In-beam study of multiparticle excitations in the semi-magic nucleus 140Ce

Abstract Excited states of 140 Ce, especially those with spins J ⩾ 6, have been studied in the reaction 138 Ba(α, 2n) 140 Ce. Using in-beam spectroscopic methods energies, excitation functions, coincidences, angular distributions and linear polarization values of the γ-rays depopulating 140 Ce levels have been measured. The level scheme of the 140 Ce could be extended by 17 new levels with spin values up to J = 13 and with excitation energies up to E x = 5336 keV. The measurement of the time distributions of the γ-radiation using the γ-r.f. method revealed a new isomeric state ( J π = 9 − ) with a half-life of 1.7 (2) ns. The experimental data on excitation energies and electromagnetic properties of levels in 140 Ce are in rather good correspondence with theoretical results, which have been obtained by diagonalizing the same hamiltonian in a shell-model basis for positive-parity states and in a particle-core coupling basis for states of negative parity. A particle-core description of the 3 − 1 states in even-mass ( N = 82, 136 ⩽ A ⩽ 146) nuclei suggests that the steep decrease of the energy of these levels in the mass region considered is caused only by the increasing proton number.