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

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.