Micelle Effects in Liquid Scintillation Counting

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/6πη₀R

where 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.

We undertook series of DLS measurements designed to better characterize the size of micelles in the types of LS cocktails regularly employed for radionuclide metrology at the National Institute of Standards and Technology (NIST).  Five different commercially available LS cocktails were studied over a range of aqueous fractions and acidities; in all cases the hydrodynamic diameters were substantially smaller than the previous 8 nm estimate.  In addition, calculations were performed in order to quantify the effects of using erroneous micelle diameters in efficiency calculations.

Because the micelle sizes in cocktail prepared from commercial scintillants are much smaller than in Triton X-100/toluene solutions, the micelle effect on LS counting efficiencies is smaller than previously estimated.  While 8 nm diameter micelles would appreciably impact counting efficiencies for low-energy Auger electron emitting nuclides such as 55Fe, the smaller micelles (generally ≤ 4 nm) indicated in our measurements minimally (≤ 0.12 %) affect efficiencies even for 55Fe.  These results were published in Applied Radiation and Isotopes in 2012 (v. 70, pp. 2164 – 2169).

Ongoing studies are addressing the impact of metal ions on micelle size distributions, the experimental determination of how counting efficiencies vary with micelle size (for comparison with the calculations described above), and how the optical effects of micellization affect counting efficiencies.