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
.
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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