Observation of abnormally large radii of nuclei in excited states in the vicinity of neutron thresholds
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CLEI Experiment
Observation of Abnormally Large Radii of Nuclei in Excited States in the Vicinity of Neutron Thresholds A. A. Ogloblin1)* , A. N. Danilov1) , T. L. Belyaeva2) , A. S. Demyanova1) , S. A. Goncharov3), and W. Trzaska4) Received December 21, 2010
Abstract—Differential cross sections for inelastic scattering leading to the excitation of some nuclear states situated near neutron-emission thresholds were analyzed. With the aid of a modified diffraction 13 С nucleus at 3.09 MeV, for the first model, abnormally large radii were found for the 1/2+ 1 state of the 9 + levels of positive-parity rotational bands in the Be (1/2 level at 1.68 MeV and 5/2+ level at 3.05 MeV) 14 Be and 11 Be (5/2+ level at 1.78 MeV and 3/2+ level at 3.41 MeV) nuclei, and for the 2+ 1 state of the − 12 nucleus at 1.54 MeV and 11 state of the Be nucleus at 2.7 MeV. All of these states possess signatures typical of neutron halos. DOI: 10.1134/S1063778811110147
1. INTRODUCTION The discovery of neutron halo in some neutronrich light nuclei [1] was among the most outstanding discoveries at the end of the past century. The term “exotic nuclei” itself was originally associated with precisely these nuclei. The existence of a halo manifests itself in the presence of a diffuse surface region surrounding a core of normal nuclear density and containing only neutrons. The physical reason behind this phenomenon is that the binding energy of valence neutrons is small, which results in the formation of a long tail of their wave function and, accordingly, in the increase in the nuclear radius. In such two-component nuclei (11 Be, 11 Li, 14 Be, etc.), the average distance between valence neutrons and the nuclear-core center is 6 to 7 fm, and this leads to a considerable increase in the root-mean-square radii in relation to normal nuclei of the same mass. Until recently, a neutron halo was observed almost exclusively in the ground states of some radioactive nuclei. As far as we know, there is only one article [2] that reported an observation of a halo in the first excited state (1/2+ state at 3.09 MeV) of the 13 С 1)
Russian Research Centre Kurchatov Institute, pl. Kurchatova 1, Moscow, 123182 Russia. 2) ´ ´ Universidad Nacional Autonoma de Mexico (UNAM), ´ 01000 Mexico, Mexico. 3) Skobeltsyn Institute of Nuclear Physics, Moscow State University, 119991 Russia. 4) ¨ ¨ PO Box 35, FIN-40351 Jyvaskyl ¨ ¨ University of Jyvaskyl a, a, Finland. * E-mail: [email protected]
nucleus and in the 1− state of the 12 В nucleus at 2.62 MeV. At the same time, the possibility of halo formation in excited states near neutron-emission thresholds was discussed more than 50 years ago [3]. Observation and investigation of a halo in excited states may extend substantially the existing ideas of nuclear exotic phenomena, since they may stem from totally new types of nuclear structures. The absence of direct methods for measuring radii of nuclei in short-lived (T1/2 10−10 s) states is the main reason for which a reliable observation of a halo in excited states presents significant
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