Absolute Scintillation Efficiency of Anthracene Crystals

Abstract

The absolute scintillation efficiency of anthracene crystals excited with fast electrons is measured. The experimental method used avoids the errors existing in previous measurements which are due to the anisotropic refraction, spatial distribution and re-absorption of fluorescence in these crystals. At a temperature of 290°K the intrinsic scintillation efficiency (in the absence of re-absorption of fluorescence) is found to be 0 062 ± 0.004; this is reduced to 0 050±0 005 for the crystal by self-absorption of fluorescence. Reasons for this low efficiency are discussed. A detailed description is given of excitation conditions produced in organic crystals by ionizing particles with a discussion of possible quenching mechanisms in the excitation column. For electrons these are considered to be due to internal molecular dissipation of electronic excitation energy, to mutual interaction between excited molecules and to the strong electric fields surrounding ions. For the heavy particles these processes are more pronounced and further quenching occurs due to thermal fields and to damaged molecules. The role of migration of excitation energy is also considered. This description of the scintillation process satisfactorily explains the low efficiencies and the decay shapes of fluorescence observed experimentally.