The Discovery of a Prolate-Oblate Shape Triplet of Spin 0+ States in the Atomic Nucleus 186PB

The fundamental excitations in many-fermion systems remain one of the most exciting subjects of today's physics. In even-even atomic nuclei, pair breaking, vibrations and rotations generally form the low-lying excitation spectrum. However, for specific numbers of protons and neutrons, a subtle rearrangement of only a few nucleons among the orbitals at the Fermi surface can result in a different elementary mode: a macroscopic shape change [1,2]. We have identified for the first time in a nucleus (¹⁸⁶Pb) three different shapes (spherical - oblate - prolate) as the lowest three states in the energy spectrum [3]. The parent ¹⁹⁰Po nuclei were produced in the ¹⁴²Nd(⁵²Cr,4n)¹⁹⁰Po complete fusion reaction, studied with the velocity filter SHIP [4] at the UNILAC heavy ion accelerator (GSI, Darmstadt). Nuclei of interest after separation were implanted into a position-sensitive silicon detector (PSSD), where their subsequent a decays were measured. In front of the PSSD six similar silicon detectors were mounted (Si-box), facing the PSSD, which were used to detect conversion electrons in prompt coincidence with α-particles. A 4-fold segmented Ge Clover detector was installed behind the PSSD to record prompt α-X and α-γ coincidences. The whole set-up has been optimized to observe fine structure in the α decay that leads, as studied for the heavier even-even Pb nuclei [5], to the identification of low-lying 0⁺ band heads, which will decay predominantly by E0 conversion electron transitions to the ground state. Further details on the experimental method can be found in [3,6,7]...