Study on the 208Pb(α, 6He) 206Pb reaction at Eα = 109 MeV

Abstract

The 208Pb(α, 6He) 206Pb reaction induced by 109 MeV α particles has been studied with 70 keV energy resolution. Twenty-five transitions up to 6.2 MeV in excitation energy have been analyzed. The shapes of the angular distributions are reproduced well by the zero-range distorted-wave Born approximation (DWBA) calculations. The spin and parity of the level at 6.10 MeV are proposed to be 9 −. It is interpreted as a weakly-coupled state of the collective 3 − state and the two-hole 6 + shell-model state. The energy dependence of the total cross sections of the (α, 6He) and (p, t) reactions has been studied systematically by using our present data and other data from the literatures. It reveals that there exists an empirical rule which allows one to compare quantitatively the ratios of the transition strengths of both reactions. The transition strength of the (α, 6He) reaction is found to be about 35 times smaller than that of the (p, t) reaction for the transitions to the natural parity states, while it is about 15 times smaller for the transition to the unnatural parity 3 + state. These small transition strengths are explained by the fact that this reaction occurs at the surface region of the nucleus.