Page 174 - Radiochemistry and nuclear chemistry

158 Radiochemistry and Nuclear Chemistry

electric power nor cooling also renders them more suitable for field applications. Further,
their independence from effects of temperature, pressure, and many other factors results in
higher reliability compared to X-ray generators. Finally, and, perhaps as important as any
of the other factors, they are in general much less expensive than accelerators.
To counterbalance these advantages is the disadvantage of the inability to turn off the
radiation from radionuclides. This often requires that the radiation source be well shielded,
adding to its weight and cost. An additional drawback to the use of radionuclides is that
they have to be replaced after a few half-lives. The seriousness of this disadvantage depends
upon the lifetime of the particular nuclide and is unimportant in cases where longlived
sources can be used.
The extensive use of radionuclides in industry is illustrated by Table 6.3, which
summarizes the various studies undertaken by I.C.I., UK, in "a typical year'.

6.9.1. Radionuclide gauges

Radionuclide gauges are a measurement system consisting of two parts, a radioactive
source and a detector, fixed in some geometry to each other. They are used mainly for
control in industrial processes but can also be applied for specific analyses. The gauges
come in two types. In one type the radiation source and the detector are on opposite sides
of the technical arrangement to be measured; these are known as transmission or absorption
instruments. In the second type, known as reflection or back-scattering instruments, the
radiation source and the detector are on the same side. The instruments are also classified
with respect to the kind of radiation involved. For example, -/-transmission,/~-reflection,
secondary X-ray instruments, etc. These radioisotope gauges are used for measurements of
thicknesses, densities, etc.

TABLE 6.3. Radioisotope based studies undertaken annually by a large chemical company
,
Technique (a) Number of applications
Level and interface measurements:
9 y-ray absorption 2 I0
Neutron backscatter 480
7-raY backscatter (storage cavities) 71
Blockage detection and deposition:
7-ray absorption 132
Neutron backscatter 129
Entrainment and voidage:
9 ,/-ray absorption 86
Thickness and corrosion measurements 15
Distillation column scans 108
Flow measurements:
Pulse velocity 483
Dilution techniques 84
Leak detection 90
Residence-time studies 21
Carry-over studies (tracer) 6

(') Less commonly used techniques have not been included.