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

particular feature is proportional to the concentration of that corresponding 
specie. 

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53 
In these scintillators, the fluor concentrations are small (< 2.5%), and hence the 
features observed coincide with the base structure only. Therefore, the same peak 
features are seen in polystyrene based UPS923A and TileCal samples.
Similarly, the same features are seen among the polyvinyl toluene based EJ200, 
EJ208, BC408 and EJ260 samples. Owing to the similarity in structure of PVT 
and PS, many of the peaks observed in the PVT based samples are also present 
in the PS based ones. The EJ260 spectrum has an additional fluorescence 
background due to its higher wavelength shifting capabilities, thus leading to a 
lower spectral resolution. This background has been subtracted out in Figure 
5-14.  
The respective peaks were assigned to their corresponding vibrational modes 
using the Raman Peak assignment datasheet from Horiba Jobin-Yvon (Refer to 
Appendix F). These are summarised in Table 5-1.  
Figure 5-14: Raman spectra for the un-irradiated plastic scintillator samples

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54 
Table 5-1: Summary of peak allocation to functional and vibrational groups
5.5.1.
 
Structural damage as a function of dose 
This section refers to the spectra shown in Appendix E. An example of the spectra for the 
EJ208 type scintillator is shown below. Similar features are observed for the different 
scintillator types.
Figure 5-15: Raman spectra for EJ208 samples.
Peak number
Assigned Vibrational Mode
1-2
δ(C-C) aliphatic
3-7 , 9-14
ν(C-C) alicyclic or aliphatic chain vibrations
8
ν(C-C) aromatic ring chain vibrations
15
δ(CH
3
)
16
δ(CH
2
) or δ(CH
3
)asymmetric
17
ν(C=C)
18-19
ν(C-H)
20-21
ν(=(C-H))

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55 
At doses of < 1 MGy, the scintillators maintain their structural characteristics 
and the same spectral features are observed. As the do se exposure progresses to 
8 MGy, additional peak features begin to appear in the regions of 500 -650 cm
-1
, 
1800-2250 cm
-1
and 2250-2500 cm
-1
.These peak features become more prominent 
as dose is increased to ~25 MGy.
The 500-650 cm
-1
peak corresponds to that of (C-C) alicyclic or aliphatic chain 
vibrations. A strong peak in the region of 2100-2250 cm
-1
, corresponds to a 
𝜈(𝐶 ≅ 𝐶)
bond, however no assignment data could be found to correlate for the 
region of 2250-2500 cm
-1
. Since these peaks are broad, they could have arisen 
from an additional fluorescence process that was not accounted for in the 
background subtraction.
These peaks are most prominent in the EJ260 and EJ208 data, both of which 
contain wavelength shifting fluors that absorb and emit light to higher 
wavelengths. The Raman spectra for EJ260 samples irradiated to 25 MGy could 
not be obtained since fluorescence had saturated the signal.
A possible explanation for the formation of the additional peak at 500 -650 cm
-1
, 
can be made by considering bond breakage in the benzene ring. If the C-H bonds 
are broken, this could result in dehydrogenation (loss of hydrogen) or even loss 
of larger mass C
n
H
n
structures. This effect was observed by Torrisi [17] upon 
conducting in situ mass spectroscopy on polyvinyl toluene undergoing proton 
irradiation. In particular, stripping the benzene of its hydrogen would result in 
the formation of alicyclic rings, aliphatic chains or even 
𝐶 ≅ 𝐶 
bonds.