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

Figure 30 The ortho-positronium intensity (I
3
) dependence on time in temperatures: 123 K (black diamonds),
173 K (violet dots) and 298 K (blue triangles) in scintillator based on polystyrene, containing 2 % of PPO and
0.03 % of 2-(4-styrylphenyl)benzoxazole. 
In the room temperature (298 K) I
3
has relatively high value, equal to about 42 %. 
Then the sample was cooled down to 123 K and stored in this temperature for 15 hours. 
During this time, o-Ps lifetime (τ
3
) was stable (Fig. 29) but its intensity decreased 
significantly. Also in 173 K o-Ps production intensity is unstable in time. Time constants 
are equal to 2 h in 123 K, 8 h in 173 K and 10 h in 298 K. Time constants determine an 
amount of time which is needed to stabilize I
3
intensity. It is shorter for lower 
temperatures. In the article [100] in modified polystyrene similar effects were observed. 
Described time constants vary with temperature in range from 3 h to 30 h.
Fast decrease of intensity with time may be caused by competition between two 
processes: creation and decay of free radicals [100]. In case of our research, the o-Ps 
intensity in polymeric material is affected also by the temperature difference between the 
last point before jump and the temperature of long measurement. In the article [97] similar 
research were carried out, however the increase of I
3
in low temperature were observed, 
contrary to our research. The differences are caused probably by different polymeric 
material.
The size of free volumes was calculated assuming that they are spherical and using 
the Formula (4), and the results of these calculations are shown in Fig. 31. In the room 
temperature in which scintillators will be used, the volume of free voids is about 0.1 nm
3
. 
With increasing temperature, sizes of pores increase. At temperature of 300 K the size of 

---

63 
free volume is doubled. In calculations it was assumed that volumes are spherical. 
However, in plastic empty voids can have different shapes, so obtained results are 
indicative and averaged over voids of different sizes and shapes.