On the Relationship Between Scintillation Anisotropy and Crystal Structure in Pure Crystalline Organic Scintillator Materials

Abstract

The scintillation anisotropy effect for proton recoil events has been investigated in five pure organic crystalline materials: anthracene, trans-stilbene, p-terphenyl, bibenzyl, and diphenylacetylene. These measurements include characterization of the scintillation response for one hemisphere of proton recoil directions in each crystal. In addition to standard measurements of the total light output and pulse shape at each angle, the prompt and delayed light anisotropies are analyzed, allowing for investigation of the singlet and triplet molecular excitation behaviors independently. This work provides new quantitative and qualitative observations that make progress toward understanding the physical mechanisms behind the scintillation anisotropy. These measurements show that the relationship between the prompt and delayed light anisotropies is correlated with crystal structure, as it changes between the pi-stacked crystal structure materials (anthracene and p-terphenyl) and the herringbone crystal structure materials (stilbene, bibenzyl, and diphenylacetylene). The observations are consistent with a model in which there are preferred directions of kinetic processes for the molecular excitations. These processes and the impact of their directional dependencies on the scintillation anisotropy are discussed.