Physics and Engineering of Radiation Detection

338 Chapter 6. Scintillation Detectors and Photodetectors

A typical emission and absorption spectra of an organic scintillator is sketched
in Fig.6.2.2. In general, the higher the Stoke’s shift the smaller the probability
of re-absorption of scintillation light since the overlap area will be smaller. Hence
Stoke’s shift can be used as a measure of appropriateness of a scintillator for certain
application.

λmax,a λmax,s

Li

gh

t I

ntens

ity

Stoke’s Shift

Absorption Emission

Wavelength

Figure 6.2.2: Typical

absorption and emission

spectra of an organic scin-

tillator. The wavelength

difference of the peaks of

the two spectra is gener-

ally used to quantify the

amount of Stoke’s shift.

The above mentionedSband transitions are not the only transitions possible in
organic scintillators. Another mode of electron relaxation is through the vibrational
levels in the tripletT 1 band. The process is graphically depicted in Fig.??.As
before, the incident radiation transfers the electrons from theS 0 ground level to the
S 1 vibrational levels. These electrons first decay into theS 1 ground level through
radiationless transitions. Now, instead of falling into theS 0 level directly, the elec-
trons can also first go to the availableT 1 levels. These triplet levels are much more
stable than the singlet levels and consequently the electrons can be thought of being
trapped there for an extended period of time. From theT 1 ground level, into which
all electrons eventually decay, they fall into theS 0 levels. This also results in the
emission of light but in this case it is calledphosphorescenceordelayed fluorescence,
since it is emitted after a substantial delay, which is more than 100msfor typical
organic scintillators.
Now that we understand the basic scintillation mechanism in organic scintillators,
let us discuss the different forms in which organic scintillating materials are available.
In radiation detectors the following three forms of organic scintillators are widely
used.

Plastic scintillators

Liquid scintillators

Crystalline scintillators

In the following sections we will discuss each of these in some detail.