Physics and Engineering of Radiation Detection

6.2. Organic Scintillators 337

in differentS 1 vibrational levels to go to theS 1 ground level through radiationless
transitions. These transition are very fast and are completed within a few tenths
of a nanosecond. From there the electrons fall into theS 0 ground and vibrational
levels with decay times on the order of a few nanoseconds. The excess energy of the
electrons is emitted in the form of fluorescence photons, which for most materials
lie in the ultraviolet or visible region of the electromagnetic spectrum. The decay of
electrons from theS 1 vibrational states toS 1 base level is a favorable process spe-
cially for scintillation detectors. The reason is that during such decays the electrons
loose some of their energy. Consequently the energy emitted during their transitions
from theS 1 ground level to theS 0 levels is less than the energy absorbed by them
during the radiation induced transitions fromS 0 ground level toS 1 levels. This
essentially implies that the absorption and emission spectra of such materials do
not match, hence disfavoring the re-absorption of the scintillation light in the ma-
terial. This phenomenon was first observed by Sir George G. Stokes, who later on
formulated aStoke’s lawthat says that the wavelength of fluorescence light is always
greater than the wavelength of absorption. Of course, this law does not really hold
for all wavelengths and all materials since there is always some overlap of emission
and absorption spectra. However the so callStoke’s shiftis a reality, which simply
characterizes the fact that the peak of emission spectrum is shifted from the peak
of absorption spectrum. The amount of Stoke’s shift can be defined as

λStokes=λmax,s−λmax,a. (6.2.1)

Hereλmaxrepresents the peak of the spectrum with subscriptsaandsreferring to
the absorption and scintillation (fluoroscence) respectively.

S 2

S 1

S 0

Incident

Radiation

λmax,a

λmax,s>λmax,a

Inter−state Transitions

Fluoroscence

E Singlet States

Figure 6.2.1: Typical singlet energy level diagram of organic

scintillators. The incident radiation transfers the electrons

fromS 0 ground level toS 1 levels. These electrons first decay

intoS 1 ground level and then fall into theS 0 levels and as

a result emit scintillation light. The most probable wave-

length of emissionλmax,sis greater than the most probable

wavelength of absorptionλmax,a.