NaI(Tl) is 0.25msec and that for anthracene is 0.026msec. The faster decay
time permits the use of organic scintillators at higher count rates.
Solid Scintillation Detectors
See Table 8.1 for a summary of the various characteristics of the following
detectors.
NaI (Tl) Detector
Pure sodium iodide does not produce any scintillation after interaction with
g-radiations at room temperature. However, if it is doped with a trace
amount (0.1–0.4%) of thallium as an activator, NaI(Tl) becomes quite effi-
cient in producing light photons after g-radiations interact with it. NaI(Tl)
molecules are excited or ionized by interaction with g-rays or x-rays, and
the high-energy states return to ground states by emitting light photons.
Approximately 20–30 light photons are produced per 1 keV of energy.
The choice of NaI(Tl) crystals for g-ray detection is primarily due to the
high density (3.67 g/cm^3 ) of the detector and the high atomic number of
iodine (Z =53), compared to organic scintillators. However, NaI(Tl) crys-
tals are hygroscopic, and absorbed water causes color changes that reduce
light transmission to the PM tubes. Therefore, the NaI(Tl) crystals are her-
metically sealed in aluminum containers. Also, the entrance and sides of the
crystals are coated with a reflective substance (e.g., magnesium oxide) so
that light photons are reflected toward the photocathode of the PM tube
(see later). These crystals are fragile, and must be handled with care. Room
temperature should not be changed abruptly, because such changes in tem-
perature can cause cracks in the crystal.
Bismuth Germanate Detector
The bismuth germinate (Bi 4 Ge 3 O 12 or BGO) detector has a higher density
and effective atomic number and so higher attenuation coefficient (hence,
higher stopping power) for 511 keV photons than NaI(Tl). But it has
a slightly longer scintillation decay time (300 ns) compared to NaI(Tl)
(250 ns) and its light output is relatively small causing poor energy resolu-
tion. However, energy resolution has minimal effect on the spatial resolu-
tion of PET, which is mainly determined by the size of the detectors.
Moreover, BGO crystals are not hygroscopic. Because of these factors,
BGO is preferred to NaI(Tl) for most positron emission tomography (PET)
cameras.
Barium Fluoride Detector
Barium fluoride (BaF 2 ) is an inorganic crystal that has a very fast decay
time (0.8 ns) and offers a suitable detector for time-of flight PET. The
photon yield of this crystal is relatively small and it is slightly hygroscopic.
82 8. Scintillation and Semiconductor Detectors