=8.3%
The energy resolution depends on the photon energy. The higher the
photon energy, the better the energy resolution (i.e., smaller FWHM),
because of the decrease in the percentage of statistical variations in the
pulse production. The FWHM (%) of NaI(Tl) detectors is about 7% to 10%
for the 662-keV g-ray of^137 Cs and 10% to 14% for the 140-keV g-ray of
99mTc. In contrast, the FWHM (%) in Ge(Li) detectors is about 0.42% for
140-keV g-rays and about 0.2% for photons of more than 1 MeV.
Detection Efficiency
The detection efficiency of a counter is given by the observed count rate
divided by the disintegration rate of a radioactive sample. The count rate
of a sample differs from the disintegration rate because of several factors.
Radiations from a source are emitted isotropically around 4psteridians, but
only a fraction of all photons emitted strikes the detector, depending on the
solid angle subtended by the detector on the source. Only a fraction of all
photons striking the detector may interact in the detector and produce
pulses. Only a fraction of all pulses produces a single photopeak. Further-
more, the count rate is affected by the abundance of a particular radiation
Energy resolution %()=×55
662
100
96 8. Scintillation and Semiconductor Detectors
Fig. 8.7. The full width at half maximum (FWHM) of the 662-keV g-ray of^137 Cs in
a NaI(Tl) detector.