Phase transition in the rotational band of a nonaxial nucleus

A macroscopic theory is presented of phase transitions in the yrast band of nonaxially deformed nuclei with spins I > 50. The transition is related to the smooth variation of deformation; it is a consequence of shell effects in nuclei in which subshells with large angular momentum j near the Fermi surface begin to be filled up. A change in sign of the nonaxial deformation results in instability of rotation of the nucleus about the axis with an intermediate moment of inertia. The transition through the critical point I0 is therefore accompanied by rotation of the angular momentum vector through 90 deg relative to the body fixed system of coordinates. The phase transition parameter defining the nature of the rotational spectrum of the nucleus is the quantity |γ| I02. The phase transition region is characterized by intersection of the bands, approach of the side hands to the yrast band and by strong reduction of E2 strengths in the yrast band near the critical point. The effects are calculated for rare earth elements with neutron numbers N between 92 and 96: 154Sm, sup(162,164)Er, 164Yb