Arrangement of Detectors

The production of short-lived positron emitters has been described in section 12.2. By interaction with electrons, the positrons are annihilated and two y-ray photons of 511 keV each are emitted simultaneously in opposite directions. By measuring these photons by means of a suitable arrangement of detectors, exact localization of the radionuclides in the body is possible. This is the basis of positron emission tomography (PET), which has found broad application in nuclear medicine. The most frequently used positron emitters are listed in Table 19.2. They are preferably produced by small cyclotrons in the hospitals or nearby. 

Figure 3.16 A modern arrangement of detector and preamplifier within the cryostat housing - an exploded view of the ORTEC Pop-Top detector capsule with a horizontal dipstick cryostat and a 201 Dewar. Reproduced by permission of ORTEC...

DETECTOR GEOMETRY A description of the geometric arrangement of detector and sample (distance, sizes of detector and sample), which is sometimes expressed as a soUd angle. It influences the efficiency of counting. 

Schematic diagram of a muitichannei atomic emission spectrometer, showing the arrangement of muitipie exit siits and detectors for the simuitaneous anaiysis of severai eiements.

FIGURE 3.7 Optimizing arrangement of LC modules for very complex systems. Four-detector (DAD, ELSD, CLND, and SQ-MS) LCMS system. The capillary connections from the diode array detector have been highlighted for better visibility. (Courtesy of Kenneth Lewis, OpAns Pic.)... 

Trojanowicz, M., Electrochemical Detectors in Automated Analytical Systems , in Modem Techniques in Electroanalysis, Vanysek P. (Ed.), Wiley-Interscience, New York, 1996, pp. 187-239. This chapter contains a fairly thorough discussion of the possible arrangements of electrodes within flow systems, and for a variety of applications. 

FIGURE 7.6 Schematic of a laser-induced fluorescence detector. A lamp with focusing optics and an appropriate band-pass filter could be used in place of the laser excitation when tightly collimated light is not required. The emitted fluorescence is detected by a PMT that can be operated in current mode or photon counting mode. Inset shows the mutually perpendicular arrangement of excitation, capillary, and detection optics. 

The 2.3 A structure of the achromobacter NIR (Godden et al., 1991) has been determined from an MIR map based on native and two derivative data sets collected on an area detector. In the crystal the tightly packed arrangement of monomers around the crystallographic threefold axis suggests that the molecule must be a trimer, rather than a dimer, as originally determined from gel filtration studies. The chain tracing substantiates this. 

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