Understanding the peak asymmetry in alpha liquid scintillation with beta/gamma discrimination

The peak evaluation in alpha liquid scintillation is known to be easy, most ly due to the gaussian shape of the peaks. However, we often observed a hig h-energy tail in addition to a pure gaussian function. This effect is only detectable with a high resolution alpha liquid scintillation spectrometer s uch as the PERALS(R) system. Indeed, its intrinsic resolution (180 keV for a 4 MeV alpha particle) is better than that obtained for conventional LSC s pectrometers. The peak asymmetry was quantified using the Fisher's coeffici ent gamma (1) (symmetry factor). We show that the main effect responsible f or the asymmetry is internal conversion. Indeed, most of the even-even nucl ides have low a intensity transitions leading to excited levels of their da ughter nuclides. The internal conversion is almost equal to 100% and conseq uently produces a sum peak at higher energy. No generalization is possible for odd-even nuclides, but the knowledge of their disintegration scheme all ows to explain the experimental values obtained and the differences observe d (e.g. between Am-241 and Am-243). The experimental determinations of gamm a (1) are given for polonium, radon, radium, thorium, uranium, plutonium, a mericium, and curium isotopes. We show the necessity to take into account t he L and M shell contributions for few nuclides like thorium isotopes to ge t a maximum accuracy in the activity measurements.