The strange behaviour of nuclei at high spin states

Abstract

(i) The deexcitation into particles and y rays of a high angular momentum compound nucleus formed in a fusion reaction is studied. In the energy region where particle emission is not possible the competition between statistical and collective transitions is taken into account. The side feeding pattern and the γ multiplicities can only be explained if one assumes that each state is a member of a rotational band and can decay by an E2 transition of about 150 s.p.u. along this band and with about 10⁻³ s.p.u. by a statistical E1 transition. The y cascades run therefore through an irregular mixture of statistical and collective transitions before hitting the yrast line. (ii) In the second part it is shown that in typical rare earth nuclei one finds no rotation around a strongly deformed oblate symmetry axis as predicted by the liquid drop model. But calculations of the deformation energy surface show that [in front and after] the rare earth region one finds rotation around a symmetry axis of nuclei with a slightly oblate shape above a doubly closed shell nucleus (¹⁴⁶ ₆₄Gd₈₂) and with a slightly prolate shape below a doubly closed shell nucleus (²⁰⁸ ₈₂Pb₁₂₆). This behaviour can be understood by the MONA effect, which yields for a few particles or holes outside a closed shell a strong correlation between the high angular momentum of the nucleus, its shape and the direction of the rotational axis.