Proton induced radiation damage studies on plastic scintillators for the Tile calorimeter of the atlas detector

Figure 3-1: Energy level diagram of an organic molecule with π -electron structure, adapted 
from [1]. 
The fluorescence process occurs for transitions from the lowest vibrational first 
excited state (S
10
) to the ground state, where energy is emitted as a photon of a 
characteristic wavelength. This process forms the primary mechanism for 
scintillation in organic plastic scintillators. Fluorescence occurs within a matter 
of nanoseconds which gives plastic scintillators their fast re sponse capability.
Sometimes, transitions called inter-system crossing can occur whereby excited 
singlet states are converted to triplet states. The lifetime of the T
10
triplet state is 
longer than that of the S
10
state and so the de-excitation to the ground state from 
the triplet state takes longer. As a result, a delayed light emission called 
phosphorescence occurs. The wavelength of the light emitted in phosphorescence 
is longer than that of light emitted by fluorescence.

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16 
A final energy loss mechanism that may occur is the process of delayed 
fluorescence. In this process, molecules in the triplet state may undergo thermal 
excitation and excite back into the singlet state before undergoing fluorescence 
to the ground state. [1] Phosphorescence and delayed fluorescence are considers 
as quenchers of fluorescent light since these three radiative processes can 
compete with each other.
The energy of fluorescent light is generally less than the energy required for 
absorption because the fluorescence transition can occur to any of the ground 
state’s vibrational levels and because absorption causes a change to the 
equilibrium internuclear potential
[15]. However, a small amount of overlap may 
occur between the absorption and emission wavelength ranges resulting in re -
absorption of scintillation light. Typical plastic scintillator bases have a very low 
fluorescent yield and therefore aren’t very transparent to their own scintillation 
light.
Primary fluor dopants are thus added in small concentrations (typically < 3% by 
weight). Primary fluors are chosen such that their absorption spectra match the 
emission spectra of the base and generally contain a high quantum yield of the 
energy transfer transition. Light can be transferred between base and fluor via 
either radiative re-absorption, or by a non-radiative coulombic interaction called 
Forster resonance energy transfer. [15] 
Forster energy transfer is limited by the distance between the interacting states 
and is therefore more likely to occur with increasing fluor concentrations until a 
saturation is reached. Light is then emitted by the fluors at higher wavelengths, 
generally in the UV range of 340-360 nm.
Since this wavelength is still below the peak efficiency of common 
photomultipliers, a secondary fluor is added at concentrations of < 0.1% by 
weight. The secondary fluor acts as a wavelength shifter and prevents re-
absorption of scintillation light by the primary fluor. It also helps to increase the 
bulk attenuation length of the emitted light. Energy transfer between the primary 
and secondary fluors occurs via radiative exchange [15]. A schematic of the 

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17 
radiative transfer of energy from polymer base to the primary fluor and secondary 
fluor is shown in Figure 3-2.