector with a 1 mm anode wire distance and a total area of 200 mm x 200 mm is currently in use for measurements of muscle diffraction patterns and for X-ray crystallography The spatial resolution is about (2,5x2,5)mm FWHM. 

The rise-time technique has also been applied to two-dimensional detectors. A spatial resolution of 2 mm by 1 mm FWHM has been reported . As with linear detectors, the count rate capability of this method is lower than with the delay-line read-out, making it less useful for synchrotron radiation experiments. 

In this method, the signal originating at each wire (both the anode wires and the cathode wires) is processed by an individual amplifier-discirminator circuit. This requires many amplifiers. For instance, for a two-dimensional detector with a resolution of 256x256, a total of 512 amplifier-comparator systems are necessary. Each circuit on its own, however, is not complicated, and large scale fabrication is not too difficult. 

The device of Fig. 14 is only a one-dimensional detector. For two-dimensional detectors, several schemes for the localization of avalanches are feasible. However, this needs a careful study of the signal distribution as is shown by the results obtained at CERN... 

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