Level structure of the odd-odd nucleus ¹⁵⁶Eu

The reaction chain ¹⁵³Eu(n, γ)¹⁵⁴Eu(n, γ)¹⁵⁵Eu(n, γ)¹⁵⁶Eu was studied using ¹⁵³Eu 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 ¹⁵⁴Sm(n, γ)¹⁵⁵Sm{A figure is presented} ¹⁵⁵Eu(n, γ)¹⁵⁶Eu, we assigned 95 transitions to ¹⁵⁶Eu, 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 ¹⁵⁶Sm to ¹⁵⁶Eu indicates a 0.7% β-branch to the 1^- level at 266.95 keV, and log f{hook}t = 7.1. The 434.230(2) keV energy is proposed for the level 448 ± 15keV, 3^-, p 5 2⁺[413]↓-n 11 2^-[505]↑, earlier known from the ¹⁵⁴Eu(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 ¹⁵⁶Eu which results in observation of parity doublets. It can be estimated that the ¹⁵⁶Eu level scheme is complete up to about 240 keV excitation energy for all spin values.