Liquid scintillation counting (LSC) techniques underlie many of the primary activity measurements at NIST and other national metrology institutes. The microscopic properties of LSC samples affect counting efficiencies in important ways. In recent years, NIST scientists have investigated the effects of sample composition on micellar properties and counting efficiencies.
Commercially available scintillants (often referred to by manufacturers as "cocktails") for liquid scintillation (LS) counting are complex concoctions of alcohols, phosphates, polymers, and salts in organic solvent. Various surfactants are used to suspend the aqueous fraction in the different cocktails, and it is in the resultant reverse micelles that the aqueous materials (typically metal ions or, in the case of tritium, water molecules) of interest in radionuclide metrology reside. The properties of these reverse micelles can impact LS counting experiments in several ways.
The "micelle size effect" arises when an electron is emitted from a radionuclide and 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.
Dynamic light scattering (DLS) measurements at NIST1 revealed that the ≈ 8 nm micelle diameter assumed (based on studies with a surrogate surfactant solution) was much larger than the diameter of even the largest reverse micelles observable in commercial LS cocktails. Thus, the micelle size effect was being overestimated.
In a follow-up study,2 it was demonstrated that the micelle size effect was not experimentally observable in even the most extreme case achievable. A low-energy Auger electron-emitting radionuclide (55Fe) was found to exhibit no measurable change in counting efficiency with changing micelle size.
"Other" micelle effects on LS counting experiments have also been studied at NIST. Since the transmission of optical scintillation photons will be affected by the presence or absence of reverse micelles, it is important to be able to discriminate between cocktail compositions that do or do not contain micelles. A technique for identifying micellar phase boundaries in the contexts of LS experiments, based on Compton spectrum quenching, has been recently developed at NIST.3 The technique has been applied recently to measure the impact of cosurfactant additions and will be used in future cases.