  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.
