Decay time measurement for Bi₂O₃ nanoparticle-loaded plastic scintillator using scintillation pulse-height distribution

Decay time spectra of a scintillator can be measured precisely using synchrotron radiation. In such measurements, one should maintain the constraint that a photodetector detects less than one scintillation photon per excitation event. This is required to ensure the random detection of scintillation light. The scintillation decay times of 20 and 0 wt% Bi2O3 nanoparticle-loaded plastic scintillators (Bi-PLSs) were measured recently using a synchrotron X-ray beam. Phosphor of 2-(4-tert-butylphenyl)-5-(4-biphenylyl)-1,3,4-oxadiazoleutyl (butyl-PBD) was mixed with 1 mol% of polyvinyltoluene-based Bi-PLSs. The scintillation pulse-height distribution of the samples was investigated using a cooled SiPM for the decay time measurement. The cooled SiPM detector demonstrated a good energy resolution and could distinguish the number of scintillation photons emitted during one X-ray pulse excitation. The experiments were executed using 67.41 keV X-rays at beamline BL-14A of the Photon Factory ring. From the pulse-height measurements, we determined that a smaller aperture covering the scintillator was crucial for decreasing the frequency of multi-photon events per X-ray event. The decay time values of 20 and 0 wt% Bi-PLS with butyl-PBD were finally obtained as 1.33 +/- 0.01 ns and 1.42 +/- 0.01 ns under an adjusted experimental condition. A statistical discussion is presented for a timing profile that the single- and multi-photon events form in a time spectrum.