Level Structure in ²⁰⁹Fr

Abstract: The high-spin states of ²⁰⁹Fr have been studied in the ¹⁹⁷Au(¹⁶O,4n)²⁰⁹Fr reaction at ¹⁶O energies from 90 to 105 MeV using techniques of in-beam γ-ray spectroscopy. Measurements of γ-ray excitation functions,γ-γ-t coincidences and γ-ray angular distributions were performed with 4 BGO(AC)HPGe detecters. A level scheme for ²⁰⁹Fr with 21 γ rays was established for the first time,including a high-spin isomer with a measured half-life of (52±20)ns. Comparing with the low-lying level structure in ²⁰⁸Rn,it may conclude that the low-lying levels in ²⁰⁹Fr are formed by coupling the h_9/2 proton to the neutron hole excitations in ²⁰⁸Rn. Thus,negative parities could be assigned tentatively to the low-lying states. The nucleus ²⁰⁹Fr,with four valence neutron holes and five valence protons outside the closed-shell nucleus ²⁰⁸Pb,also presents a competition between neutron-hole excitations and proton excitations in its low-lying level spectrum. Additionally,it should be pointed out that the higher-lying level structure in ²⁰⁹Fr does not follow the excitation pattern of ²⁰⁸Rn. We will pursue shell model calculations to interpret the level structure in ²⁰⁹Fr.