Possible Shears Bands
204
206
in At and Fr
Evan Seyfried
Motivation
S. Frauendorf, Z. Phys.
A 358, 163–167 (1997)
• Shears bands known along Z = 82 Pb nuclei
• Predicted along N = 120 series of nuclides
N=120
Background
S. Frauendorf, Nuc. Phys.
A 601 (1997) 41-55
• Quick Shears Band Review
High-j, proton-particle / high-j, neutron-hole interaction:
Perpendicular at lowest energies  Off-axis total angular momentum
Large magnetic dipole moment develops
Magnetic Rotation:
Tilted Axis Cranking (TAC) model:
Quantized precession of the magnetic
dipole moment, μ┴) about the total
angular momentum ( J )
Closing of Shears  Shows I(I+1) rotation-like
behavior in near-spherical nuclei
Experiment
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Argonne National Lab
ATLAS accelerator, GAMMASPHERE
Fusion-evaporation reaction
30Si + 181Ta  211Fr*
Beam Energy: 152 MeV
15E6 γ-γ coincidences
20 hrs beam time
Nuclei of interest produced:
204At  α3n
206Fr  5n
Experiment
W. Reviol, D.G. Sarantites, et. al.,
Nuclear Instruments and Methods in
Physics Research A 541 (2005) 478-500
WashU’s HERCULES
High-efficiency Evaporation-Residue Counter Under
Lots of Elastic Scattering (ratio σE-R/σfission ~ 1/200)
64 thin fast-plastic scintillators in 4 concentric rings
Data
Projection of Coincidence Matrix
X-rays used for
Isotopic ID
Data
Francium (86 keV) & Astatine (79 keV) X-ray gates
Fr
205Fr
206Fr
207Fr
At
203At
204At
Data
Shears Bands
206Fr
Shell Model
States
Shell Model
States
204At
Level Schemes
Why Shears?
• Characteristic low-energy energy-level spacing
• Band sits high in energy above the shell model states
• Possible alignment:
J (ħ)
ħω (MeV)
Shears Bands
from light Pb
and Bi to the
N=119 isotones
Bi
Pb
Rn
At
Po
J (ħ)
ħω (MeV)
Fr
Future Direction
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Evidence for shears bands, floating above expected
odd-odd shell model states in both 204At and 206Fr
(N=119 isotones)
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More evidence needed, but we consider them
possible shears bands
1. Longer run to find linking transitions to shell model states
– Angular correlation for M1 confirmation
– Confirm configurations for the band
2. Lifetime measurement to determine B(M1)’s
Thanks to:
• U.S. Naval Academy:
D.J. Hartley
• Washington University:
W. Reviol, D.G. Sarantites, C.J. Chiara, O.L. Pechanaya
• CSNSM:
K. Hauschild, A. Lopez-Martens
• Argonne:
M.P. Carpenter, R.V.F. Janssens, D. Seweryniak, S. Zhu
Sponsored by grants from the NSF and DOE
Nilsson Diagrams
• Level Schemes:
Data
S. Chmel, S. Frauendorf, and H. Hubel,
Phys. Rev. C 75, 044309 (2007)
Shears Bands
• Demonstrates back-bending seen in shears bands in
the light-Pb nuclei
• B(M1)/B(E2) values limited to >___ (μ┴)2/(eb)2
• Longer data run needed to observe B(E2) values
– S. Chmel, et. al. report same problem with light Pb nuclei
• Lifetime measurements required to get B(M1) values