Nuclear Seminar

Order, Ergodicity and Chaos in Superdeformed Nuclei

Dr. Teng Lek Khoo
Argonne National Laboratory

Monday, November 20, 2006   4:00 p.m.   NSH 124
(Refreshments served prior to seminar in NSH 124)

 

     Shell structure in nuclei can lead to the creation of an excited minimum characterized by large deformation.  The “life” of a superdeformed nucleus reflects an unusual double cycle of chaos-to-order transition, as well as a new regime of chaos-to-order transition.
     As the hot, chaotic compound nucleus cools, a small fraction of gamma cascades populates the superdeformed (SD) well, flowing through excited rotational bands, which exhibit properties never before observed.  The spectrum of cooling, statistical E1 γ rays evolves into a broad E2 peak, indicating a hot, elongated nucleus.  In ¹⁹⁴Hg, Eg-Eg correlations reveal exceptionally narrow (FWHM ~ 10 keV) ridges, parallel to the diagonal.  The ridges exhaust nearly all of the 1-dimensional E2 strength, are constituted from 30-150 bands, and have a J⁽²⁾ moment of inertia identical to that of the yrast SD band.  The situation is very different from that in nuclei with normal deformation, where the E2 strength in excited states exhibit both a weak (<10% intensity) narrow (~30 keV) component and a broad (~200 keV) one.  The multitude of bands and theory both suggest large mixing among the excited ¹⁹⁴Hg SD bands, with wave functions having many (2-8) basis components.  Thus, a chaotic regime is approached, where transition strengths would be expected to have fluctuating (Porter-Thomas) strengths.  Yet, rotational coherence and flow are preserved, with E2 matrix elements connecting unique states differing in spin by 2ħ.  This situation suggests that the transition from chaos to order evolves through an ergodic regime. 
     Cascades trapped inside the SD well reach the “ground” band of the minimum, where cold ordered rotation gives rise to impressive equi-spaced sharp lines.  Here ordered states exist, with good quantum numbers, even though they are embedded in a dense sea of chaotic states from the primary minimum.  Tunneling out of the superdeformed well precipitates a sudden decay out of the minimum, with a reprise of a statistical spectrum.  Finally, the nucleus settles to the ground state through ordered levels.