Molecular Design Considerations for Different Classes of Organic Scintillators

The purpose of this chapter is to review the distinguishing characteristics of different classes of organic scintillators with respect to the performance requirements of typical use cases. Discussion of the relevant physical and photophysical parameters will be provided in the context of the rational design of radiation detection materials. A partial list of these properties is as follows: scintillation light yield, emission wavelength and anisotropy, timing characteristics, ionizing particle discrimination, optical attenuation length, mechanical and environmental robustness, detector volume, and cost. Material design considerations relative to these properties are one of the main objectives of this Chapter. Aromatic materials can produce scintillation light in response to ionizing radiation. This characteristic has been observed in crystalline, liquid, and amorphous scintillators, owing to a process that includes ionization recombination, population of excited electronic states, and radiative de-excitation. Organic scintillators from each category are employed today to satisfy the specific requirements of a wide range of applications. The photophysical and scintillation properties of molecules used in organic scintillators vary across a wide range. This has resulted in over 70 years of research aimed at developing scintillators that possess specific physical and scintillation characteristics. In 1956, Sangster and Irvine Jr. published an in-depth survey describing the scintillation properties of more than fifty organic molecular crystals. This work was significant since it described several important considerations that continue to govern contemporary scintillator development efforts: (1) the scintillation efficiency is dependent upon the electronic structure and fluorescence properties of the constituent molecules, (2) the orientation, shape, and steric properties of the molecule impact the mechanisms of energy transfer giving rise to scintillation, (3) the scintillation efficiency is sensitive to defects and the presence of molecular impurities. Later studies also addressed practical considerations such as detector fabrication/scale-up and the relationship between molecular structure and mechanical properties. These categories provide a general framework from which to approach the development of improved organic scintillators.