Understanding the peak asymmetry in alpha liquid scintillation with β/γ discrimination

Abstract

The peak evaluation in alpha liquid scintillation is known to be easy, mostly due to the gaussian shape of the peaks. However, we often observed a high-energy tail in addition to a pure gaussian function. This effect is only detectable with a high resolution α liquid scintillation spectrometer such as the PERALS® system. Indeed, its intrinsic resolution (180 keV for a 4 MeV α particle) is better than that obtained for conventional LSC spectrometers. The peak asymmetry was quantified using the Fisher´s coefficient γ1 (symmetry factor). We show that the main effect responsible for the asymmetry is internal conversion. Indeed, most of the even-even nuclides have low α intensity transitions leading to excited levels of their daughter nuclides. The internal conversion is almost equal to 100% and consequently produces a sum peak at higher energy. No generalization is possible for odd-even nuclides, but the knowledge of their disintegration scheme allows to explain the experimental values obtained and the differences observed (e.g. between 241Am and 243Am). The experimental determinations of γ1 are given for polonium, radon, radium, thorium, uranium, plutonium, americium, and curium isotopes. We show the necessity to take into account the L and M shell contributions for few nuclides like thorium isotopes to get a maximum accuracy in the activity measurements.