Position-sensitive counter

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. 

There are two possibilities for the detection of the interferences the film detection and the registration of x-ray counts with scintillation counters or position-sensitive detectors. However, the SAXD method does not detect interferences from which the interlayer spacings can be calculated. It rather makes it possible from the sequence of the interferences to decide the type of liquid crystal [13,14]. 

Specific detectors are also available for quantification of radiopharmaceuticals. These detectors use a position-sensitive proportional counter. These detectors are sensitive to the beta and gamma nuclides listed in Table 3.5. The detector analog output can also be represented as an analog curve. Various other detection procedures have also been used, such as flame ionization [66], mass spectrometry [27,67], and infrared (IR) [68,69]. 

Figure 22.4. Schematic diagram of a synchrotron X-ray diffractometer. PSPC position sensitive proportional counter SR beam synchrotron radiation beam. (Reproduced with permission from Ueno et al., 2000.)...

The Diethorn expression has been applied in a detailed analysis of commercially available proportional chambers by R. W. Hendricks Fig. 2 shows the test results for a series of counters, the parameters of which are given in Table I. The tested devices are not position sensitive detectors, but normal cylindrical proportional counters. 

Interferences can be detected in two ways (i) the film detection and (ii) the registration of X-ray counts with scintillation counters or position-sensitive detectors. 

A position sensitive detector (PSD) employs the principle of a gas proportional counter, with an added capability to detect the location of a photon absorption event. Hence, unlike the conventional gas proportional counter, the PSD is a line detector that can measure the intensity of the diffracted beam in multiple (usually thousands) points simultaneously. As a result, a powder diffraction experiment becomes much faster, while its quality generally remains nearly identical to that obtained using a standard gas proportional counter. ... 

Set the sensitivity of the detector monitor to about 50% by turning the scale knob approximately five full clockwise turns from the fully counter position. 

Circuits (1) and (2) may be used with ordinary diffractometers to increase the peak/background ratio of diffraction lines. They are by no means necessary quite adequate diffraction patterns can be obtained from a wide variety of specimens with no other discriminator than a Kp filter. Circuit (3) is required only in x-ray spectroscopy (Chap. 15), in a very special kind of diffractometry (Sec. 7-10), and with a position-sensitive proportional counter. Any one of these circuits is more effective, the better the resolution of the counter with which it operates. 

This is a very recent development. It involves a side-window position-sensitive proportional counter (Sec. 7-5), a multichannel analyzer, and the rpeasurement of the angular positions of many diffraction lines simultaneously. The anode wire of the counter, which is long and curved, coincides with a segment of the diffractometer circle and is connected, through appropriate circuits, to an MCA. The powder specimen is in the form of a thin rod centered on the diffractometer axis. The geometry of the apparatus therefore resembles that of a Debye-Scherrer camera (Fig. 6-2), except that the curved film strip is replaced by a curved counter. 

A more recent, and very promising, development is due to Cohen and James, who adapted a position-sensitive proportional counter to stress measurement. This counter has the great advantage that it can measure the 29 position of a diffracted beam without a 29 movement of the counter (Sec. 7-5). Preliminary work was done with the counter mounted on a standard diffractometer [16.10]. Later a portable instrument was made, in which the counter and a miniature, air-... 

SAXS measurements were made at the National Center for Small Angle Scattering Research (NCSASR) at Oak Ridge National Laboratory using the 10m SAXS camera. The instrument used a rotating anode CuK (X" 0.1542 nm) x-ray source, crystal monochromatization of the incident beam, pinhole collimation, and a two dimensional, position sensitive, proportional counter. Sample to detector distance was 1.12 m. Disc-shaped samples were cut from the molded films and dried in a vacuum oven at 65 °C for four weeks prior to the SAXS analyses. 

WOL 83] WOLFEL E.R., A novel curved position sensitive proportional counter for x-ray diffractometry ,/. Appl. Cryst, vol. 16, p. 341-348, 1983. 

Fleischmann and co-workers have carried out a number of in-situ x-ray diffraction studies at roughened electrodes.Their experiments are different from those previously discussed in that instead of a synchrotron source they employ a conventional x-ray tube ( 1.5 kW) with a position-sensitive proportional counter for the measurement of the diffracted intensity. They have carried out experiments in both transmission (Laue) and reflection (Bragg) modes. 

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. 

A further increase in speed of data taking is expected by segmentation of detectors, i.e. each detector segment has its own read-out system. A linear position-sensitive detector develop recently at EMBL consists of 128 anode wires, with each of them acting as an individual counter. The integral count rate is 10 MHz of statistical events. A similar development has been started for area counters. The new diffractometer for X-ray resonance scattering in HASYLAB is equipped with three area counters (Fig. 5). This system can handle nearly a million statical events per second... 

In a typical experiment we want to record the scattering pattern as a function of 5-30 different wavelengths near an X-ray absorption edge. Using three position-sensitive area counters shown in Fig. 5, 65536 data items represent one scattering pattern. One scan typically produces 20 x 65536 = 1310720 data items in about one hour. On-line data reduction then becomes necessary. 

The spatial resolution of the position-sensitive area counters can be characterised more readily. 

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