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

5.4.2.
 
Photo-bleaching vs time correlation 
The photo-bleaching effect was assessed by acquiring several spectra along the 
same spot with continuous laser beam exposure over a period of ~30 seconds. An 
example of the bleaching effect observed is shown for an un-irradiated EJ200 
sample in Figure 5-10.  
The photo-bleaching vs time correlation graphs were plotted by taking the ratio 
between the integrated spectra for each measurement to that of the ~0 sec 
measurement. The photo-bleaching correlations for both the irradiated and un -
irradiated regions of each sample are shown in Appendix D, alongside their 
respective fluorescence spectra. The curves have been fitted with the equation 
𝑓(𝑥)
 = 
𝑦
0
+ 𝑎𝑒
−𝑡𝑏
. 

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49 
Figure 5-10: Spectra showing the photo-bleaching of fluorescence light s with laser 
exposure time for an un-irradiated EJ200 sample.
The trend observed in the correlations graphs indicate that the rate of photo-
bleaching occurs more rapidly as irradiation exposure increases for all of the 
scintillator types besides BC408. BC408 shows a slowing down in the bleaching 
rate of the irradiated samples. The bleaching occurs over both the base and fluor 
emission regions.
There are several factors that could influence the rate of photo-bleaching 
observed, however, more investigations are needed to fully understand this effect. 
Potential factors to consider include the types of fluors present and their 
concentrations by weight; as well as structural changes influenced by dose rate 
dependant damage. 
The photo-bleaching correlation curves were used to deduce a correction factor 
for the fluorescence data used in the comparative study. Figure 5-11 shows how 
the correction factor ‘
C’
was calculated, with the relevant equations used. The 
correction was calculated for 
t = 3 seconds
.

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