Measurement geometries used for medical X-ray CT scanners of 1 st , 3 rd th and 4 generation. Also shown is the instant configuration in which all ray-sum measurements are carried out simultaneously [7]. 

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... (atomic number) of the attenuating material (see Figure 6). This can be utilized to provide even higher sensitivity than what is possible at high energies. Low energy meters are only used for permanently installed gauges whereas high-energy meters also may be used for clamp-on installation, for instance, in process diagnostics. Most gamma-ray transmission tomography systems are based on one or several fan-beam collimated point sources each facing an array of detectors on the opposite side of the process or object under investigation, Figure 2. These systems fall into two categories; those using one source and mechanical scanning to obtain multiple projections (3 rd or 4 th generation), and those using multiple sources and thus avoiding mechanical scanning (instant). For imaging of fairly static objects or processes, or where temporal averaging of their dynamics is satisfactory, the first is preferred because of the much lower cost. However, instant imaging has to be used on dynamic objects or processes to avoid inconsistency in the measurement cross section. Typical examples on dynamic processes with time constants in the sub second region are multiphase flows and processes involving separation, sedimentation, filtration, mixing, etc. Instant tomography is also needed for imaging of objects carried on production lines such as a conveyer belts. 4. SCANNING TRANSMISSION TOMOGRAPHY : rd There are many gamma-ray tomography developments based on 3 generation configuration in Figure 2 and temporal averaging measurements. For process diagnostics this has also been applied with only one detector acquiring data at several positions. Full data acquisition is thus obtained by scanning both source and detector. This has proved to be successful and revealed process conditions [9], which would not be detectable with one or two measurements at each level in a scan of, for example, a chemical reactor. The performance of tomography systems is experimentally evaluated using phantoms to simulate the process of interest and thus provide accurate reference conditions. The system shown in Figure 3 is designed for imaging of liquid flow distribution in trickle bed reactors, and the example uses 17 phantoms with different gas fraction. A 300 mCi 137 Cs source and 32 BGO scintillation detectors are used, and by scanning a maximum number of 16000 transmission measurements, so called ray-sums, are obtained. The reported gas fraction (or liquid retention) error is 3% at ...

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... (atomic number) of the attenuating material (see Figure 6). This can be utilized to provide even higher sensitivity than what is possible at high energies. Low energy meters are only used for permanently installed gauges whereas high-energy meters also may be used for clamp-on installation, for instance, in process diagnostics. Most gamma-ray transmission tomography systems are based on one or several fan-beam collimated point sources each facing an array of detectors on the opposite side of the process or object under investigation, Figure 2. These systems fall into two categories; those using one source and mechanical scanning to obtain multiple projections (3 rd or 4 th generation), and those using multiple sources and thus avoiding mechanical scanning (instant). For imaging of fairly static objects or processes, or where temporal averaging of their dynamics is satisfactory, the first is preferred because of the much lower cost. However, instant imaging has to be used on dynamic objects or processes to avoid inconsistency in the measurement cross section. Typical examples on dynamic processes with time constants in the sub second region are multiphase flows and processes involving separation, sedimentation, filtration, mixing, etc. Instant tomography is also needed for imaging of objects carried on production lines such as a conveyer belts. 4. SCANNING TRANSMISSION TOMOGRAPHY : rd There are many gamma-ray tomography developments based on 3 generation configuration in Figure 2 and temporal averaging measurements. For process diagnostics this has also been applied with only one detector acquiring data at several positions. Full data acquisition is thus obtained by scanning both source and detector. This has proved to be successful and revealed process conditions [9], which would not be detectable with one or two measurements at each level in a scan of, for example, a chemical reactor. The performance of tomography systems is experimentally evaluated using phantoms to simulate the process of interest and thus provide accurate reference conditions. The system shown in Figure 3 is designed for imaging of liquid flow distribution in trickle bed reactors, and the example uses 17 phantoms with different gas fraction. A 300 mCi 137 Cs source and 32 BGO scintillation detectors are used, and by scanning a maximum number of 16000 transmission measurements, so called ray-sums, are obtained. The reported gas fraction (or liquid retention) error is 3% at ...

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... HIGH SPEED TRANSMISSION TOMOGRAPHY : Computer tomography (CT) theory implicates that objects exhibiting some symmetry and homogeneity characteristics can be reconstructed successfully from very few views [13]. This inherent property has an important potential in instant non-scanning geometry systems like the one shown in Figure 2; where a very large number of sources and detectors would make such systems very expensive. An 85 ray-sum gas/ liquid flow imaging system has been constructed [14] see Figure 5. This comprises five 500 mCi 241 Am sources and a total of 85 CdZnTe detectors each 1x1 cm 2 . The design is for an 80 mm inner pipe diameter and the spatial resolution is about 5 mm. The temporal resolution is a few ms corresponding to an image rate of several hundred frames per second. 6. DUAL ENERGY AND MULTIPLE SCHEMES METHODS : The need for component fraction measurements in multiphase processes lead to the development of what is called dual energy meters. Here energy sensitive detectors are used and the transmission is measured at two energies. The highest energy is chosen where Compton scattering is the dominant attenuation mechanism in the mixture, Figure 6. The linear attenuation coefficients of the components are then proportional to their densities. The lowest energy is in the range dominated by photoelectric absorption where the linear attenuation coefficients are strongly dependent on the effective atomic number or composition. Thus the difference in attenuation is proportional to the density in the Compton dominant region (for absorbers with Z/A ≈ 0.5 ), and proportional to Z 4 to Z 5 in the photoelectric region, where Z and A are the effective atomic number and the effective atomic weight of the composition, respectively. The result is two independent measurements enabling determination of the volumetric fractions of three components in a closed system. This principle was first developed for gas/oil/water measurements [15], and later also for ash in coal measurements [16]. The difference in dependency of photoelectric and Compton attenuation to the process density and atomic composition may also be measured by the dual modality principle using one low radiation energy (e.g. 241 Am sources). Here one transmission measurement responding to both photoelectric and Compton attenuation, are combined with one scatter measurement from which the Compton response is derived [17]. A dual modality system based on scatter and annihilation radiation for measurement of ash in coal has also been developed [18]. Low- energy systems require low attenuation vessel walls (or windows therein) and are ...