Development of novel plastic scintillators based on polyvinyltoluene for the hybrid j-pet/mr tomograph

3.4. Applications of 2-(4-styrylphenyl)benzoxazole 
 
In this thesis for the first time application of 2-(4-styrylphenyl)benzoxazole as
a plastic scintillator dopant is described. The aim of the dopant is shifting of the 
scintillator's emission spectra towards longer wavelengths to match it to the quantum 
efficiency of SiPMs. 
2-(4-styrylphenyl)benzoxazole can be obtained in relatively simple and cost-
effective chemical synthesis. Method of synthesis is described in patent [49] and in the 
review article [50]. The substance was synthesized for the first time by A. E. Siegrist and 
collaborators in 1960 and applied as optical brightener.
2-(4-styrylphenyl)benzoxazole was used also as an emission material in OLED 
(organic light - emitting diode), electroluminescent diodes [51] or as NLO (nonlinear 
optics) material [52]. It was applied in cosmetology, dentistry and in photographic 
materials [53]. The substance was utilized in photosensitive coating [54] as well as
a whitening agent [55] and in optical disks for lasers [56]. 
 
 

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4. Experimental methods 
Polymerization process, in which plastic scintillators are obtained is conducted in
a furnace in a specially designed glass reactors or in a form. Scintillating dopants were 
dissolved in liquid purified monomer (styrene or vinyltoluene). Scintillators were obtained 
by bulk polymerization of such prepared samples. The mechanism of this kind of 
polymerization is free radical. This is a process occurring in pure monomer. To avoid 
contamination of the material, the polymerization is thermally initiated, without any 
chemical initiators. High concentration of monomer enables high rates and degrees of 
polymerization. However, there is a problem with increasing viscosity of the mixture when 
the polymerization proceeds. Bulk polymerization reaction is highly exothermic and 
increasing viscosity inhibits heat flow leading to formation of regions of local overheating. 
As a result, empty voids can be generated in a block of polymer because of the internal 
shrinkage. 
Bulk polymerization allows to produce scintillator characterized by high light 
output due to e.g. its homogeneity [57]. The temperature schedule was adjusted to obtain 
optically homogeneous scintillator samples and eliminate effects of polymerization 
shrinkage. Production of scintillators and optimization of their composition maximizing 
scintillator light output was the first stage of the research.
Two scintillating dopants were dissolved in the monomer: primary and secondary 
fluor. In prepared scintillators, primary fluor is a commercially available compound:
2,5-diphenyloxazole (PPO). As a secondary fluor 2-(4-styrylphenyl)benzoxazole was used. 
It is a substance chosen amongst three chemical compounds which was synthesized and 
tested 
as 
wavelength 
shifter 
in 
plastic 
scintillator. 
Only
2-(4-styrylphenyl)benzoxazole posseses exceptionally good scintillating properties. The 
use of this substance as a scintillator dopant and the novel scintillator composition are 
subject of patent application [58]. 
Measurements of light yield were carried out in detector laboratory. Charge spectra 
were registered irradiating scintillators with 
22
Na source of gamma quanta with energy of
511 keV originating from annihilation of positron with electron (Fig. 1). The source was 
placed in the lead collimator providing narrow beam of gamma quanta, about 1 mm wide. 

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24 
Interaction of gamma quanta with the scintillator results in production of light. To both 
sides of scintillator photomultipliers are connected (see Fig. 2). They play a role of 
converters of scintillation light into electrical signals. Then signals are collected and 
processed by the oscilloscope. Determining the position of the middle of the Compton 
edge on the charge spectrum histogram, light output of manufactured and purchased 
scintillators were appointed and compared. Light output is the most important parameter of 
the scintillator and determines the number of photons emitted per unit of energy deposited 
in the scintillator. It gives information about the effectiveness of conversion of the incident 
radiation into photons.
Optimal 
concentration 
of 
the 
novel 
scintillating 
dopant:
2-(4-styrylphenyl)benzoxazole was set maximizing light output of the scintillator. 
Scintillators with different concentrations of the dopant were prepared and light output of 
the samples were measured. 
Based on measurements conducted with the setup enabling determination of the 
light output, characterization of signals arising in synthesized and commercial scintillators 
were done. Rise and decay times of signals were determined and compared with 
commercially available scintillators. The shorter the decay time, the better is the scintillator 
concerning application in J-PET/MR scanner. 
Scintillators were subject of tests in order to measure the emission spectrum. 
Emission spectra of thin samples of J-PET were registered and compared with quantum 
efficiency of silicon photomultipliers to check if they are matched to each other. A proper 
matching of these quantities is necessary for an effective light conversion into electrical 
pulses by photomultiplier. 
Characterization of scintillators structure by analyzing sizes and fraction of free 
volumes in particular samples were carried out using Positron Annihilation Lifetime 
Spectroscopy (PALS). This technique enables very accurate analysis of free volume sizes 
in the scintillator and any structural transitions occurring with the temperature changes. 
Glass transition temperature and temperatures of some structure changes correlated to 
organization of molecules can be observed. 
Samples of plastic scintillators were analyzed also using Differential Scanning 
Calorimetry (DSC). This is the method which can be considered as a complementary to 
PALS. It is based on measurement of the amount of heat released or absorbed during 

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physical or chemical process. DSC allows to describe thermal transitions in polymers.
In case of polystyrene or polyvinyltoluene, the most significant is glass transition (T
g
) 
temperature. It is a point of transition of amorphous brittle polymer into rubbery one. T
g
is 
also visible in PALS measurement, therefore results obtained in both methods were 
compared.