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)