Eur. Phys. J. A 7, 167–176 (2000) T HE EUROPEAN PHYSICAL J OURNAL A c ° Societ`a Italiana di Fisica Springer-Verlag 2000 Properties of N=84, even-even nuclei populated in the spontaneous fission of 248 Cm A. Korgul 1 , W. Urban 1 , T. Rz¸aca-Urban 1 , M. Rejmund 2 , J.L. Durell 3 , M.J. Leddy 3 , M.A. Jones 3 , W.R. Phillips 3 , A.G. Smith 3 , B.J. Varley 3 , N. Schulz 4 , M. Bentaleb 4,a , E. Lubkiewicz 4,b , I. Ahmad 5 , L.R. Morss 5 1 Institute of Experimental Physics, Warsaw University, ul.Ho˙ za 69, 00-681 Warszawa, Poland 2 Centre de Spectrometrie Nucleaire et de Spectrometrie de Masse, bat. 104, 91405 Orsay Campus, France 3 Schuster Laboratory, Department of Physics and Astronomy, University of Manchester, Manchester M13 9PL, UK 4 Institut de Recherches Subatomiques, CNRS-IN2P3 et Universite Louis Pasteur, 67037 Strasbourg, France 5 Argonne National Laboratory, Argonne, IL 60439, USA Received: 10 November 1999 / Revised version: 22 December 1999 Communicated by D. Schwalm Abstract. Excited states in the neutron-rich, N=84 nuclei 134 Sn, 136 Te and 138 Xe, populated in the spon- taneous fission of 248 Cm, were studied to medium spins using the EUROGAM2 array. OXBASH code calculations support the experimental identification of maximum aligned configurations in these isotopes. Empirical shell model calculations agree with the proposed excitation energy of the neutron h 9/2 excitation in the 132 Sn region. A discrepancy between the observed and calculated excitation energy of the I π =12 + level in 136 Te indicates possible admixtures of collective excitations in this nucleus. Clear signs of collective excitations are observed in 138 Xe. PACS. 21.60.Cs Shell model – 23.20.Lv Gamma transitions and level energies – 27.80.+w 190 ≤ A ≤ 219 – 25.70.-z Low and intermediate energy heavy-ion reactions 1 Introduction Nuclei with a few valence particles outside a doubly-magic core provide useful information on the single-particle exci- tation energies and nucleon-nucleon effective interactions. In some cases such nuclei can provide information which can not be obtained from studies of nucleiwith only a single valence nucleon. The structure ofnucleiwith just a few valence particle may be simple enough to allow ex- traction of a precise information on fundamental nuclear properties. This should be especially true for nuclei in the vicinity of 132 Sn, which is a good double-closed shell nu- cleus. A recent study of the 134 Sb nucleus [1], provides an example. In 134 Sb proton-neutron configurations involving the i 13/2 neutron level have been identified and give the first estimate of the neutron i 13/2 excitation energy. Be- cause it is unbound, the νi 13/2 level has not been observed in the single-valence neutron nucleus 133 Sn. Nuclei with a few valence-particles have a reason- ably high density of excited states at medium spins and are good subjects for γ-ray spectroscopy measurements. Thanks to recent developments in γ detection techniques [2], nuclei in the 132 Sn region can now be studied through a Present address: Universit´e de K´enitra, K´enitra, Morocco b Present address: Department of Physics, Jagiellonian Uni- versity, Krak´ow, Poland prompt-γ spectroscopy of fission fragments. This method allows investigations of excited states up to ∼ 15~ in spin and ∼ 8 MeV in excitation energy and advanced analy- sis techniques [1] make possible studies of γ-ray yields as small as 10 −8 of the total intensity. The present work concerns the N=84 isotones 134 Sn, 136 Te and 138 Xe, which have two valence neutrons out- side the 132 Sn core.Nuclei close to the 132 Sn core are well described by the shell modelbut there are indica- tions that nucleiwith a few valence particles may acquire collective properties. It is therefore of interest to investi- gate the N=84 nuclei in order to test the limits of the shell model in this region. Partial results of this work have been recently reported in [3–5]. 2 Experiment, data analysis and the results In the present work, prompt γ radiation following spon- taneousfission of 248 Cm was measured using the EU- ROGAM2 array of Ge anti-Comptonspectrometers. About 2×10 10 γγγ coincidences were collected. For more details on the experiment and data analysis see [6]). Partial levelschemes of 134 Sn, 136 Te and 138 Xe were obtained from the analysis of triple-γ coincidences. Where possible angular correlations between γ rays in these nu-