Figure 21 Averaged signals from photomultiplier connected to BC-420 (top panel) and J-PET scintillator (bottom
panel). Superimposed dashed-red lines indicate result of the fitting of sum of functions given by formulas 4 and 5.
Signals were normalized to 1 V.
Decay time of signals registered by photomultipliers connected to BC-420
scintillator determined by the method described above is equal to about 1.49 ± 0.02 ns.
This is the value consistent with 1.5 ns, which is declared by the producer [12].
50
However, there is a discernible difference between decay time of signals in BC-420 and
0.05J-PET scintillator in which it is equal to 1.91 ± 0.03 ns.
Decay time is an important parameter characterizing plastic scintillators, especially
these which will be applied in experiments based on measurements of signals time,
like J-PET/MRI. It is one of the most important factors determining time resolution, which
is a measure of the time interval in which two signals can be distinguished. The shorter the
decay time is, the better is the scintillator.
BC-420 is one of the fastest plastic scintillator manufactured by Saint Gobain, what
means that its decay time has very low value. However, decay time of signals in other
plastic scintillators e.g. BC-400, BC-404, BC-408 are equal to 2.4 ns, 1.8 ns, and 2.1 ns,
respectively. Therefore we can claim that decay time of signals in J-PET scintillator, which
is approximately equal to 1.9 ns, is comparable with BC commercial scintillators and they
are suitable for application in PET/MRI devices.
8. Structure of J-PET scintillators
8.1. Molecular weight
Condition of polymerization process and Trommsdorff effect influence the
molecular weight of polymers. As it is apparent from article [84], there is a dependence of
plastic scintillators light output on the polymer molecular weight (Fig. 22). Up to a value
of about 10
5
u, the light output increases with increasing polystyrene molecular weight.
Above this value, light output reaches plateau.
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