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
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.
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