Level structure of the odd-odd nucleus 156Eu

Abstract The reaction chain 153 Eu(n, γ) 154 Eu(n, γ) 155 Eu(n, γ) 156 Eu was studied using 153 Eu targets. The γ-ray and internal conversion electron spectra were measured with high resolution spectrometers. Using several subsequent measurement series, as well as the radiation spectra from the reaction-decay chain 154 Sm(n, γ) 155 Sm 155 Eu(n, γ) 156 Eu, we assigned 95 transitions to 156 Eu, and 70 multipolarities were determined for them. 31 levels with spin values from 0 to 5 were found up to 515 keV. These levels are assigned to 13 rotational bands interpreted using proton orbits p 5 2 + [413]↓, p 5 2 − [532]↑, p 3 2 + [411]↑ and neutron orbits n 5 2 + [642]↑, n 3 2 − [521]↑, n 5 2 − [523]↓, n 11 2 − [505]↑ . Comparison with the decay of 156 Sm to 156 Eu indicates a 0.7% β-branch to the 1 − level at 266.95 keV, and log ƒt = 7.1 . The 434.230(2) keV energy is proposed for the level 448 ± 15 keV , 3 − , p 5 2 + [413]↓− n 11 2 − [505]↑ , earlier known from the 154 Eu(t, p) measurements. Model calculations are in agreement with the Gallagher-Moszkowski rule for four doublets Ω p ±Ω n . The calculated mixing amplitudes explain origin of some two-particle transitions. Comparatively intense E1 transitions between levels of bands with ΔK = 0 and opposite parities allow to propose existence of octupole deformation in 156 Eu which results in observation of parity doublets. It can be estimated that the 156 Eu level scheme is complete up to about 240 keV excitation energy for all spin values.