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

Figure 3-3: (a) Pulse height spectra for BC505 samples before and after the 5 Mrad 
irradiation. (b) The transmission spectra for the BC505 sample. (c) The transmission 
spectra for the undoped base of BC505 before and after the 5 Mrad irradiation. Obt ained 
from [20]. 
When this additional absorptive component extends to the wavelength region 
where radiative transfer between fluors occurs, this forms a competitive process 
to the regular light shifting mechanism and hence reduces the intrinsic light 
output of the scintillator. Furthermore, if the induced absorption extends to even 

---

19 
higher wavelengths, it can greatly reduce the attenuation length and cause 
additional losses in bulk samples.
The formation of new absorptive centres, is proportional to the absorbed dose 
and can be related to degradation of the polymer base. A study was conducted by 
Torrisi [17] that investigated the types of species desorbed by the polymer 
polyvinyl toluene (~200 µm thick) during proton bombardment. This was 
assessed using “in situ” mass quadrupole spectroscopy. It was found that C-H 
bond breaking, hydrogen degassing and free radical formation occurred for high 
stopping power irradiations. In these studies, both free radical formation as well 
as their recombination occurred at a rapid rate, with a diffusion of reco mbined 
species being observed towards the polymer surface.
It should be noted that plastic scintillators show a discolouration along with 
damage. For blue emitting scintillators, this is typically observed as a yellowing, 
increasing with absorbed dose until subsequent browning ensues. Orange and 
green discolouration have also been observed in some scintillators [15]. These 
discolouration’s can be linked to free radical production or to trapped electrons 
and conjugated double bonds [18]. Free radicals in particular absorb light and 
therefore cause a reduction to the amount of scintillation light emitted.
A fraction of the radiation induced absorption i s recoverable and this ‘healing’ 
of damage may be accelerated by exposure to oxygen or heating under vacuum. 
Free radicals react with oxygen to produce peroxide radicals which generally do 
not absorb visible light and therefore result in bleaching
[18].
If irradiation occurs under oxygen exposure, this influences the amount of 
damage undergone by the scintillator and the rate of oxygen diffusion into the 
sample has been shown to play a role in dose rate dependant effects . If oxygen is 
absent, ‘healing’ can still occur when radicals interact with hydrogen or when 
two radicals annihilate. These second order processes can occur via cross -linking, 
disproportionation or recombination and therefore modify the polymer structure
[19].

---

20