Programme 3: High Spin Phenomena (Oi, Walker, Regan, Podolyak)
The key objective of this work package is to study high-spin phenomena
through the application of state-of-the-art techniques, both theoretical and
experimental. With increasing angular momentum, nuclear many-body systems
undergo structural changes from the superconducting phase. Unlike infinite systems,
structural transitions in nuclei happen gradually as a reflection of the finite-size effect.
Until the normal-conducting phase emerges at very high spin (the Mottelson-Valatin
effect), nuclei show a variety of many-body states as a consequence of the interplay
between collective and single-particle degrees of freedom. One such interesting form
of excited state is the oblate-deformed state at high spin. Prolate deformation is
generally favoured as the equilibrium shape of low-spin nuclear states. Angular
momentum can be an effective probe for the investigation of why oblate deformation
is hard to produce. The structural change associated with the onset of oblate
deformation is predicted to appear as a “giant backbending” [Xu00,Oi01a,Wa06] at
high spin (about 20 ℏ ) in neutron-rich hafnium (Z=72) isotopes. This area will be
investigated to provide motivation for future experiments in this regime which will be
available following developments of radioactive beam technology.
Fusion-evaporation reactions using neutron-rich radioactive beams can be
used to produce ultra high-spin states. Together with the 4π gamma-ray detectors,
such as AGATA and Gammasphere, the search for the excited states above the yrast
line becomes possible. Such side-bands include wobbling motion, chiral twin bands,
magnetic rotations, and nuclear tidal waves. These high-spin phenomena are 3D nuclear rotations, for which the theoretical understanding is still at a preliminary
stage though some progress has been made, led by Oi [Oi03, Oi05a, Oi05b, Oi06].
This will continue as an area of theoretical focus by Oi in the upcoming period.
Isomers can be a major tool for the study of nuclear structure [Wa99]. High-K
isomers are abundant in the Hf-W-Os region. This type of isomer has an axially
symmetric shape and possesses a quantum number “K”, corresponding to the
projection of the total angular momentum onto the symmetry axis. The quantum
selection rule for K allows a nucleus to be isomeric before decaying to the K=0
ground state. When high-K bands cross low-K bands, a different type of backbending
happens [Oi01b,Wa07b]. The original back bending explanation is based on rotational
alignment, where the structural transition happens between two low-K structures (gand s-bands). However, high-K components dominate when the Fermi level is in the
upper part of the shell. Therefore, by shifting the Fermi level, the new “tilted”
backbending may be produced. So far, a unified understanding of this effect has not
been achieved. Increasing Tz, through the use of neutron-rich beams, will allow an
experimental study of the evolution of the backbending mechanism in an isotope
chain, for comparison with theoretical calculations. The relevant band crossing
involves a drastic change in the K quantum number, that is, the isomeric “Kforbidden” transitions. To allow the transitions, the symmetry needs to be broken.
Currently, two modes are known to be important: tilted rotation and γ vibration.
Quantum mechanical approaches, such as the generator coordinate method (GCM),
are essential in dealing with these dynamical modes. Our theoretical study, led by Oi,
uses the GCM and the 3D cranked Hartree-Fock-Bogoliubov method, with quantumnumber projection.
We have an excellent track record of gaining access to experimental beam
time at leading accelerators (GSI, GANIL, LNL, JYFL, ANL and TRIUMF). Specific
near-term projects include the inelastic excitation of a 178Hf beam at ANL using
Gammasphere in order to probe ΔK=16 mixing; using a radioactive 14C beam at ANL
to study high-K vibrations in 186Os; and using deep-inelastic reactions to gain access
to the doubly-mid-shell 170Dy region (LNL, experiment performed mid-2007, data
analysis in progress). In the longer term, we plan to exploit radioactive beams at GSIFAIR, GANIL-SPIRAL2 and TRIUMF-ISAC2 to study high-spins in neutron-rich
nuclei. Indeed, we have already initiated such studies with a 8He beam led by
Podolyak and Walker [Po03,Ga05]. This activity is strongly linked to the
accompanying programme 2. The exploitation of the AGATA gamma-ray
spectrometer will be a key feature.
In the near-to-medium term, we plan to lead and collaborate in a number of
AGATA experiments using heavy, stable beams to induce deep-inelastic collisions
(DIC). It is experimentally established that deep-inelastic reactions performed at
energies of about 10-20% above the Coulomb barrier are well suited to producing
neutron-rich nuclei at high-spins. The PRISMA magnetic spectrometer in Legnaro has
been specifically designed for the study of the products of multi-nucleon transfer and
deep-inelastic reactions. A programme of experiments has already taken place using
PRISMA in conjunction with the CLARA array including the reaction 82Se+170Yb led
by Regan, aimed at the study of nuclear collectivity approaching the theoretically,
'valence-maximal' nucleus 170Dy [Re02]. During the rolling grant period, the AGATA
demonstrator will be
-ray
detection efficiency. In addition to the continuation of the analysis from the Surreyled PRISMA+CLARA, 82Se+170Er experiment (which ran took place in early 2007),
members of the experimental group, specifically Regan, Podolyak and Walker, will be
heavily involved in the running of these and other deep-inelastic experiments to study
near-yrast phenomena in stable and neutron-rich systems during the initial Legnaro
and GANIL phases of AGATA.
AGATA will be located in GANIL for 12 month starting in the middle of
2010 and coupled to existing instruments such as VAMOS. The heavy beams such as
238
U and 208Pb which are available at GANIL at near-Coulomb barrier energies,
offers the unique opportunity to study the structure of exotic nuclei using deepinelastic reactions in inverse kinematics at high spins. Experiments led by Regan
[Re03] using GAMMASPHERE and CHICO have shown that discrete states in
projectile-like residues in such DIC reactions can be routinely populated to at least
20ħ. One of the important AGATA-plus-VAMOS experiments which will be carried
out at GANIL is to search for evidence of a new region of nuclear superdeformation
at high spins, centred around the -stable nucleus 100Mo. Such structures have been
predicted for more than a decade [Sk97] and are based on the presence of doublymagic superdeformed (SD) shell gaps at Z=42 and N=58. Previous, thin-target deep
inelastic studies, led by Regan, have populated discrete states up to 20ħ in 100Mo,
close to the spins where the predicted SD minimum becomes yrast [Re03]. Related
studies are
expected to be performed at Argonne
using the
GAMMASPHERE+CHICO, earlier in the rolling grant period. Such analysis requires
an experienced PDRA, who can both develop and apply the required analysis
algorithms to the later AGATA-based work at Legnaro and GANIL. One
experimental PDRA is therefore requested to lead the data analysis and contribute to
the experimental preparation of the deep-inelastic work from this work-package for the
duration of the rolling grant period.
Theory: Oi (20h), PDRA (1)
Experiment: Regan (7h), Walker (4h), Gelletly (4h), Podolyak (2h), PDRA (1), PhD
(1), Prog/phys (0.33), Phys/exp (0.33)
REFERENCES
[Re02]
P.H. Regan et al., Phys. Rev. C 65, 037302 (2002)
[Re03]
P.H. Regan et al., Phys. Rev. C 68, 044313 (2003)
[Sk97]
J. Skalski et al., Nucl. Phys. A617, 282 (1997)