First Observation Of Excited States In Te-137 And The Extent Of Octupole Instability In The Lanthanides

Excited states in Te-137, populated in spontaneous fission of Cm-248, were studied by means of prompt-gamma spectroscopy, using the EUROGAM2 multidetector array. This is the first observation of excited states in Te-137. The yrast excitations of Te-137 are due to the three valence neutrons. occupying the nu f(7/2) and nu h(9/2) orbitals, similarly as observed in its heavier N = 85 isotones. Systematic comparison of excited levels in the N = 85 isotones shows inconsistencies in spin and parity assignments in Xe-139 and Ba-141 nuclei. The new data for Te-137 do not confirm earlier suggestions that octupole correlations increase in the N = 85 isotones, close to the Z = 50 closed shell.