First observation of medium-spin excitations in the Cs-138 nucleus

T Rzaca-Urban, W Urban, M S Sarkar, S Sarkar, J L Durell, A G Smith, B J Varley, I Ahmad

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    Medium-spin, yrast excitations in the Cs-138 nucleus, populated in the spontaneous fission of Cm-248, were observed for the first time. Cs-138 was studied by means of prompt ray spectroscopy using the EUROGAM2 array. The newly observed yrast cascade, built on the known 6-isomer at 80 keV, was successfully described by shell model calculations. Analogously to the 136I isotone, the 6(-) isomer in Cs-138 has the ( pi g(7)(4) (/2) d(5/2)vf(7/2) ) (6-) dominating configuration and the 7(-) excitation, located 175 keV above, corresponds to the ( pi g(7/2)(3)d(5/2)(2)vf(7/2))(7-) as dominating configuration. Similarly as in I-136, changing the position of the d(5/2) proton orbital improves the reproduction of the data. However, in Cs-138 the energy of this orbital should be increased compared to its energy in Sb-133, to get the best description, in contrast to I-136 and Sb-135, where it had to be decreased. The best reproduction of excitation energies in Cs-138 is obtained assuming that the pd(5/2) orbital in Cs-138 is located about 100 keV higher than in Sb-133. These observations suggest that the lowering of the d(5/2) s. p. energy in Sb-135 is not a physical effect due to the appearance of a neutron skin, as proposed by other authors, but rather an artifact due to some de. ciency of the input data used in the shell model calculations in the region of the doubly magic Sn-132 core.
    Original languageEnglish
    JournalEuropean Physical Journal A
    Issue number1
    Publication statusPublished - 1 Apr 2007


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