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Abstract Atomic nuclei serve as prime laboratories for investigations of complex quantum phenomena, where minor nucleon rearrangements cause significant structural changes. 190Pb is the heaviest known neutron-deficient Pb isotope that can exhibit three distinct shapes: prolate, oblate, and spherical, with nearly degenerate excitation energies. Here we report on the combined results from three state-of-the-art measurements to directly observe these deformations in 190Pb. Contrary to earlier interpretations, we associate the collective yrast band as predominantly oblate, while the non-yrast band with higher collectivity follows characteristics of more deformed, predominantly prolate bands. Direct measurement of the $$E0({0}_{2}^{+}\to {0}_{1}^{+})$$ E 0 ( 0 2 + → 0 1 + ) transition and γ-e − coincidence relations allowed us to locate and firmly assign the $${0}_{2}^{+}$$ 0 2 + state in the level scheme and to discover a spherical $${2}_{3}^{+}$$ 2 3 + state at 1281(1) keV with $$B(E2;{2}_{3}^{+}\to {0}_{1}^{+})=1.2(3)$$ B ( E 2 ; 2 3 + → 0 1 + ) = 1.2 ( 3 ) W.u. These assignments are based purely on observed transition probabilities and monopole strength values, and do not rely on model calculations for their interpretation.

Neutron-proton pairing is a topic of continuous interest in nuclear physics and open questions remain. The authors experimentally observe direct proton decay from the ground state of odd-odd 116La, providing support for the presence of strong neutron-proton pair correlations in this exotic, neutron deficient system.