A triplet of differently shaped spin-zero states in the atomic nucleus Pb-186

Andreyev, AN Huyse, M Van Duppen, P Weissman, L Ackermann, D Gerl, J Hessberger, FP Hofmann, S Kleinbohl, A Munzenberg, G Reshitko, S Schlegel, C Schaffner, H Cagarda, P Matos, M Saro, S Keenan, A Moore, C O'Leary, CD Page, RD Taylor, M Kettunen, H Leino, M Lavrentiev, A Wyss, R Heyde, K

An. Andreyev et al., A triplet of differently shaped spin-zero states in the atomic nucleus Pb-186, NATURE, 405(6785), 2000, pp. 430-433

28

INGLESE

Article

Multidisciplinary,Multidisciplinary,Multidisciplinary

NATURE

0028-0836 → ACNP

405

6785

2000

430 - 433

ISI

0028-0836(20000525)405:6785<430:ATODSS>2.0.ZU;2-L

Understanding the fundamental excitations of many-fermion systems is of sig nificant current interest. In atomic nuclei with even numbers of neutrons a nd protons, the low-lying excitation spectrum is generally formed by nucleo n pair breaking and nuclear vibrations or rotations. However, for certain n umbers of protons and neutrons, a subtle rearrangement of only a few nucleo ns among the orbitals at the Fermi surface can result in a different elemen tary mode: a macroscopic shape change(1-3). The first experimental evidence for this phenomenon came from the observation of shape coexistence in O-16 (ref. 4). Other unexpected examples came with the discovery of fission iso mers(5) and superdeformed nuclei(6). Here we find experimentally that the l owest three states in the energy spectrum of the neutron deficient nucleus Pb-186 are spherical, oblate and prolate. The states are populated by the a lpha-decay of a parent nucleus; to identify them, we combine knowledge of t he particular features of this decay(7) with sensitive measurement techniqu es (a highly efficient velocity filter(8) with strong background reduction, and an extremely selective recoil-alpha-electron coincidence tagging metho d(8-10)). The existence of this apparently unique shape triplet is permitte d only by the specific conditions that are met around this particular nucle us.