  We demonstrated the feasibility of measuring the axial coordinate of a gamma
quantum interaction point in a plastic scintillator bar via the detection of
scintillation photons escaping from the scintillator with an array of
wavelength-shifting (WLS) strips. Using a test set-up comprising a BC-420
scintillator bar and an~array of sixteen BC-482A WLS strips we achieved a
spatial resolution of~5\,mm~($\sigma$) for annihilation photons from a
$^{22}$Na isotope. The studied method can be used to improve the spatial
resolution of a plastic-scintillator-based PET scanner which is being developed
by the J-PET collaboration.
