Levels of ¹⁵⁵Eu from thermal neutron capture

The γ-ray and conversion electron spectra of ¹⁵⁵Eu from the (n, γ) reaction were studied at the Institut Laue-Langevin using the curved crystal spectrometers GAMS 1, GAMS 2/3, the β-spectrometer BILL and the pair spectrometer. The γ-transition multipolarities were determined in the energy region 40-1200 keV. The level scheme of ¹⁵⁵Eu was developed with 51 levels up to the excitation energy of 1.7 MeV; these levels were depopulated via 140 transitions out of 360 transitions observed in the γ-ray spectrum. The levels were grouped into 13 rotational bands, and of these 3 2^-[541], 1 2^-[550], 5 2⁺[413]+Q₂₀, 3 2⁺[411]+Q₂₀ were entirely new for ¹⁵⁵Eu was found B_n = 8151.3(3) keV. It has been determined that the E1 transitions of ΔK = 0 type were enhanced in comparison with these of the ΔK = 1 type. The Nilsson + RPC, cranked shell model and odd RV model calculations have indicated the presence of strong collectivization, even for the ¹⁵⁵Eu lowest states.