Lifetimes of low-lying levels in light rare earth nuclei around A=135

Abstract

The lifetimes of 20 nuclear energy levels have been measured in ^132,134Nd, ^133,135Pm and ^134,136Sm using the recoil distance technique. The nuclear reactions employed for studying these nuclei were ⁹²Mo+⁴⁶Ti and ⁹²Mo+⁵⁰Cr at beam energies of 210 MeV and 230 MeV respectively. The measured transition strengths in the yrast bands in ^132,134Nd and ¹³⁵Pm show a marked decrease over the expected rotational values, whilst the equivalent values in the ^134,136Sm isotopes show very little decrease in collectivity. Nuclear deformations have been deduced for the low-lying states using axially symmetric rotational model assumptions. The results are compared with theoretical ground-state deformation predictions, and also with the results of the Grodzins' phenomenological formula which is based on the energy of the first 2⁺ state. The nuclear deformations are consistent with a transition to well deformed rotors as the number of neutrons decreases and also as the number of protons increases towards Z=64.