Gamma backscatter

The next process variable to be controlled is film thickness (gauge). Either on-line or off-line measurements allow us to monitor film gauge. On-line devices usually employ a radiation source, such as a gamma backscatter system. These devices measure thickness by emitting radiation that reflects back to the sensor from both the near and far surfaces of the film. They can be mounted to measure a fixed location on the bubble or on a carriage to traverse around the bubble (Fig. 3.21). Also, a unit traversing back and forth across the flattened web can be used, but this measures two-layer thickness. Off-line devices typically operate on a capacitance principle, so they don t require the safety practices employed with radiation sources. 

Figure 18-7. Principle of operation—gamma backscatter gauging technique. (Courtesy of NDC Systems.)...

Sales and Technical literature on Gamma Backscatter, Beta Transmission, X-Ray Fluorescence, NDC Systems. Irvindale, CA. 

Ti content in the polymer films was measured with a Princeton Gamma Tech System 4 x-ray Fluorescence Spectrometer. The conditions employed were Cr target, 50 keV source operating at 3 mA, 0.75 mm aperture, 4.8 mm beam stop, helium atmosphere and 100 sec. counting time. A calibration curve was constructed by plotting the fluorescence counts versus the amount of Ti in HB-HPR 206 films determined by Rutherford Backscattering Spectroscopic (RBS) analysis. 

Fig. 5.12. Energy of Compton edge and backscattered peak for gammas of energy <0.7 MeV.

FIGURE 21>22 Diagram ol Everhart-Thomley secondary electron delector. Paths of secondary electrons (SE) and backscattered electrons (BE are shown. The scintillator is a phosphor that emits light when struck by energetic particles such as electrons, gamma rays, or radioactive particles. 

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). 

Backscattering is minimal for alpha particles because they are mostly stopped in the backing material (NCRP 1985b). A typical factor is 1.03. Backscattered gamma rays add to the Compton continuum a broad peak at an energy of about 0.2 MeV. 

IPM can be used simultaneously with RBS (Rutherford backscattering spectrometry), NRA (nuclear reaction analysis), PIXE (particle induced X-ray emission) or PIGE (particle induced gamma ray emission). More specialized examples include the field ion microscope (FIM), which gives better then atomic resolution in the study of high melting point materials. 

The nuclear backscatter densitometer may be used either as a conventional gamma-gamma log of the type used for petroleum exploration (Threadgold, 1980) or in a smaller version for use by itself in bays and harbors to determine the bulk density of the very soft top soils or in conjunction with a cone penetrometer (Nieuwenhuis and Smits, 1982). 

The density may be measured by direct transmission, backscatter or backscatter/air-gap method of gamma radiation. The measurements of moisture content are taken, in most devices available, in backscatter mode, regardless of the mode being used for density. 

In the direct transmission method, the gamma source extends through the base of the gauge into a preformed hole to a desired depth. In the backscatter or backscatter method, the gamma and neutron sources and the detectors are in the same plane (see Figure 1.17). 

The economic benefits that may be derived from the use of radioisotope technology in petrochemical industry are large. In this chapter we tried to present the state-of-the-art in major techniques used in petrochemicals such as gamma-scanning as a diagnostic tool for distillation columns and pipes, neutron backscattering for level and interface detection in... 

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