exotic nuclei structure and reaction noyaux exotiques ... - IPN - IN2P3

eference » [5] for the yrast band in rotational nuclei.
By distinguishing the angular momentum and
energy changes due to an increase in the rotational
frequency, and due to changes in the intrinsic
structure of the deformed core, it is found that the
best reference is given by the above formulae but
with the value of J 1 that is obtained from simply
fitting the spin being replaced by J 1 /3.
The idea of introducing a reference in the first
place is to subtract from the total spin and energy
changes, the contributions from the core, and the
J 1 /3 term in the above expressions reflects extremely
well the smoothly changing core structure.
The remainder can be directly related to the more
rapid changes in the single-(quasi)particle structure
due to band crossings. In particular, the spin
and energy changes associated with the 2-
quasineutron crossing can be very reliably extracted.
These are shown in Fig. 2 as functions of
mass number for the even-even osmium isotopes.
Also shown are J 0 and the inferred position of the
aligning i 13/2 levels with respect to the Fermi energy
λ. This direct extraction of λ allows a simple interpretation
of the evolution of the nuclear structure
along the isotopic chain, without recourse to detailed
theoretical calculations. The techniques developed
in Ref. [5] also give an excellent means of
extracting the properties of higher bands, especially
through band-mixing calculations.
Towards the perfect rotor
An important quantity in the VMI model is found to
be 2G=2E-Iω, which is a measure of the nonrotational
energy of the core. It is independent of J 0
and is directly related to J 1 through 2G≈J 1 ω 4 /2. For
certain systems, this can be very small compared
with the core rotational energy, and such nuclei
behave as almost perfect rotors. This is demonstrated
in Fig. 3 for the nuclide 178 Yb. Its even-even
neighbour 180 Yb has no known excited state but is
thought to be the best nuclear rotor that exists. An
experimental proposal has been accepted at
iThemba LABS in South Africa [6] to study this system
via an incomplete fusion reaction.
Superdeformed nuclei
The appearance of this extraordinary “perfect rotor”
behaviour seems to depend on the position of
the Fermi level which determines several properties
of the system. It should lie far enough above
the aligning levels in order to give a high 2-
quasineutron energy, thereby delaying the corresponding
crossing until much higher frequencies,
but at the same time close enough for them to still
give a significant contribution to J 0 .
In this respect, the superdeformed bands in eveneven
nuclides in the mass-190 region are quite
exceptional. Their larger deformations and larger
single-particle angular momenta j lead to moments
of inertia which are around a factor of 2 larger than
the largest seen in the above mass≈150 region.
Like the case shown in Fig. 3, very few of these
nuclides show any sign of a specific interaction
(crossing) with higher bands, and the VMI model
is capable of fitting the experimental spectra up to
very high spin with a quite remarkable precision.
Fig. 3. For a pure VMI band, the quantity 2G=2E-Iω is
the non-rotational energy of the core. In the 178 Yb
nuclide, this represents only 7% of the total energy,
even at the highest spins. Thus the system behaves
almost like a perfect rotor.
So much so, that the deviations observed for certain,
particular spin values may be related to the
very weak interactions with normal-deformed
states, that may be responsible for the “decay-out”
phenomena observed in these systems (see for
example, Ref. [7]). Information on band crossings
and on the “identicality” [8] of various bands may
also be obtained from these analyses.
References:
[1] S.M. Harris, Phys. Rev. B 138, 509 (1964); Phys.
Rev. Lett 13, 663 (1964)
[2] D.R. Inglis, Phys. Rev. 96, 1059 (1954); 103,
1786 (1956)
[3] M.A.J. Mariscotti, G. Scharff-Goldharber and B.
Buck, Phys. Rev. 178, 1864 (1969);
[4] G. Scharff-Goldharber, C.B. Dover and A.L.
Goodman, Ann. Rev. Nucl. Sci. 26, 239 (1976)
[5] N. Rowley, J. Ollier and J. Simpson, Phys. Rev.
C 80, 024323 (2009)
[6] J. Ollier et al., in progress
[7] A. Lopez-Martens, Eur. Phys. J. A 20, 49 (2004)
[8] C. Baktash, B. Haas and W. Nazarewicz, Annu.
Rev. Nucl. Part. Sci. 45, 485 (1995)
73