Figure 19 Relative and absolute light output of J-PET plastic scintillators. The relative light output was determined with respect to the BC-420 plastic scintillator. Based on data presented in Fig. 19 and Tab. 8, one concludes that the highest light
output is achieved for J-PET scintillator containing 0.05 ‰ of novel WLS, so called
0.05J-PET. It corresponds to 10318 ± 139 photons/MeV. These values meet the light
output of the light output of BC-420, which is equal to 10240 photons/MeV.
The light output of J-PET scintillators of lower and larger concentrations of
2-(4-styrylphenyl)benzoxazole, is lower than in case of 0.05J-PET. Up to the concentration
of 0.05 ‰, the amount of wavelength shifter is inefficient to enable an effective energy
transfer. Considering values larger than 0.05 ‰, light loss is due to concentration
quenching [79]. 0J-PET scintillator does not contain wavelength shifter, only polymeric
matrix and primary fluor. Wavelength of the light produced by this sample lies within UV
range which is not matched to the quantum efficiency of photomultipliers and is strongly
absorbed in the material. That is why less light is detected and the measured light output of
the scintillator has significantly lower value in comparison to other samples.
Optimization of wavelength shifter concentration was carried out for scintillators
samples with dimensions 14 mm × 14 mm × 20 mm. However, according to
the article [31], concentration of wavelength shifter in plastic scintillator is not a fixed
characteristics. It is dependent on its length and even the shape. In larger J-PET scintillator
strips, the optimal concentration of WLS will be probably lower than in tested 2 cm long
47
scintillators. Results described in the thesis prove that 2-(4-styrylphenyl)benzoxazole can
act as an effective wavelength shifter in plastic scintillator and its action is comparable
with commercially used substances.
