Solid scintillators include materials such as sodium iodide, lithium iodide, anthracene, naphthalene and
'loaded' polymers. Sodium iodide detectors are by far the most important, and subsequent discussions
will be restricted to these. Crystals of NaI which have been activated by the incorporation of 1% of TlI
were the cornerstone of γ-ray spectrometry for many years. On striking a crystal γ-rays interact in a
number of different ways – the most important are photoelectric absorption and Compton scattering. In
the former the γ-photon is completely absorbed, and excites electrons to energies directly related to the
photon energy. Thus, ultimately the intensity of the light scintillation, and the height of the voltage
pulse produced, are proportional to the energy of the incident photon. In this way bases for pulse height
analysis and γ-ray spectrometry are provided. When Compton scattering occurs, the γ-photon loses
some of its energy in preliminary collisions before being photoelectrically absorbed. It follows that γ-
spectra will show distinct 'photopeaks' and a background of lower energy derived from the Compton
effect (Figure 10.11). Simplicity and high detection efficiencies are the valuable features of NaI(TlI)
detectors, which are to be contrasted with the relative complexity, lower efficiencies but exceptional
resolution of semiconductor systems (Figure 10.11(a)).
Liquid scintillators are employed largely for measurements on pure negatron emitters (^14 C,^3 H,^32 P,^35 S)
and are especially valuable when the negatron carries a low energy, e.g.^3 H(0.018 MeV),^14 C(0.16
MeV). Ideally the scintillator and the sample are dissolved in the same solvent to ensure intimate
contact between the radionuclide and the scintillator. Xylene, toluene or dioxan are suitable solvents.
The latter will retain the scintillator in solution with up to 20% of water. Insoluble samples may be
measured in suspensions stabilized with silica gel. The energy of the particles is absorbed by the solvent
and transferred to the scintillator, finally being re-emitted as a pulse of UV radiation which activates the
photomultiplier. A typical 'cocktail' for liquid scintillation counting contains a secondary scintillator,
which shifts the scintillations to longer wavelengths. In so doing it will shift the emission peaks into a
more sensitive region for the photomultiplier. Typical scintillators are conjugated aromatic molecules
whose characteristics are shown in Figure 10.12. The intensity of a scintillation is related to the energy
of the incident ionizing particle, whence pulse height analysis may be employed to provide a measure of
spectrometric discrimination. However, reference back to Figure 10.3 will re-emphasize the problems
of negatron spectrometry and it will be seen why only two component mixtures can be satisfactorily
analysed. Moreover, this can be accomplished only if there is a large difference in the negatron energies
(e.g. ten-fold). The important interferences associated with liquid scintillation counting are
chemiluminescence, bioluminescence, chemical quenching and colour quenching. The first arise when a
chemical or biochemical reaction within the matrix stimulates the emission of radiation. Refrigeration
of the sample