Page 246 - Radiochemistry and nuclear chemistry

230 Radiochemistry and Nuclear Chemistry

In nuclear installations like uranium mines, nuclear reactors, reprocessing plants, etc., it
is necessary to continuously monitor gas and liquid effluents. Figure 8.21 shows an
arrangement for monitoring of radioactive aerosols (e.g. Tc, Ru, actinides in air). Two
detectors are used, so that some activity (e.g. mother or daughter activity) is allowed to
dezay between the two detectors. The delay time is adjusted by varying the length of the
paper strip between the detector and the rate of movement of the paper strip. The detector
may be energy sensitive or simple GM, proportional or scintillation devices. In monitoring
of water the detectors (e.g. GM tubes) may dip into the streaming water, or the water may
flow around the detector as in Figure 8.8(a).
Radioactive gas flows can be measured by using capillaries or packed columns of 2 - 20
ml volume containing scintillating material, either for monitoring purposes or, commonly,
combined with a gas chromatograph for obtaining "radio-chromatograms" of 3H or 14C
labeled organic substances. These detectors, which are commercially available, are viewed
by PMTs connected to Personal-Computer (PC) based analysis and display systems.
Radioactive liquidflows can be monitored in several ways depending on the decay mode,
energy and liquid film thickness. Liquid flow GM-counters (Fig. 8.8.a), or other types of
flow cells combined with solid state detectors, are used at high/3- and "y-energies. Glass
scintillators embedded in tubings (alternatively tubings containing scintillators) are used for
low-energy B-emitters (e.g. 3H, 14C, 35S). The flow cells have typically volumes from 0.01
to 5 ml. By using peak analysis (i.e. analyzing each scintillation intensity versus time, so-
called "time-resolved technique") it is possible to measure cx-emitters in high/3-~ fluxes and
even to obtain crude a-spectra, as well as to discriminate different B-emitters and to reduce
background. A weakness of this technique is the memory effect in the detectors caused by
radionuclide sorption. Therefore, the commercial systems have easily exchangeable flow
cells. This demand is circumvented in the LISOL-system, in which the liquid flow is

Air
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Oeo 9 Detectors
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9
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9 "'"
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Filter strip ~i'.-" I

FIG. 8.21. Monitor for radioactive aerosols with prompt and delayed measurements. (From
H. Kiefer and R. Maushardt.)