News releases

Discovery of superdeformed shape in 40Ar
-Approaching to the origin of anomalous deformation in neutron-rich nuclei-

Mar. 11 , 2010

Presenters:
Eiji Ideguchi (Center for Nuclear Study, Graduate School of Science, University of Tokyo)
Masumi Oshima (Japan Atomic Energy Agency)
Tsuneyasu Morikawa (Department of Physics, Kyushu University)


Abstract:
We discovered nuclear superdeformation in a highly excited state of 40Ar, one of the stable isotopes of element argon. The discovery was made for the first time using Japanese nuclear experimental facility (Tandem accelerator in Japan Atomic Energy Agency). Superdeformation means an ellipsoidal shape with a two-to-one ratio for the major to minor axes. Among many superdeformed states identified so far, our finding corresponds to the neutron-richest superdeformation.
Excited states of 40Ar were produced via a fusion-evaporation reaction, 26Mg(18O, 2p2n) using a 70-MeV 18O beam in the Tandem accelerator. By using a germanium detector array, GEMINI-II, we successfully observed a series of gamma-ray transitions in 40Ar (see figure), which clearly indicates a rapid rotation of superdeformed nucleus. This result implies that the proton number (Z=18) and the neutron number (N=22) can be both “magic numbersEfor superdeformation, and also this is the first case suggesting that the superdeformed magic number (N = 22) can persist in neutron-rich nuclei.
Based on a nuclear model calculation, we consider that the onset of the superdeformation is produced by simultaneous excitations of two protons and two neutrons to the higher-lying levels. This feature is a typical characteristic in atomic nuclei as a finite many-body system, that is, only a small fraction of the constituent nucleons (10% in the case of the 40Ar superdeformation) can influence the whole nuclear structure. A similar situation was found in the anomalous deformation of unstable neutron-rich nuclei produced by multi-nucleon excitations. These strongly deformed states can be explained through a common mechanism, which may shed a light on new aspects in the nuclear many-body physics.


Paper information:
E. Ideguchi, S. Ota, T. Morikawa, M. Oshima, et al.,
Physics Letters B 686, 18-22 (2010).
“Superdeformation in asymmetric N>Z nucleus 40ArEBR>

Figure:



Caption:
Gamma-ray spectrum of 40Ar. Red peaks in equidistant spacing indicate a rapid rotation of a superdeformed nucleus. Blue peaks are gamma-ray transitions in low-lying spherical states, while green peaks correspond to transitions from the superdeformed states to spherical states.


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