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

Figure 3-2: Radiative transfer of energy between polymer base, primary and secondary 
fluors adapted from [16]. 
3.2.
 
Radiation damage in plastic scintillators 
Whilst plastic scintillators provide an efficient way to detect ionizing particles, 
it is this same interaction that causes its subs equent damage. When ionizing 
particles pass through the scintillator medium, they leave behind them a wake of 
excited particles, some of which undergo fluorescence to emit light. At high 
incident doses however, this radiation may also break chemical bonds therefore 
modifying the polymer properties.
The damage process is complicated and can be influenced by a number of factors 
including the rate of irradiation, the amount of dose absorbed, and the nature of 
the impinging particle (particularly its stopping power), as well as the 
environment under which the radiation occurs [17]
[18]
[19]. Structural properties 
of the plastic polymer can also influence the extent to which t he damage occurs.

---

18 
The biggest effect of radiation damage is that it alters the optical properties of 
the scintillator. This can manifest in two ways, either a decrease in actual 
scintillation light output due to damage to the fluorescent component, or thr ough 
a degradation in its transmission character as a result of the formation of optical 
absorption centres. The change in transmission character further affects the light 
attenuation length of the scintillator. [15]
Primary and secondary fluor dopants are in general relatively radiation hard. It 
is therefore believed that majority of the radiation damage arises in the polymer 
base. This was demonstrated in studies performed on an organic doped liquid 
scintillator (BC505), after exposure to 5 Mrad (50 kGy) of irradiation from a 600 
Ci 
60
Co gamma source [20].
In these studies, large losses were observed in the intrinsic scintillation output in 
response to a radioactive source despite the fairly small transmission losses seen. 
Upon investigation of the un-doped liquid base, a transparency loss was observed 
to occur, indicating the formation of an absorptive front which tails off at higher 
wavelengths. The relevant spectra of this study are shown in Figure 3-3.