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

Figure 4-12: (a) Back view showing the path travelled by the laser into the LabRAM HR, 
(b) front view of the spectrograph, (c) zoomed view of the sample undergoing 
fluorescence. 
The main obstacle to overcome during testing, was the effect of photo-bleaching 
of the fluorescent light. For excitation wavelengths below 250 nm, photo-
bleaching occurs more prominently since the probability of exciting the electron 
to the triplet state increases. This is a stable state with a long lifetime and can 
interact with other molecules to produce irreversible covalent modifications. 
Photo-bleaching therefore results in a decrease to the fluorescence yield since 
molecules undergo photon induced chemical damage.
The destruction of the molecule is proportional to the emission intensity, the 
emission time and the number of excitation and fluorescence cycles undergone. 
In order to reduce the effect of photo -bleaching undergone during testing, the 
laser was scanned over a 20x20 µm
2
area and the acquisition time was limited to 

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35 
one second per spot tested. Three spots along the irradiated region and three spots 
along the un-irradiated regions of each sample were tested in order to gauge the 
ratio of loss to fluorescence yield over the wavelength range of 350 -500 nm.
Each test required manually starting the acquisition and switching the laser onto 
the sample simultaneously. An uncertainty in the time of exposure from laser 
switch on to acquisition start time was therefore estimated to be ~2 -3 seconds. 
Photo-bleaching vs time correlation curves were therefore made over a 30 second 
time span. These were used to correct the data. Fluorescence experiments were 
conducted in the year after irradiation was performed.
4.2.4.