particular, an increase in transmission is observed over this region for the blue
scintillators.
Since these regions correlate to absorption by the fluors, less absorption could
lead to a breakdown in the light transfer mechanism and hence a loss in final light
output of the scintillators. This effect is further enhanced in the ~8 MGy Tilecal
samples, and partially present in the UPS923A samples, both of which contain
the polystyrene base as well as a larger fraction of fluors versus the other
scintillator types.
As the exposure dose is increased, a shifting of the absorptive edge to higher
wavelengths is observed. This new absorptive component may mask the effect of
absorption loss by the fluors. The features are a result of free radical production
caused by the radiation damage. These free radicals compete for light absorption
and therefore quench the eventual light yielded by the scintillator. Since this
additional absorptive component corresponds to the blue wavelength region, this
accounts for the yellow discolouration observed in the scintillators.
In addition, a certain portion of the shifted absorptive tint is seen to recover over
time. A significant amount of this recovery occurs within the first day after
irradiation. This correlates with the visual fading of the discolouration observed
in the samples. Samples were stored out of direct contact with visible light
between irradiation and testing, but were exposed to air. It is believed that th e
healing is therefore mostly driven due to free radicals interacting with oxygen
thereby causing their bleaching.
Although the scintillator transmissions were monitored over a day to day bases,
a time concise correlation was not made. Hence a comparison b etween the rates
of recovery between the different scintillator types could not be drawn. The
transmission spectra taken 4 weeks after irradiation were used for the
comparative study.
