Commercially available scintillates (often referred to by manufacturers as “cocktails”) for liquid scintillation (LS) counting are complex concoctions of alcohols, phosphates, polymers, and salts in organic solvent.
This is because of the “micelle effect” on scintillation efficiency. When an electron is emitted from a radionuclide, it loses energy while traversing the aqueous material within a micelle. This energy is not deposited in the scintillation material, and so does not result in scintillation light. The distance that an electron must travel through an aqueous medium prior to interacting with the organic scintillator is therefore of interest when calculating theoretical scintillation efficiencies for a particular radionuclide in a particular scintillation cocktail. This problem can be particularly acute for low-energy Auger electron-emitting radionuclides.
When researchers have attempted to account of the micelle size effect on scintillation efficiency, they have taken micelle size estimates from a set of dynamic light scattering (DLS) measurements made by Rodríguez et al. in 1998. In those experiments, the formation of aqueous micelles in solutions of toluene and Triton X-100 (ethoxylated p-tert-octylphenol, a nonionic surfactant) was studied by DLS. If the viscosity of a solution is known, DLS can be used to measure the average hydrodynamic radius (R) of suspended particles by means of the Stokes-Einstein equation:
D = kT/6πη0Rwhere D is the diffusion coefficient measured via DLS, k is Boltzman’s constant, T is the absolute temperature, and η0 is the solvent dynamic viscosity. Based on the Rodríguez results, a value of ≈ 8 nm has been adopted as an estimate for the diameter of micelles in LS cocktails. The appropriateness of the toluene/Triton-X100 system as a model for commercial LS cocktail is suspect, however, especially for cocktails that include ionic surfactants.