Reflection-Asymmetric Shapes in Nuclei

Abstract

Soon after the collective nuclear model [1,2] was established, low-lying negative parity states were observed [3] in even-even Ra and Th nuclei from alpha-particle spectroscopic studies. These negative parity levels formed a rotational band with spin sequence 1, 3, 5, and K quantum number 0. Since these states had energies much lower than the expected two quasi-particle states, these were interpreted as octupole vibrations about a spheroidal equilibrium shape. Ever since the discovery of the octupole vibrations, scientists have been thinking about the possibility of octupole deformations in nuclei, that is, nuclei with octupole equilibrium shape. These octupole shapes are symmetric about the Z axis but reflection asymmetric about the XY plane and resemble a Pear.• --. w i : , . £ ,,,.,.. -, ' ; ,.,,_• , T he submitted manutcnpt has been authored bv a contractor of the U. S. Government under contract Na. W-31-109-ENG-3S. Accordingly, the U. S. Government retains a nonexclusive, royalty-free license \n publish or reproduce the published form of this contribution, or allow others to do so, for IK S. Government purposes. MASTERThe octupole deformation or octupole vibration is produced by the long range octupole-octupole correlations. These correlations depend on the r 3 Y3*r^Y3 matrix elements between single particle states with A2. = Aj -3 and the energy difference between them. A look at the shell states ( Fig. 1 with the binding energy of ~10 MeV due to the quadrupole deformation (Fig. 2).It is well known that the rotation of symmetric and asymmetric molecules generate different rotational bands. In the case of the former, the band has 0 + , 2 + , 4 + --level sequence and the latter has 0 + , 1", 2 + , 3", --sequence.Thus the rotation of reflection symmetric and reflection asymmetric nuclei will produce different spectra and these can be used as signature of octupole shape. OCTUPOLE DEFORMATION THE ACTINIDE REGIONIn odd mass nuclei, the two members of the parity doublet have very similar wave functions except the parity. Thus one would expect similar properties for both bands. The data on odd mass nuclei in the mass 224 region can be summarized as follows:1. Parity doublets have been observed in several odd-mass Ra, Ac and Pa nuclei [12][13][14][15]. An example [14] is shown in Fig. 4. The members of the doublets are connected by fast El transitions [15].The B(E1) values in these nuclei are larger than l.OxlO" 3 Weisskopf units (see Fig. 5).3. Alpha decay rates are enhanced to the parity doublet partner of the favored band.4. Coriolis matrix elements and Ml transition rates are attenuated.5. Octupole deformation has also been found at moderate spins [16] in 221 Th and 223 Th.In even-even nuclei, levels with alternating parity have been observed in several Ra and Th nuclei. In general, these are connected by fast El transitions. OCTUPOLE DEFORMATION IN N-88, Z-56 NUCLEIAs shown in Fig. 1, octupole correlations are also expected in the Z~56, N~8Snuclei. However, in these nuclei, spacings for the A2. = 3, Aj =3 le...