Page 580 - Instrumentation Reference Book 3E

562 Measurements employing nuclear techniques

backscattered and measurable by the detector.
Once a coating has been applied to the roll the
intensity of the backscattered radiation returning
to the detector will change. This change is a meas-
ure of the thickness of the coating. Typical appli-
cations of this gauge are the measurement of
rubber and adhesives on calenders, paper on roll-
ers, or lacquer, paint, or plastics coatings applied
to sheet steel.

Measurement of coating and backing by X-ray
fluorescence X-rayfluorescenttechniques,employ- Figure 23.1 8 Preferential absorption gauge. Courtesy
ing radioisotope sources to excite the char- Nuclear Enterprises Ltd.
acteristic fluorescent radiation, are normally
used to measure exceptionally thin coatings. The The preferential absorption gauge (Figure 23.18)
coating-fluorescence gauge monitors the increase This gauge is used when the coating material
in intensity of an X-ray excited in the coating as has a higher mean atomic number than the base
the coating thickness is increased. The backing- material. The gauge employs low-energy X-rays
fluorescence gauge excites an X-ray in the back- from a sealed radioisotope source which are
ing or base material and measures the decrease in absorbed to a much greater extent by materials
intensity due to attenuation in the coating as the with a high atomic number, such as chlorine, than
coating thickness is increased. The intensity of by materials such as paper or textiles, which have
fluorescent radiation is normally measured with a low atomic number. It is thus possible to moni-
an ionization chamber, but a proportional or tor variations in coating thickness by measuring
scintillation counter may sometimes be used. the degree of preferential X-ray absorption by the
By the use of compact geometry, high-effi- coating, using a single measuring head. The instru-
ciency detectors, and a fail-safe radiation shutter, ment is used to measure coatings which contain
the dose rates in the vicinity of the measuring clay, titanium, halogens, iron or other substances
head are kept well below the maximum permitted with a high atomic number which have been
levels, ensuring absolute safety for operators and applied to plastics, paper, or textiles.
maintenance staff.
Figure 23.17(a) illustrates the principle of the
coating-fluorescence gauge, which monitors the 23.4 Miscellaneous
increase in intensity of an X-ray excited in the measurements
coating as the coating thickness is increased.
The instrument is used to measure tin, zinc, 23.4.1 Field-survey instruments
aluminum, and chromium coatings applied to In prospecting for uranium, portable instruments
sheet steel, or titanium coatings to paper or plas- are used (1) in aircraft, (2) in trucks or vans, (3)
tics sheet. hand-held and (4) for undersea surveys. Uranium
is frequently found in the same geological forma-
tions as oil, and uranium surveys have been used to
supplement other more conventional methods of
surveying, such as seismic analyses. The special
case of surveying for beryllium-bearing rocks was
discussed in Section 4.2.1.
As aircraft can lift large loads, the detectors used
in such surveys have tended to be relatively large
NaI (Tl) groups of detectors. For example, one air-
craft carried four NaI (Tl) assemblies, each 29.2cm
diameter and lOcm thick, feeding into a four- or
five-channel spectrometer which separately moni-
tored the potassium, uranium, thorium, and the
background. Simultaneously a suitable position-
Figure 23.17 X-ray fluorescence gauge. Courtesy finding system such as Loran-C is in operation, so
Nuclear Enterprises Ltd. (a) Coating-fluorescence gauge that the airborne plot of the radioactivities, as the
which monitors the increase in intensity of X-rays excited in aircraft flies over a prescribed course, is printed
coating as its thickness increases; (b) backing-fluorescence
gauge monitors the decrease in intensity of radiation excited with the position of the aircraft onto the chart
in the backing materials as the coating thickness increases. recorder. In this way large areas of land or sea,