Directional anisotropy in organic scintillation crystals

Abstract

The scintillation pulse height responses and scintillation pulse shape discrimination (PSD) properties of anthracene crystals have been investigated for protons at six different energies in the range 1–22 MeV. Similar investigations were made for stilbene, terphenyl and quaterphenyl crystals at 8 MeV and for stilbene and naphthalene crystals at 22 MeV. The pulse height and PSD responses were investigated as a function of proton direction using recoil protons released within the crystals by monoenergetic incident neutrons. The pulse height anisotropy of anthracene crystals varies from 33% at 1 MeV to 8% at 22 MeV. The pulse height anisotropies of the other crystals are similar to that of anthracene at the same energy. The PSD anisotropy of anthracene expressed as a percentage of the PSD dispersion between Compton electrons and recoil protons varies from 3% at 1 MeV to 92% at 22 MeV. The PSD anisotropy of naphthalene is about half that of anthracene at 22 MeV and the PSD anisotropies of the other crystals are 3–7 times smaller than that of anthracene at 8 MeV. The pulse height and PSD anisotropies of anthracene are consistent with a model which assumes that the transfer of excitation energy during the scintillation process occurs preferentially in directions lying within the ab-planes of the crystal lattice.