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

Figure 25 The scheme of plastic scintillators samples arrangements in the experimental setup. Positrons emitted 
from sodium source interact with scintillating material forming positronium atom in empty voids between and 
around polymeric chains. 
 
Figure 26 Block scheme of the vacuum part of the experimental setup for positronium lifetime measurements.
V1 and V2 denote valves, MA and MB denote measurers of vacuum. 
 
The sample and source of positrons are placed in sample chamber enabling 
stabilization and control of vacuum and temperature. The apparatus shown in Fig. 26 
consists of pump with measurers and system controlling supply valves. The turbomolecular 
pump enables obtaining vacuum of 2.1×10
-3
Torr. 
Sample chamber is made of copper. In the bottom of chamber thermocouple is 
placed and below the heater is situated. It is wound on the copper rod which can be 
immersed in liquid nitrogen. Such setup with heater power supply connected with 
thermoregulatory device Shimaden FP 21 enables an automatic temperature control. 
In measurements of positronium lifetime START signal corresponding to the time 
of the atom origin is regarded as registration of gamma quanta coming from 
22
Na source 

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57 
with energy of 1274 keV (Fig. 27). On the other detector STOP signal is registered, which 
is detection of gamma quanta of energy 511 keV. This is the energy of gamma quanta 
coming from annihilation. Therefore, positronium lifetime is the time interval between 
START and STOP signal. 
Figure 27 The scheme of 
22
Na source decay in which gamma quanta of energy 1274 keV are produced just after 
emission of positron. Gamma quanta production is preceded by creation of non stable excited state of