Figure 12 Absorption (green solid line) and emission (red dashed line) spectra of POPOP in cyclohexane [69]. In the case of 2-(4-styrylphenyl)benzoxazole, the Stokes shift resulting from the
difference between peaks of 402 nm and 349 nm, is equal to 53 nm. This value is
comparable
to
Stokes
shift
of
POPOP.
Therefore
Stokes
shift
of
2-(4-styrylphenyl)benzoxazole is sufficient to use the substance as a scintillator dopant.
Another issue which is necessary to consider is adjustment of the primary additive
to the 2-(4-styrylphenyl)benzoxazole to provide an effective energy transfer. According to
Fig. 5, in plastic scintillator the energy is transferred from primary to secondary fluor in the
way of fluorescence. However to provide an effective transfer of energy, emission
spectrum of primary fluor has to overlap with absorption spectrum of secondary fluor. In
Fig. 13 and Fig. 14 absorption and emission spectra of plastic scintillating additives are
presented. As a primary additive 2,5-diphenyloxazole (PPO) and p-terphenyl (PTP) were
considered while 2-(4-styrylphenyl)benzoxazole (novel WLS) is assumed to act as
a wavelength shifter, which is secondary additive.
In both sets of mixture: PPO + novel WLS and PTP + novel WLS, the emission
spectrum of the primary fluor overlaps with the absorption spectrum of secondary fluor. In
case of PPO, their maxima fits almost perfectly to each other, while PTP and novel WLS
maxima are slightly shifted within few nanometers.
