ABSTRACT
Different scintillating materials (organic, inorganic, and organometallic materials) have the property of emitting visible light after being excited by one of the many possible sources. This property is related to a generic term, luminescence, which is defined as a prompt emission of light following the excitation of the substance. The book The Glossary of Terms Used in Photochemistry gives a more precise definition in refining the physical characteristics of the substance itself and in relation with its environment: Luminescence is a “spontaneous emission of radiation from an electronically excited species or from a vibrationally excited species not in thermal equilibrium with its environment” (Braslavsky 2007, p.367). Several sources of excitation can be used to generate luminescence. For example, photoluminescence is associated with an excitation process using visible photons. In addition to the mode of excitation, physical characteristics of the electronic states involved in the luminescence process can split the latter in different categories. Taking again photoluminescence as an example, two subdivisions appear, characterized by the spin multiplicities implied in the electronic states of the luminescent transition: fluorescence and phosphorescence (Knoll 2010). These two phenomena are of particular interest when depicting the scintillation process in organic materials. The former arises from transitions in the energy levels of a molecular entity with retention of spin multiplicity. The latter is however a luminescent process characterized by a change in spin multiplicity, typically from triplet to single states. Moreover, phosphorescence gives a light emission of longer wavelength than fluorescence due to the smaller energy difference of the transition implied in the process. Both definitions of fluorescence and phosphorescence apply to organic molecules, which is not the case for nanocrystalline semiconductors and metallic nanoparticles, because of the irrelevance of the concept of spin multiplicity to these materials. Thus, more general definitions are needed for these emitters. Fluorescence can be defined as the emission from an excited state reached by direct photoexcitation, whereas phosphorescence is an emission from another excited state, the transition associated having the particularity of being forbidden. In this section, the scintillation process of organic compounds will be discussed by overseeing the main concepts involved. At first, the scintillation physics will be addressed with a description of the excitation and de-excitation processes. Physical concepts related to Stokes shift and wavelength shifters will also be presented. Section 1.2 will end with an overview of other de-excitation modes possible in organic scintillators. Section 1.3 will get into the notions surrounding the scintillation light emission such as the rise time and signal falloff, and the light–energy relationship. Then, a brief comparison of scintillators and scintillating fibers will be made, presenting the main characteristics of each type of organic scintillators. A presentation of the properties and spectrum of selected organic scintillators will further be presented.
