Page 176 - Radiochemistry and nuclear chemistry

160 Radiochemistry and Nuclear Chemistry

The use of radioisotope gauges in density measurements is dependent upon (6.26) in
which r and x are constant while the absorption coefficient is density dependent (i.e.
dependent on the average atomic composition of the absorber). A practical arrangement is
illustrated in Figure 6.28, E, where the density of a medium in a pipeline is measured. This
medium may be a mixture of gas and liquid such as water and water liquids with different
composition and different amounts of dissolved substances as, for example, oil, salts or
acids in water, process solutions in general or sludges, and emulsions such as fruit juices,
latex emulsions, etc. From the variation in density the concentration and composition may
be determined. Such density gauges are also used for control in filling of soft drink bottles
and cans and submerged in rivers and lakes for measuring the depth of the bottom silt, etc.
Density gauges are used in the production and the fabrication of such diverse products as
automobile tires and cigarette packages.
Thickness gauges are the most common type of instrument using the absorption technique.
In this case x in (6.26) is varied. Measurements can be carried out on all kinds of materials
with thicknesses of _< 100 g cm -2 and is independent of the temperature and of whether the
material is stationary or in motion. Figure 6.29, F, illustrates the application of a thickness
gauge in a rolling mill where material of constant thickness is produced by using the signal
from the detector for control purposes. Thickness gauges are used in the fabrication of
glass, metal, paper, plastic, rubber, candy bars, etc. They have been used for measuring
the thickness of snow in polar regions, icing on airplane wings, and other applications in
which it is necessary to use remote operation.
In order to measure very thin layers of material such as coatings of paint, wax, and plastic
films on papers or other material, two thickness gauges are used with a differential coupling
so that one detector measures the uncovered and the other the covered or treated portion
of the material. Thickness gauges also are used in industry to measure the degree of wear
in industrial machinery. For surface measurements of thicknesses < 0.8 g cm -2 most
thickness gauges use radiation sources with/3-emitters while for thicknesses of 0.8-5 g
cm -2 bremsstrahlung radiation sources are most suitable. For even thicker materials
7"emitters are used.
Use of reflection gauges depends on the fact that the intensity of the scattered radiation
under conditions of constant geometry depends on the thickness of the scattering material
and its electron density (if/~- or -y-sources are used). If neutrons are used the mass number
of the scattering material is of prime importance. The electron density of the scattering
material varies with the particular element and the chemical composition. Frequently, it is
possible to determine the thickness and the nature of a surface layer by means of
/~-scattering. Reflection gauges have been applied to on-line analysis of tin-covered iron
plates, metal coatings on plastics, paint layers, and to measuring the protective coating
inside pipelines (Fig. 6.29, G). In some instances 7-radiation sources are preferred over
/~-emitters in measurements of material with greater wall thicknesses, particularly when
transmission measurements are not feasible. Steel thickness from 1 to 20 cm has been
measured with 5 % accuracy using backscattering from 6~ or 137Cs sources of 20/zCi
intensity.
Scattering and reflection are dependent on the electron density of the absorber, which is
approximately proportional to the value of Z/A. Backscattering of/~-particles from organic
compounds is therefore very dependent on the hydrogen concentration (Z/A = 1) but fairly
independent of the concentration of C, N, and O (Z/A = 0.5). This has led to the
development of sensitive instruments for hydrogen analysis for various organic and