Source-detector geometry

For practical source-detector geometry, a compressed filter sample can improve the detection efficiency by about a factor of five over an uncompressed filter. To eliminate the need for sample compression, several solutions could be pursued. 

Another type of summing, referred to as true coincidence summing, is a function of the nuclide decay scheme and the source/detector geometry and will be dealt with in some detail in Chapter 8. All of the other features in the spectrum can be attributed to unavoidable interactions of gamma-rays from the source with the surroundings of the detector - the shielding, cryostat, detector cap, source mount, etc. 

Intrinsic efficiency (full energy peak or total) relates the counts in the spectrum to the number of gamma-rays incident on the detector. This efficiency is a basic parameter of the detector and is independent of the source/detector geometry. 

The source-detector geometry has changed, or the sample did not fully reach the measuring position. 

Figure 4 shows the coordinate systems associated with the example shown in Figure 3 The horizontal axis is x, and the vertical direction is y. The conveyor belt is perpendicular to the y axis and moves in a direction into the page. The disk rotation angle, 9, is measured counter-clockwise from the y-axis. This example has 501 detectors in a straight hne, which is defined as the 5 direction. The straight hnes running from the source to the detectors represent rays of radiation detected at each detector location. There are 501 such rays that the figure represents with 21 hnes. (The detector geometry is often modified to place individual detectors along an arc of a circle centered on the X-ray source.)...

Figure 7. Effect of external source and geometry on count rate. Source used was 1-fACi sodium-22 at end of detector...

This section is based on the description published in reference [40], The geometry of the data acquisition for XRDCT is shown in Fig. 18. A fan beam illuminates the object, and the transmitted fan (in the plane of the drawing) reaches the 2-D detector at its central row. These data can be used for attenuation correction and also for a conventional transmission-CT image. The detector columns are indexed by the variable t, whereas the relative angle between measurement system (source, detector, etc.) and suitcase is described by the variable 4>. In conventional CT, a 2-D data set, I(f, ), is measured for many angular positions, 4>, of the object with respect to the device. 

The detector geometries also result in different energy resolutions especially for lower y-ray energies. This is shown in Fig. 5.33. Typical absolute efficiency curves for various Ge detectors in the Marinelli-beaker configuration are shown in Fig. 5.34, while Fig. 5.35 shows typical absolute efficiency curves for various Ge detectors with 2.5 cm source to the end-cap spacing. 

Assuming that our absorber contains enough Fe57 to produce an effect of 2.5% we can estimate the time of measurement if we know the counting rate N per sec of the 14.4 keV /-radiation (which depends on the activity of the source and on the geometry of the experimental set-up), and if we limit the accuracy of the measured effect to 5%. Under this condition we need N counts in each of the 400 channels of the multichannel analyzer and from Fig. 6 we see that the total error of the measured effect is given by 2 ]/lV/0.025 N = 0.05, leading to N=2.5 106 counts per channel. With a source activity of 100 mCi, a source detector distance of about 12 cm, a detector window of 2.54 cm diameter, and an absorber with effective thickness of about 0.5 g/cm2... 

In reflectivity experiments, the source and detector geometry are chosen so that the scattering vector Q is parallel to the z-axis (perpendicular to the plane of the monolayer), as shown in Fig. 7. The reflectivity R(Q) is then given by ... 

Fluorescence detectors can give improved selectivity over ultraviolet absorption detectors because fewer compounds fluoresce than absorb (Chapter 16). Sensitivities at least as good as and perhaps better than the UV detector are achieved, depending on the geometry of the excitation source-detector arrangement, the intensity of the source, and the quantum efficiency of the fluorophore. The amper-ometric detector (see Chapter 15) is useful for detecting electroactive substances and has found considerable use in biological applications, for example, in the HPLC separation and detection of trace quantities of catecholamines from the brain. 

Absolute total detector efficiency is the probability that a gamma emitted from a specific source will be recorded in the detector. The geometry assumed for the absolute efficiency is shown in Fig. 12.12. The intrinsic efficiency (Fig. 12.11) depends on the energy of the gamma E and the size of the detector L. The absolute total efficiency (Fig. 12.12) depends on, in addition to E and L, the radius of the detector R and the source-detector distance d. Therefore the absolute total efficiency, as defined here, is the product of intrinsic efficiency times the solid angle fraction (see also Chap. 8). 

The third listed calibration approach involves the separate determination and subsequent combination of all factors that constitute the counting efficiency. These factors depend on detector volume, source-to-detector geometry, and the extent of radiation attenuation and scattering. They have differing degrees of impact in different detectors. Some approximate counting efficiency and background values for the detectors discussed here are listed in Table 8.1. The listed PERALS (Photon... 

The practical counting efficiency e represents the probability that any particular photon or particle of radiation emitted by the sample source will be recorded by the detector. As explained in Section 8.2, its value may depend on many factors, including the detector, the type and energy of the radiation, the composition of the source, and the geometry of the source-detector configuration. It includes the loss factor in the pulse analysis system and attenuation and scattering fractions associated with the sample-detector system. All of these factors are discussed further in Section 8.2. 

Insufficient absorption of the X-ray flux incident on the scintillator could have various deleterious effects. First, the efficiency of the X-ray detection is diminished when X-ray photons are allowed to pass through the scintillator, without absorption and creation of excitons that excite luminescent centers. Second, in the common detector geometry where a photodiode is attached to the side of the scintillator opposite from where the X-rays enter, an X-ray that is not absorbed in the scintillator can be absorbed by the diode. This will cause the formation of electronic defect in the diode, so that an additional source of noise will be created, which in turn can degrade the performance of the whole detection system. Therefore, the scintillator material should be able to absorb aU the incident X-rays ideally. Various detector designs have been proposed to minimize this effect, e.g., by placing the diode surface away from the direct path of the incoming X-ray beams [73, 77-79]. 

Conventional instrument geometries incorporate a source, prism or grating to disperse the light from the source, a detector, and necessary accessories such as lenses, mirrors, sample holders, and movable slits. If the spectral range of the instrument extends beyond the visible into the near-infrared and ultraviolet, dual sources, detectors, and ancillary electronics are required. The Perkin-Elmer Lambda 19, diagramed in Fig. 2, is an example of this technology (37). 

The optical design of all spectrophotometers is very limited. Many types of geometrical errors are involved in even the simplest designs (49). Whereas an observer constantly views objects under changing conditions, a spectrophotometer is carefully assembled with usually one view of specimens. The instrument s view is restricted to specified industry standards that describe conventional optical geometries, sources, detectors, data acquisition, and calculation algorithms. These restrictions are necessary to communicate solar parameters of a specimen without confusion. 

---