Gamma-Ray Absorption Techniques

Microwave-based meters have also been used to monitor water content in emulsions (39). Microwave techniques can be used in two ways Either the attenuation of the microwave radiation due to absorption by the water phase is measured, or capacitance or resonance changes in a microwave cavity are noted. The capacitance-change method is much more sensitive, although both, like the gamma-ray absorption method, are limited in that solids content must be constant or zero in order to accurately interpret the information obtained. Both of these techniques are applicable to field situations and on-line monitoring. 

Using nucleonic level detectors These usually employ gamma ray absorption or neutron backscatter techniques, but may also use gamma ray backscatter (Sec. 14.5). A case where a gamma ray absorption indicator solved a level control problem at the base of a column has been described (71). 

The gamma-ray transmission technique previously employed by Vermeulen et al. (5) has been used to determine the point gas holdup in mechanically agitated tank by Calderbank, in plate columns and in packed columns (I). The principles of the application of gamma-ray absorption to holdup measurements depend on the use of the relationship. 

Based on the examinations of sensitivity, precision, accuracy and valid range, they concluded that this is a promising technique to measure cake thickness of biological particles. This method has the benefits of low cost, easy installation and accuracy compared to gamma-ray absorption, NMR microimaging and computerized axial tomographic scans (CATSCAN) techniques. The fact that this method can only be applied to a flat plate membrane module is its major limitation. 

On-line moisture monitors, which are based on infra-red absorption, nuclear magnetic resonance, capacitance and microwave attenuation are used in numerous process industries. The applicability of these techniques to the coal preparation industry has been reviewed (13,14). The CONAC and "Elemental Analyzer" units use a microwave attenuation method for the moisture measurement. This information is used to correct the PNAA hydrogen assay data in order to calculate the hydrogen content of the coal. Kay-Ray, Inc., (Arlington Heights, Illinois) has developed an instrument that measures the moisture content of a coke stream and uses a combination of microwave and gamma ray attenuation measurements. 

The most widely used NDA instruments rely on the detection of nuclear radiation such as gamma rays and/or neutrons. Physical measurement techniques are also used with available instruments that measure heat, weight, liquid volume, thickness, and light emission/absorption. These physical techniques may be applied by themselves, or they may be used in combination with other nuclear measurements to provide quantitative measurements of the nuclear material. The general reference on the theory and application of passive NDA (PANDA) is given by reference (Reilly et al. 1991) and its addendum (Reilly et al. 2007). 

Neutron activation analysis (NAA) is the general term used to describe a nuclear-based technique in which a solid or liquid sample is irradiated with neutrons. Capture or absorption of a neutron excites the nuclide that returns (promptly or after a delay) to ground state by emission of an energetic photon (gamma ray) and/or other particles from the nucleus (Figure 1.20). 

The first example of the recoil-free resonant absorption of gamma rays was observed by Mossbauer using Ir (Mossbauer, 1958) and the Mossbauer effect has now been detected in over one hundred isotopes. However, most of the practical applications of the effect in the technique known as Mossbauer spectroscopy have been with a much smaller number of isotopes in which the conditions for observing the Mossbauer effect and obtaining useful information from it are particularly favourable. 

The discovery of the resonant absorption of gamma-rays in lr by Rudolf Mossbauer in 1958 was a milestone in nuclear physics because it was formerly assumed that such a phenomenon could never occur due to the large recoil energies involved. The interest for this new finding was nevertheless still limited in the first years, but, a real breakthrough emerged from the fact that many isotopes showed a larger and much more sensitive effect. So, a new technique, called Mossbauer spectroscopy, was bom. The most important feature of this spectroscopic method is the extreme sharpness of the emission line which can easily be... 

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