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

Raman Spectroscopy 
Raman spectroscopy was conducted in order to identify if a change in species 
occurs within the scintillators when subjected to radiation damage. The Raman 
Effect is the result of inelastic scattering of light, whereby incoming photons 
interact with lattice phonons to cause virtual excitations. These virtual 
excitations then de-excite by releasing a photon of wavelength corresponding to 
a specific virtual mode. These Raman active modes are characteristic of certain 
bonding structures. Thus, from a Raman spectrum, one can identify the specie 
and bonding structure within a solid by identifying their corresponding peaks.
Raman spectra were obtained for the un-irradiated control samples as well as on 
the 0.8 MGy, 8 MGy and 25 MGy irradiated samples using the LabRAM HR 
Raman spectrograph. A 514 nm Argon laser was used to excite the Raman modes. 
The spectrograph was calibrated using the 579 cm
-1
peak (yellow doublet) 
expected from a mercury discharge lamp.
The Rayleigh peak that results from elastic light scattering was eliminated by 
using a notch filter. The noise of plasma lines that arise from the laser were 
filtered out with an interference filter. Raman studies for the 0.8 MGy samples 
were conducted 10 days after irradiation while 8 MGy and 25 MGy samples were 
tested 4 weeks and 6 weeks after irradiation respectively. Spectra obtained were 
analysed using the LabSpec5 software tool.

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36 
5
 
Results and analysis 
5.1.
 
Observations 
After irradiation, samples obtained a yellow to brown discolouration as dose 
exposure increased. After removal from the microprobe, samples were exposed 
to air and a fading of the discolouration was observed. For the 800 kGy irradiated 
samples, the fading occurred within several minutes from removal and irradiated 
regions needed to be marked quickly so as to discern the irradiated spot.
The higher dose irradiated samples had a more gradual fade, with a region of 
permanent discolouration remaining after several days. Figure 5-1 shows the 
discolouration after irradiation in EJ208 samples of different doses. Figure 5-2 
shows two ~25 MGy irradiated samples after they have healed and some of the 
discolouration has faded.