Lifetime measurements by the recoil path method. Application to the yrast states of 154Er

Lifetimes of high spin yrast states in 154Er have been measured, using the recoil distance method. The special mode of feeding occuring through (heavy ion, xn) reactions has lead to write an analysis code (code BACLE) which allows the extraction of lifetimes of levels within a cascade that contains at most ten levels and for fifteen different targets to plunger distances. This code uses Bateman equations, in order to calculate the population of nuclei having a defined excited state, and also uses a non linear fitting procedure. The strengths of E2 transitions have been found of the same order of magnitude than the Weisskopf estimates. The experimental results about the nucleus 154Er are discussed within the framework of the 'oblate coupling scheme' of A. Bohr and B.R. Mottelson