Possible Shears Bands in 204 At and 206 Fr Evan Seyfried
Jan 03, 2016
Possible Shears Bands in 204At and
206FrEvan Seyfried
Motivation• Shears bands known along Z = 82 Pb nuclei• Predicted along N = 120 series of nuclides
S. Frauendorf, Z. Phys. A 358, 163–167 (1997)
N=120
Background• Quick Shears Band Review
High-j, proton-particle / high-j, neutron-hole interaction:Perpendicular at lowest energies Off-axis total angular momentumLarge 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
S. Frauendorf, Nuc. Phys. A 601 (1997) 41-55
Experiment
• 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 α3n206Fr 5n
WashU’s HERCULESHigh-efficiency Evaporation-Residue Counter
Under Lots of Elastic Scattering (ratio σE-R/σfission ~ 1/200)
64 thin fast-plastic scintillators in 4 concentric rings
ExperimentW. Reviol, D.G. Sarantites, et. al., Nuclear Instruments and Methods in Physics Research A 541 (2005) 478-500
Data
Projection of Coincidence Matrix
X-rays used for Isotopic ID
DataFrancium (86 keV) & Astatine (79 keV)
X-ray gates
Fr
At203
At204
At
205Fr206Fr207Fr
DataShears Bands
206Fr
204AtShell Model States
Shell Model States
Level Schemes
Why Shears?• Characteristic low-energy energy-level spacing• Band sits high in energy above the shell model
states• Possible alignment:
ħω (MeV)
J (ħ
)
Pb
Bi
At
Po
Rn
FrShears
Bands from light Pb and
Bi to the N=119 isotones
ħω (MeV)
J (ħ)
Future Direction• Evidence for shears bands, floating above
expected odd-odd shell model states in both 204At and 206Fr (N=119 isotones)
• 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
Data• Level Schemes:
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
S. Chmel, S. Frauendorf, and H. Hubel, Phys. Rev. C 75, 044309 (2007)