Observation of the 𝐽≤7/2 low-spin states in 213Fr populated in the electron capture of the 1/2− ground state of 213Ra

Abstract

A detailed level scheme of 21 Fr126 following the EC/𝛽+ decay of the 1/2− 213Ra parent ground state was built in an experiment performed at the ISOLDE Decay Station, CERN. The fragmented total 𝛽 decay strength favours the direct population of several low-spin (𝐽≤7/2) excited states. The analysis of the 𝛾-singles spectrum and 𝛾−𝛾 coincidences allowed us to identify many new 𝛾-ray transitions and excited states in 213Fr up to about 3.6 Me Vexcitation energy. The spins and parities of the newly established levels, on top of the (7/2−1) state, were mainly assigned based on the systematics of the 𝑁=126 isotones and further compared with shell-model calculations. The level scheme displays a structural pattern, with several groups of states with negative parity, emerging from the well defined, simple, 𝜋⁡(ℎ59/2), 𝜋⁡(ℎ49/2⁡𝑓17/2) configurations or from their configuration mixing. The strength of the E2 transitions within the multiplets is compared with shell-model theoretical calculations performed with the KHPE and H208 effective interactions. A new (3/2−) isomer with a half-life of 26(3) ns has been identified. An upper limit of 35 ps was determined for the half-life of the first excited state, 7/2−. The possibility of a mixed 𝑀⁢1 +𝐸⁢2 character is discussed for the 7/2−1→9/2−gs decay in 213Fr, which leads to an 𝑙-forbidden nature of the 𝜋⁢𝑓7/2→𝜋⁢ℎ9/2 transition.