Direct observation of E0 transitions in 188Pb through in-beam spectroscopy

Competing configurations, assigned with three different shapes, are mixed at low angular momentum in the neutron-deficient ¹⁸⁸Pb nucleus. Here, we present a simultaneous conversion electron and γ-ray in-beam spectroscopic precision measurement employing the sage spectrometer. The level energy of the first excited state, $0_2^{+}$, has been determined at 591(1) keV through direct measurement of conversion electrons. By using the intensity of the observed $0_2^{+}\to 0_1^{+}$ transition to the ground state, the feeding of the $0_2^{+}$ state has been determined for the first time, which suggests the $0_2^{+}$ state is the head of the predominantly prolate band. The compositions of the $4_2^{+}\to 4_1^{+}$ and $2_2^{+}\to 2_1^{+}$ inter-band transitions have been determined, indicating configuration mixing between the bands.