Page 237 - Radiochemistry and nuclear chemistry

Detection atut Measurement Techniques 221

dE/dx. However, the light yield is, at best, an order of magnitude lower than that of
NaI(TI). This has mostly limited their use to counting of c~ and other energetic multiply
charged particles. Gas-flow detectors are discussed in w

8.5.2. Liquid scintillator detectors

Liquid scintillators have a wide use for routine measurement of B-emitters, particularly
low-energy ones like 3H and 14C, in liquid samples, especially in biochemistry and in
hospitals. Since these isotopes are very important in biochemical applications, most of the
development of liquid scintillation technique has been focused on them.
The sample is directly dissolved in the liquid scintillator solution (scintillator cocktail) and
the light output measured by PMTs. Normally two PMTs are used in order to eliminate
much of their internal noise by only accepting coincident pulses from both tubes. Liquid
scintillation counting offers several advantages when measuring low-energy B-emitters
compared to most other detectors. Problems like attenuation by the detector window, self-
absorption and backscattering are avoided. However, the introduction of the sample into the
scintillator medium often reduces the light output considerably, see Table 8.4 and Figure
8.17. This effect is called quenching and depends on phenomena such as chemical reactions
that absorb some of the deposited energy (chemical quenching) and changes in optical
properties (color quenching). A reduction in light output reduces the efficiency, especially
at low/S-energies. For low quenched samples the efficiency may approach 100% since the
B-particles almost always have to encounter t~, scintillator. The energy resolution is
sufficiently good to differentiate between e.g. ~ (Ea,ma x 18 keV) and '"C (Ea,ma x 160
keV).
Measurement of a-emitters is also feasible. In this case several MeV of energy is
deposited in the scintillation cocktail, usually yielding near 100 % detection efficiency. The
high amount of energy deposited also reduces sensitivity to quenching. The energy
resolution for a-particles is at best 5 -- 10 % and thus far inferior to that of surface barrier
detectors. By means of special electronics the difference in light-pulse decay time between
scintillations caused by c~ (long decay time) and by B,y (short decay time) can be used to
measure c~-emitters in samples with a high B,y-background. Liquid-flow detectors are
discussed in w

8.5.3. Solid scintillator detectors

ZnS(Ag) is a traditional phosphor for a-detection while anthracene and stilbene can be
used for B-particle detection. For -y-rays, sodium iodide with a small amount of thallium
impurity, NaI(TI), is the most common phosphor. CsI(TI) is another often used scintillator
because it can be formed to special shapes, e.g. thin sheets, much easier than NaI(TI).
Plastics with incorporated organic scintillators are often used in nuclear physics experiments
because they produce short light pulses and can be made in various shapes.
Detectors with scintillation crystals are used commonly for routine radioactivity
measurements, particularly of -y-emitters, because of their reliability. As compared to GM
tubes they have the advantage of shorter resolution time and higher "t-efficiency, although
they require a more stable high voltage supply. Particularly the well-type crystal shown in