Physics and Engineering of Radiation Detection

6.2. Organic Scintillators 339

S 2

S 1

S 0

Incident

Radiation

λmax,a

T 3

T 2

T 1

Inter−state Transitions

λmax,p>>λmax,a

E

Phosphorescence

Singlet States Triplet States

Figure 6.2.3: Typical energy level diagram of organic scintillators. The inci-

dent radiation transfers the electrons fromS 0 ground level toS 1 levels. These

electrons first decay intoS 1 ground level and then either fall into theS 0 levels

directly (see Fig.6.2.1) or through the tripletT 1 state. The later transition

results in the emission of phosphorescence photons after a delay that depends

on the lifetime of theT 1 state. The wavelength of phosphorescence photons

λmax,pis much larger than the absorption wavelengthλmax,a.

6.2.B Plastic Scintillators

Plastic scintillators are extensively used in a variety of applications. They are syn-
thetically produced through a process that is time consuming and highly labor in-
tensive. The basic idea, however, is quite simple: add the scintillator in a convenient
base and then polymerize the mixture. The practical situation is not that simple,
though. First of all, all the constituents must be in highly purified form and secondly
there are waiting periods involved between the different steps to let the material set-
tle down to the required configuration. The base material that is generally used for
the mixture is acrylic, polystyrene, or polyvinyltoluene monomer. A convenient
scintillator is then added to this base and mixed homogeneously. The concentration
of this main scintillator is about 1% by weight of the mixture, which is sufficient to
produce a high light yield. Most of the time another scintillator is also added to the
mixture. The individual purposes of all these materials will be discussed shortly.
The whole mixture is then polymerized to form the plastic.
A plastic scintillator produced by this method has several attractive qualities,
such as

it is chemically stable,

it has high degree of optical homogeneity, and

it can be cut and machined into virtually any shape.
We have deliberately not said anything about their light yield, decay constants,
and other important parameters. The reason is that the commercially available
plastic scintillators have such varied properties that it may be misleading to quote
one of them as a representative of all plastic scintillators.