One Dimensional Position Sensitive Detector

A data aquisition system based on a one dimensional, gas filled detector is shown in Fig. 31. The general philosophy of the system is to be as independant as possible from a central computer as the enormous amount of data demand an on line data analysis during the experiment. 

The detector operates according to the delay-hne principle. Here a 20 pm thick anode wire and a cathode which consist out of parallel metal strips connected to a delay line are used. The charge creating event induces a signal in the cathode which propagates with a velocity of about 0.2 mm ns in both directions of the 

Although the overall count rate limitation with negligible deadtime correction losses is about 10 counts s , experimental expefience shows that it is rather easy to saturate the system, especially for highly localized scattering events (e.g. strong Bragg reflections). This shows the interest in the development of new detection systems. 

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As shown in Figure 4.1.2(a), the collection of the diffracted radiation by means of a one-dimensional position-sensitive detector (1-D PSD) is made by scanning the detector over a range along the horizontal scattering vector q (=4tt sin and... 

Figure 2.16 Schematic illustration of a one-dimensional position-sensitive detector. The gas-filled detector operates as a proportional counter, and the position information is encoded in the difference in the rise time between the pulses coming out of the two ends of the anode wire.

Figure 29 further shows that the specimen position does not contribute to focusing. In a fixed specimen position, only absorption varies somewhat with diffraction angle. This setup is thus suitable when a curved one-dimensional position-sensitive detector is employed. There are detectors on the market that can measure up to 2 0 = 120° at the same time. When such a detector is used, no diffractometer is needed—only a base plate support, This technique has proven itself u.seful, particularly with programmed heating of specimens, and with in situ recording of diffraction diagrams. 

Synchrotron small-angle X-ray scattering (SAXS) measurements were carried out at 4C2 SAXS beamline in Pohang Light Source (PLS), which consisted of 2 GeV LINAC accelerator, storage ring, Si(lll) double crystal monochromator, ion chambers, and one-dimensional position sensitive detector with 2048 pixels. The... 

The area detector is an electronic device for measuring many diffracted intensities at one time. It is a two-dimensional, position-sensitive detector that records the intensity of a Bragg reflection (diffracted beam) and its precise direction (as a location on the detector) it acts like an electronic substitute for film. This detection device is now used extensively for crystals of biological macromolecules. Such a detector may involve a multiwire proportional counter coupled to an electronic device or a television imaging system both devices permit a recording of the data in a computer-readable form. Alternatively, imaging plates may be used. These have phosphorescent material layered on them and store information on the extent of X-ray exposure until scanned bv a laser, when the intensity and location of the light then emitted is recorded. 

In comparison with film cameras, diffractometers are roughly a factor of 10 more costly. They are still more expen.sive if the point counter is replaced by a one- or two-dimensional position-sensitive detector. The data acquisition time is, however, shortened by a factor of 50-100, or even more, depending on the geometry of the crystal lattice measured. 

Some measurements were performed using an X-ray source with rotating anode and a pinhole collimation [5]. The rest of the measurements were carried out using the synchrotron radiation generated at the beamline A2 of HASYLAB in Hamburg, Germany. The intensity was measured by means of one- and two-dimensional position-sensitive detectors, respectively. 

An incident synchrotron-radiated X-ray was monochromatized to X. = 1.488 A with a double-mirror monochromatizer, and focused to the position of the detector by a focusing mirror. The scattered X-ray was detected by the one-dimensional position sensitive proportional coxmter (PSPC) of the effective... 

A position sensitive detector (PSD) is employed, of which there are several types used effectively around the world. One type is essentially a square array of multianodes, as shown in Figure 1.6. By measuring the time-of-flight and the coordinates of the ions upon the PSD, it is possible to map out a two-dimensional elemental distribution. The elemental maps are extended to the z-direction by ionizing atoms from the surface of the specimens. The z position is inferred from the position of the ion in the evaporation sequence, so that the atom distribution can be reconstructed in a three-dimensional real space. 

The Chevron channel plate ion detector assembly of an imaging atom-probe can also be replaced by a position sensitive particle detector combined with a data processor, as reported by Cerezo etal.5s (A position sensitive detector was used earlier for the purpose of field ion image recording and processing.59) With such a detector both the chemical identity and the spatial origin on the emitter surface can be found for each field evaporated ion. This position sensitive atom-probe can be used to study the spatial distribution of different ion species on the emitter surface as well as inside the bulk of the emitter with a spatial resolution nearly comparable to the FIM. For such a purpose, one carries out the field evaporation at an extremely slow rate so that no more than one ion is detected from the entire field ion emitter surface in each pulsed field evaporation. From the flight time of the ion its chemical species is identified, and from the location of the detector where the ion is detected the spatial origin of the ion is located. With a fast data processor, a two-dimensional distribution of chemical species on the tip surface can be... 

Figure 4.26 Possible electronic circuit for deriving one-dimensional position information from a position-sensitive detector with a resistive strip anode. The two charges Q, and Q2 on the ends of the anode are amplified, shaped and converted to a digital signal. The mathematical operations of Q = Qt + Q2 and Q2/Qj are performed electronically, and the result is stored in a histogramming memory from which it is read into the computer. Q2/Q carries the information first that an electron has been detected and second at which position this electron has hit the detector. From [Wac85].

The third development, which is essential to the successful application of position sensitive detectors in time resolved measurements is the availability of large and fast memory systems. As an example, a two dimensional detector with 256 x 256 pixels, in an experiment with 128 consecutive time frames of one second each will produce... 

Reduction from the two-dimensional form to the one-dimensional equatorial form was a requirement of the equatorial diffractometer geometries utilizing point detectors or at most linear position sensitive detectors. Importantly, this correction is neither applicable to the Bragg-Brentano nor to the flat transmission geometry, but is only valid for the Debye-Scherrer geometry. 

If the scattering patterns are anisotropic due to a preferred orientation of molecules and crystals, it is often desirable to use two-dimensional position sensitive counters. Such area detectors, based on the principles described above, are the multiwire proportional counters (MWPC). A number of parallel tightened anode wires are mounted in front of a cathode structure, the latter consisting of two layers of parallel metal strips which are oriented horizontally in one layer and vertically in the other. One possibility of reading out the strips is to connect the strips of each... 

TCSPC with two-dimensional position-sensitive detection can be used to acquire time-resolved images with wide-field illumination. The complete sample is illuminated by the laser and a fluorescence image of the sample is projected on the detector. For each photon, the coordinates in the image area and the time in the laser pulse sequence are determined. These values are used to build up the photon distribution over the image coordinates and the time (see Fig. 3.12, page 40). The technique dates back to the 70s [312] and is described in detail in [262]. Lifetime imaging with a TCSPC wide-field system and its application to GFP-DsRed FRET is described in [162]. A spatially one-dimensional lifetime system based on a delay-line MCP is described in [509]. 

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