6.2. Organic Scintillators 349
D.1 Anthracene (C 14 H 10 )......................
Anthracene is perhaps the most widely used organic scintillator crystal. Its wide
popularity has actually made it a standard for comparing the properties of other
scintillators. The light yield of scintillators is sometimes given relative to the light
yield of anthracene. The absolute efficiency of anthracen, as we saw earlier during
the discussion on light yield of scintillators, is about 5% for blue light. Since the light
output ofNaIis superior than this, therefore some researchers prefer to compare
all scintillators with respect toNaIrather thanC 14 H 10. However the standard
practice is to compare organic scintillators with respect to anthracene and inorganic
scintillators with respect toNaI.
The peak of the pure anthracene light spectrum fall at the blue light, which has
long attenuation length in the crystal. Therefore with carefully produced crystals
one can have very good quantum efficiency. In fact, it has been observed that for
UV photons, its quantum efficiency approaches unity. The light spectrum can be
shifted by addition of a suitable impurity or more accurately a wavelength shifter
in the bulk of the material. For example addition of small amounts of naphthacene
shifts the spectrum such that the peak occurs for green photons (see Fig.6.2.10).
To remind the reader, the wavelength shifter is also a scintillator that absorbs the
photons of the primary scintillator and emits photons having a different wavelength.
Figure 6.2.10: Output light
spectra of pure (a) and doped
anthracen (b). The doping was
done with 10−^4 mole fraction of
naphthacene (11).Anthracene, being one of the earliest discovered detector-ready crystals has un-
dergone extensive studies related to effects of radiation. The fact that it has seen a
number of applications in hostile radiation environments has more with hygroscop-
icity of inorganic scintillators to do than its radiation resistance. As can be seen in
Fig.6.2.11 the light yield of anthracene whether doped or not decreases significantly
with integrated dose.