Investigation of low-lying isovector octupole excitations in spherical nuclei
Final Report Abstract
A general feature of any two-component quantum system, in nuclear physics these components are the protons and the neutrons, is that in presence of an interaction between the two subsystems the wavefunctions of the combined system form a symmetric and an asymmetric or so-called mixed-symmetric linear combination of the individual wavefunctions. As protons and neutrons are distinguished by the isospin quantum number, the symmetric combination is referred to as isoscalar and the mixed-symmetric combination as isovector. In case the interaction is attractive, the symmetric combination is shifted to a lower and the asymmetric combination to a higher energy. Characteristic signature of such a state with mixed symmetry is a strong M1 decay with a matrix element in the order of one nuclear magneton (μN) to the symmetric counterpart. Aim of the project was to confirm for the first time candidates for low-lying isovector states also for octupole excitations in the spherical nuclei 144,146Nd and 96Mo which were proposed recently. This comprises the determination of branching ratios, transitions multipolarities, multipole-mixing ratios and, eventually, absolute values for transition matrix elements. The nuclei were populated in (n,γ) reactions at the high-flux reactor of the Institut Laue-Langevin (ILL) in Grenoble (France). With EXILL, FIPPS and GAMS6 state-ofthe-art instruments for gamma-ray spectroscopy were used. EXILL was the first large array consisting Compton-suppressed HPGe detectors operated at a reactor. The high efficiency allowed for the first time the study of these reactions also in γγcoincidences. The third 3- state at 2779 keV in 144Nd fulfils all properties expected for an isovector one-phonon octupole state. Branching ratios, transition multipolarities and multipole - mixing rations have been extracted from gg-data taken with EXILL. The results were combined with the lifetime of this state, measured with GAMS6, a world-wide unique device for sub-nanosecond lifetime measurements. The transition to the first 3- state, the symmetric one-phonon octupole state, has a M1 transition matrix element of 2.2 μN, which qualifies the third 3- state as isovector one-phonon octupole state. This interpretation is supported by new QPM calculations. In 146Nd, similar candidates exist. However, reliable information on the level scheme is scarce and a (n,γγ) with the new FIPPS array, one of the first experiments with FIPPS, will correct and extent the level scheme, determine branching ratios and assign multipolarities and spins. A later run with GAMS is needed to determine absolute transition probabilities. In 96Mo, the 3178 keV candidate for an isovector one-phonon octupole state was confirmed but another enigma about the nature of a 3- level at 3024 keV appeared.
Publications
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The (n, γ) campaigns at EXILL. EPJ Web of Conferences 93, 0101 (2015)
J. Jolie, J.-M. Régis, D. Wilmsen, S. Ahmed, M. Pfeiffer, N. Saed-Samii, N. Warr, A. Blanc, M. Jentschel, U. Köster, P. Mutti, T. Soldner, G. Simpson, G. De France, W. Urban, F. Drouet, A. Vancraeyenest, P. Baczyk, M. Czerwinski, A. Korgul, C. Mazzocchi, T. Rzaca-Urban, A. Bruce, O.J. Roberts, L.M. Fraile, H. Mach, V. Paziy, A. Ignatov, S. Ilieva, Th. Kröll, M. Scheck, M. Thürauf, D. Ivanova, S. Kisyov, S. Lalkovski, Zs. Podolyák, P.H. Regan, W. Korten, M. Zielinska, M.D. Salsac, D. Habs, P.G. Thirolf, C. A. Ur, C. Bernards, R.F. Casten, N. Cooper, V. Werner, R.B. Cakirli, S. Leoni, G. Benzoni, G. Bocchi, S. Bottoni, F.C.L. Crespi, B. Fornal, N. Cieplicka, B. Szpak, C.M. Petrache, R. Leguillon, R. John, C. Lorenz, R. Massarczyk, R. Schwengner, D. Curien, R. Lozeva, L. Sengele, N. Marginean, R. Lica
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Identification of low-energy isovector octupole states in 144Nd. Journal of Physics: Conference Series 724 (2016) 012050
M. Thürauf, M Scheck, C. Bernards, A. Blanc, N. Cooper, G. De France, M. Jentschel, J. Jolie, O. Kaleja, U. Köster, T. Kröll, P. Mutti, G. S. Simpson, T. Soldner, M. Tezgel, W. Urban, J.Vanhoy, M. Werner, V. Werner, and K. O. Zell
