Nuclear deformation and neutron excess as competing effects for pygmy dipole strength

Massarczyk, R. and Schwengner, R. and Dönau, F. and Frauendorf, S. and Anders, M. and Bemmerer, D. and Beyer, R. and Bhatia, C. and Birgersson, E. and Butterling, M. and Elekes, Zoltán and Ferrari, A. and Gooden, M. E. and Hannaske, R. and Junghans, A. R. and Kempe, M. and Kelley, J. H. and Kögler, T. and Matic, A. and Menzel, M. L. and Müller, S. and Reinhardt, T. P. and Röder, M. and Rusev, G. and Schilling, K.D. and Schmidt, K. and Schramm, G. and Tonchev, A. P. and Tornow, W. and Wagner, A. (2014) Nuclear deformation and neutron excess as competing effects for pygmy dipole strength. PHYSICAL REVIEW LETTERS, 112 (7). ISSN 0031-9007

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Abstract

The electromagnetic dipole strength below the neutron-separation energy has been studied for the xenon isotopes with mass numbers A = 124, 128, 132, and 134 in nuclear resonance fluorescence experiments using the γELBE bremsstrahlung facility at Helmholtz-Zentrum Dresden-Rossendorf and the HIγS facility at Triangle Universities Nuclear Laboratory Durham. The systematic study gained new information about the influence of the neutron excess as well as of nuclear deformation on the strength in the region of the pygmy dipole resonance. The results are compared with those obtained for the chain of molybdenum isotopes and with predictions of a random-phase approximation in a deformed basis. It turned out that the effect of nuclear deformation plays a minor role compared with the one caused by neutron excess. A global parametrization of the strength in terms of neutron and proton numbers allowed us to derive a formula capable of predicting the summed E1 strengths in the pygmy region for a wide mass range of nuclides.

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