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

Loss to scintillation light yield as a function of dose
Dose (Gy)
1e+6
1e+7
1e+8
Re
lati
ve
 Ligh
t 
Yiel
d
 (
a.u)
0,0
0,2
0,4
0,6
0,8
1,0
EJ200 
EJ208 
EJ260 
BC408 
UPS923A
TileCal

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47 
5.4.
 
Results of the fluorescence studies 
The fluorescent light emitted by the plastic scintillators upon exc itation with a 
229 nm laser was measured. The photo-bleaching of this fluorescent light was 
minimised by reducing the laser exposure time as well as scanning the laser over 
the region being measured. These conditions coupled with the detector 
positioning resulted in fluorescence off the surface region being measu red rather 
than observing “bulk” effects. Photo-bleaching time correlation tests were also 
performed in order to correct the data.
The average spectra of three measurements for the un -irradiated and irradiated 
regions respectively for all the samples unde r testing are shown in Appendix D. 
Their corresponding photo-bleaching vs time correlation curves are also shown 
alongside each spectra.
5.4.1.
 
Fluorescence peak features 
Several fluorescence peak features can be observed amongst the spectra. Peaks 
falling within the wavelength region of 300 -375 nm correlate to fluorescence off 
the PVT/PS base. Since this fluorescence is predominantly from the benzene ring 
structure in both base types, the same “two-peak” feature over this region is seen 
in the un-irradiated spectra for all the different scintillator types tested.
In the wavelength region of 375-500 nm, the fluorescence correlates to that of 
the fluor dopants. These peak features therefore vary between the different 
scintillator types. The peaks reach a maximum between 400 -430 nm for all the 
blue emitting scintillators.
EJ260 shows additional peak features at ~460 nm and 490 nm, however these are 
weaker than expected. It is possible that due to the experimental conditions, the 
emitted light measured had less spatial interaction with the fluors responsible for 
shifting the emitted light into the green wavelength region.

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48 
The effect of limited spatial interaction could also explain why peaks in the base 
fluorescence region have a high intensity comparable to the fluor regions for both 
the green and blue emitting scintillators. If fluorescence was measured over the 
bulk rather than at the surface, majority of the “base emitted light” would be re-
absorbed and shifted to higher wavelengths through interaction with the fluors.
For the samples exposed to radiation damage, a general trend in the features are 
observed. For doses of ~0.8-1 MGy, an overall intensity loss occurs, with a more 
distinct loss to fluorescence in the fluor emission regions (375 -500 nm). At this 
dose, very minimal transmission losses were observed in the transmission 
spectra, although a feature indicating loss to absorption by the fluors occurred.
At progressively higher doses, the fluorescence intensity is further decreased,
with significant loss over both the base and fluor regions. At the 80 MGy dose 
range, a very weak signal is obtained with an additional peak feature appearing 
in some of the spectra. This peak feature could not be correlated to any particular 
damage effect and may be influenced by several factors such as sample thickness 
and dose rate.