Position-Sensitive Detectors

Powder diffraction studies with neutrons are perfonned both at nuclear reactors and at spallation sources. In both cases a cylindrical sample is observed by multiple detectors or, in some cases, by a curved, position-sensitive detector. In a powder diffractometer at a reactor, collimators and detectors at many different 20 angles are scaimed over small angular ranges to fill in the pattern. At a spallation source, pulses of neutrons of different wavelengdis strike the sample at different times and detectors at different angles see the entire powder pattern, also at different times. These slightly displaced patterns are then time focused , either by electronic hardware or by software in the subsequent data analysis. 

Detection of cantilever displacement is another important issue in force microscope design. The first AFM instrument used an STM to monitor the movement of the cantilever—an extremely sensitive method. STM detection suffers from the disadvantage, however, that tip or cantilever contamination can affect the instrument s sensitivity, and that the topography of the cantilever may be incorporated into the data. The most coimnon methods in use today are optical, and are based either on the deflection of a laser beam [80], which has been bounced off the rear of the cantilever onto a position-sensitive detector (figme B 1.19.18), or on an interferometric principle [81]. 

Position Sensitive Detectors. By replacing the scintillation detector in a conventional powder diffractometer with a Position Sensitive Detector (PSD), it is possible to speed data collection. For each x-ray photon received a PSD records the angle at which it was detected. Typically, a conventional scintillation detector records x-ray photons in a range of a few hundredths of a degree at a time. A PSD can measure many degrees (in 20) of a powder pattern simultaneously. Thus, for small samples, data collection, which could require hours with a conventional detector, could take minutes or even seconds with a PSD. 

XRD is an excellenr, nondestructive method for identifying phases and characterizing the structural properties of thin films and multilayers. It is inexpensive and easy to implement. The future will see more use of GIXD and depth dependent measurements, since these provide important information and can be carried out on lab-based equipment (rather than requiring synchrotron radiation). Position sensitive detectors will continue to replace counters and photographic film. 

Binnig et al. [48] invented the atomic force microscope in 1985. Their original model of the AFM consisted of a diamond shard attached to a strip of gold foil. The diamond tip contacted the surface directly, with the inter-atomic van der Waals forces providing the interaction mechanism. Detection of the cantilever s vertical movement was done with a second tip—an STM placed above the cantilever. Today, most AFMs use a laser beam deflection system, introduced by Meyer and Amer [49], where a laser is reflected from the back of the reflective AFM lever and onto a position-sensitive detector. 

Figure 2 shows the brief principle of a laser-detected FFM. A sample is put on a piezoelectrical tube (PZT), which scans X, Y plane and controls the feedback of Z axis. The laser beam from a diode is focused on the mirror of the free end of a cantilever with lens, and the reflected beam falls on the center of a position-sensitive detector (PSD), a four-quadrant photodiode. When the sample contacts with the tip and relatively moves under the control of a computer, the reflected beam deflects and changes the position on the PSD due to the twist and deflection of the cantilever caused by the changes of surface roughness, friction force, and adhesive force between the sample and the tip. The extension and re-... 

Fig. 2—Brief principle of a laser-detected FFM. 1, laser beam 2, cantilever 3, tip 4, Sample 5, piezoelectrical tube 6, position-sensitive detector.

X-ray measurements were carried out with a small-angle diffractometer with a linear position-sensitive detector. Cu Ka radiation (X = 0.154 mn) was used (Mogilevski et al. 1984). The samples were rotated with respect to the incident beam, while the intensity was registered by linear position-sensitive detector. The angular resolution of the detector was 0.01°. The curves were acquired in the 20 range of 0.3-2.0°. X-ray reflection curves are presented in Figures 22 and 23 for wild-type and recombinant proteins, respectively. The... 

A p-i-n diode has been used on glass and on polyimid as a position-sensitive detector [638, 639]. The position of an incident light spot is measured by means of the lateral photovoltage. 

PSD (1) Position-sensitive detector RIMS Resonance ionisation mass... 

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. 

Figure 1.6. Schematic diagram of the basic principle of the 3DAP using a multi-anode type position-sensitive detector. (Reproduced by permission of Hono 1999.)...

Figure 5.39. Illustrating the focussing and dispersive properties of a magnetic prism in an electron energy-loss spectrometer. In a serial spectrometer a slit at D is used to ensure that only electrons of a single energy loss enter the detector. In a parallel spectrometer, a position-sensitive detector is placed at D to collect electrons of all energies in parallel.

A Siemens Kratky camera system was utilized for small angle x-ray scattering (SAXS) measurements in conjunction with an M. Braun position sensitive detector from Innovative Technology Inc.. Wide angle x-ray diffraction was obtained utilizing a Philips table-top x-ray generator. 

The basic principles of position-sensitive detectors are published in an early review of J. Hendrix [78], A review on 2D X-ray scattering of polymers including a description of detectors has been published by Rudolf Landes [38], Wilson [79] describes the detector development at EMBL, Hamburg which contributed to the commercial success of mar research Inc. 

D Position Sensitive Detectors. Position sensitivity is accomplished by a so-called delay line. For every pulse11 arriving at the wire the time is measured that it needs to travel to each of the two ends of the wire. Thus the position of the incident photon along the wire can be computed from the time difference, i.e., the delay. Bent high-resolution ID position sensitive detectors (cf. Fig. 4.14) are advantageously used in laboratory equipment for the recording of WAXS curves. 

D Position Sensitive Detectors are multi-wire electrical-field detectors. The principal limitation of the total counting rate reduces the applicability at a synchrotron beamline in particular for 2D detectors. But even strong, narrow peaks pose a problem, because the whole image is distorted as soon as local saturation occurs. The detector response is changing, because the wires are worn out by use. 

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