The readers are referred to reference physics books for details of these
methods.
A problem with liquid scintillation counting is the noise due to sponta-
neous thermal emission of electrons from the photocathode of the PM tube.
Background noise also arises from the interaction of light with scintillation
solution. Thermal emission of electrons is reduced by refrigeration of the
counting chamber to keep the PM tubes at low temperature. But the coin-
cidence counting is the most effective method to reduce the noise.
The liquid scintillation counting systems are provided with automatic
sample changers for counting as many as 500 samples. Also, one to five
PHAs are available on a liquid scintillation counter, so that b−-particles of
different energies can be counted simultaneously by using different base-
lines and windows on each PHA. The b−-emitters,^3 H,^14 C,^32 P, and^35 S, are
commonly detected by liquid scintillation counting. Whereas the counting
efficiencies of^3 H (Emax = 0.018 MeV) and^32 P (Emax= 1.71 MeV) are
~60–70% and ~100% respectively, they are negligible for g- and x-rays.
Characteristics of Counting Systems
Detection of radiation and therefore counting of radioactive samples is
affected by different characteristics of the detector and the associated elec-
tronics. The following is a discussion of these properties.
Energy Resolution
As already mentioned, even though g-rays of the same energy are absorbed
in the NaI(Tl) detector by the photoelectric effect, pulses of different ampli-
tudes are produced because of the statistical variations in the production
of light photons in the detector and photoelectrons and electrons in the PM
tube. This results in the broadening of the photopeak. The width of the peak
or the sharpness of the peak (i.e., the energy resolution of the detector) pre-
dicts the ability of the NaI(Tl) spectrometer to discriminate between the
g-ray photons of dissimilar energies. A similar situation exists for semi-
conductor detectors where the number of ionizations may vary from one
g-ray to another of the same energy, leading to the broadening of the
peak.
The energy resolution of a system is given by the full width at half-
maximum (FWHM) amplitude of the photopeak and is expressed as a per-
centage of the photon energy as follows:
(8.1)where Egis the energy of the g-ray photon. In Figure 8.7, FWHM is 55 keV
for the 662-keV peak of^137 Cs; therefore,
Energy resolution %FWHM
()=×
Eg100
Characteristics of Counting Systems 95