52
For the integrated region of 300-500 nm, the different scintillators perform within
a 20% variation of each other. EJ260 no longer exhibits the least loss as would
be expected based on the results of the light yield study. Howeve r, it should again
be noted that only surface fluorescence is being probed in this experiment. EJ208
overall performs the best against surface fluorescence light loss in thin
scintillators.
5.5.
Results of Raman spectroscopy
Raman spectra for the different samples irradiated to ~800 kGy, 8MGy and 25
MGy were measured 10 days after, 4 weeks after and 6 weeks after irradiation
respectively. The measurements were made using a 514 nm excitation laser. The
spectra are shown in Appendix E.
As observed, an increase in dose exposure led to an increase in the fluorescent
background. For this reason, samples irradiated to ~80 MGy could not be
assessed since this background suppressed the Raman peaks. This additional
fluorescence may arise from free radicals since the formation of an absorptive
tint covering the wavelength of 514 nm increased with dose, as observed from
the transmission testing.
The fluorescence background was subtracted from the data using the LabSpec5
software tool. The background subtracted spectra are also shown in Appendix E.
All spectra were normalised to the 1000 cm
-1
peak with an intensity of 5000
counts. This peak represents the C-C aromatic ring vibration typical of benzene.
Figure 5-14 shows the Raman spectra for the un-irradiated samples of each
scintillator type. The range of the Raman shift (cm
-1
) corresponds to the
wavelength range of 514-620 nm. In the Raman technique, the intensity of a
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