3 Scintillation and Plastic Scintillators In order to understand how scintillators undergo radiation damage and how this
in turn affects their performance, it is important to understand the fundamental
mechanism behind scintillation. This Chapter focuses on explaining how
scintillation occurs and how radiation affects the scintillation mechanism. The
scintillators being investigated are then described.
3.1. The scintillation mechanism Plastic scintillators primarily consist of organic fluors suspended in a polymer
base. The polymer base that is employed generally contains some form of
aromatic ring structure which gives rise to a delocalized π-electron structure
within the molecule. When ionizing radiation impinges the scintillator, part of its
energy may be absorbed by these delocalized π-electrons, resulting in molecular
excitations. The absorption is typically exhibited in the visible and ultra -violet
regions corresponding to excitation of the singlet π-electron state. The energy
level diagram of a π-electron is shown in Figure 3-1. [1]
An excited π-electron may return to its ground state through several types of
deactivation processes, with the preferred process being that which results in the
shortest lifetime of the excited state. For excitations to higher states (S
2
, S
3
or
T
2
, T
3
), these de-excite to lower states of the same multiplicity (S
1
or T
1
) by
means of internal conversion within a short time span of the order of picoseconds.
This usually occurs when the energy levels of two excited states are close enough
that their respective vibrational modes may overlap. Excitations which have
additional vibrational energy, for example the S
11
-S
13
states, can also lose energy
through vibrational relaxation, whereby heat is dissipated and thermal
equilibrium is reached in the molecule. Both internal conversion and vibrational
relaxation are non-radiative processes.
