The aim of this work was to experimentally check the theoretical predictions of plastic scintillators? efficiency. The BC?420 scintillators have been used in the prototype of the J-PET detector. The measurement system consisted of two BaF_2 detectors, a plastic detector, an oscilloscope, 4 photomultipliers and a sodium isotope 22Na (as a source of positrons). The assessment of the plastic scintillators? efficiency was conducted by counting the number of registered and unregistered events from 511 keV gamma ray beam.
The efficiency was determined for 6 detector depths, in particular for 1.9 cm, which is the thickness of plastic scintillators used in the J-PET system. The obtained efficiency for this setup is (11.1 +- 0.3stat +- 3.0syst)%, for events with energy deposited greater than 100 keV, while theoretical prediction is 11.6 % with negligible error.