Nuclear structure of high-K isomers: implications for controlled energy release*

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Nuclear structure of high-K isomers
implications for controlled energy release
D. Cline, A.B. Hayes, University of
Rochester The K quantum number Motivation K-M
ixing in 178Hf The 48.6 keV, K5- t1/2 141
year isomer in 242mAm Nuclear Structure
Implications Implications for controlled energy
release Work supported by AFOSR and NSF
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K Quantum Number

• K is the projection of the total spin I on the
  nuclear symmetry axis
• K is a conserved quantum number for axially
  symmetric nuclei
• K-selection rule ?K ? ?
• ? is the multipole order of EM transition
• Degree of forbiddenness ? ?K - ?
• Transition is ?-times forbidden

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• Motivation for study of high-K isomers
• Nuclear physics
• High-K states have unusually simple shell
  configurations providing a powerful probe of
  structure and residual interactions in the
  nuclear many-body system
• High-K isomers probe the goodness of the K
  quantum number in nuclear structure
• Quantum electronics
• Evaluate the feasibility of using long-lived
  isomers for controllable energy storage.
• Goals
• Measure the fundamental properties of isomeric
  states by Coulomb excitation
• Ascertain the mechanism responsible for
  electromagnetic population and decay of highly
  K-forbidden isomeric states
• Elucidate the feasibility of triggered
  depopulation of isomeric states

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CHICO M.W.Simon, D. Cline, C.Y. Wu R.W. Gray, R.
Teng. C. Long Nucl. Inst. Meth. A452 (2000)
205 Work supported by the NSF
Scattering angle 12? ? 85? (Front Part)
95? ? 168? (Back Part)
Azimuthal angle total 280? of
360? Position
resolution ? 1? in ? and ? 4.6? in ? Solid
angle 69 of 4p
Time resolution ? 500 ps Mass resolution ?m/m
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Coulomb excitation of K isomers in 178Hf

• Rochester A. B. Hayes, D. Cline, C. Y. Wu, H.
  Hua, M. W. Simon, R. Teng
• ANLR. V. F. Janssens, C. J. Lister, E. F. Moore,
  R. C. Pardo, D. Seweryniak
• LBNLA. O. Macchiavelli, K. Vetter GSIJ. Gerl,
  Ch. Schlegel, H. J. Wollersheim
• WarsawP. Napiorkowski, J. Srebrny
• YaleJ. Ai, H. Amro, C. Beausang, R. F. Casten,
  A. A. Hecht, A. Heinz, R. Hughes, D. A. Meyer

• Motivation
• Highly K-forbidden Coulomb excitation of the 1147
  keV (4 sec) K8- isomer was observed by Hamilton
  et al (1982) and confirmed by Xie et al (1993)
• N.B. 8- to ground band 8 E1
  transition is K-hindered by a factor 1.9 x 1013
• Possible application of the 2447 keV (31 year) K
  16 isomer for controllable energy storage.
  Conflicting results on possible triggering
  depopulation of this isomer using X-ray radiation
  by Collins et al (1999?) which is disputed by
  work of several groups.
• Goal
• Elucidate pathways leading to Coulomb excitation
  of high-K isomers
• Physical Review Letters 89 (2002)
  242501 Physical Review Letters 96 (2006) 042505
• Physical Review C (2006) Submitted

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Coulomb excitation pathways to High-K isomer
bands in 178Hf
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Stimulated depopulation of the K16 isomer in
178Hf
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Summary for 178Hf

• Populated the K 6,8-, 14-, and 16 isomeric
  bands at 10-4 probability and measured E?
  strengths
• Elucidated pathways leading to Coulomb excitation
  of K isomers.
• Showed that there is massive break down of the K
  quantum number at high spin in the ground band
  and gamma band whereas K is conserved in high-K
  bands.
• Have identified possible Coulomb excitation paths
  to depopulate the K16 isomer in 178Hf.
• No evidence of a state required to mediate photo
  depopulation of the K16 isomer claimed by
  Collins et al.

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The 242mAm, 48.6keV, Kp 5-, (t1/2141 y) isomer
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Study of the 242mAm, 48.6keV, Kp 5-, (t1/2141
y) isomer
A.B. Hayes1, D. Cline1, K.J. Moody2, C.Y. Wu2,
J.A. Becker2, M.P. Carpenter3, J.J. Carroll4, D.
Gohlke4, J.P. Greene3, A.A. Hecht3, R.V.F.
Janssens3, S.A. Karamian5 T. Lauritsen3, C.J.
Lister3, A.O. Macchiavelli6, R.A. Macri2, R.
Propri4, D. Seweryniak3, X. Wang3, R. Wheeler4,
S. Zhu3 1) Rochester, 2) LLNL, 3) ANL,
4)Youngstown, 5) Dubna, 6) LBNL

• Motivation
• Measure coupling between K5- isomer band and
  low-K bands
• Experiment
• Coulomb excite a 98 pure isomer target, 500
  ?g/cm2 242mAm on 5mg/cm2 Ni. 104 times greater
  sensitivity to matrix elements coupled to the
  isomer band than for 178Hf
• 242mAm(40Ar,40Ar)242mAm at 170 MeV using the
  ATLAS Linac at (Argonne)
• Detect back-scattered Ar (CHICO) in coincidence
  with one photon in Gammasphere (101 Ge) 5
  LEPS detectors. Am recoils stopped in target
• Target activity 1.6 milliCi

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242mAm Coulomb excitation ?-ray spectrum
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Level SchemeNew levels are shown in
bold.Unconnected levels were not observed.
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243Am(d,t)242Am
Grotdal et al., Physica Scripta 14, 263 (1976)
Unidentified 99 keV and 171 keV states
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242Am Level SchemeNew levels are shown in
bold.Unconnected transitions were not observed.
K 0- K 3- K
5- K 6-
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Gamma yields for the K5- and K6- following
Coulomb excitation of 242Am In-band transitions
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Gamma yields for the K5- and K6- following
Coulomb excitation of 242Am Interband transitions
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Coulomb excitation of the mixed K5- and K6-
bands in 242Am

• Assumptions
• Strongly-deformed axially-symmetric rotor model
• ?K1 Coriolis band mixing
• Conclusions
• Determined band wavefunctions strongly mixed
  50-50 at I 6- to 25-75 at I 17-
• The Coriolis interaction between bands measured
  to be 6.8 keV at I 6- increasing to 24 keV at I
  17-
• Intrinsic quadrupole moment Q0 12.0 e.b
• Intrinsic ltK6-E2K5-gt -0.180 e.b
• gK-gR equals 0,080 and 0.100 for intrinsic
  K5- and K6- bands
• Intrinsic ltK6-M1K5-gt -0.280 nm,

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Neutron-proton multiplets in 242AmNew levels are
shown in bold.Previously known levels from
Salicio et al., Phys. Rev. C 37, 2371 (1988).
p5235/2- ?6311/2 p5235/2-
?6225/2 p5235/2- ?6247/2
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Known K3- ? DecaysNew levels are shown in
bold.Transitions with thin arrows from Salicio
et al.Unconnected levels were not observed.
K-forbidden transitions to K0- band have
comparable strength to K-allowed transitions to
the K5- band Explanation ? K2- / K3- Coriolis
mixing
1 ? ? ? 2
K-allowed
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Nuclear Structure and Band Mixing

• 242Am
• Complete ?K1 Coriolis mixing of K5- and K 6-
  bands due to level degeneracy
• K2- and K3- bands Coriolis mixed decay by
  comparable E2 strengths to both ground K0- and
  isomeric K5- bands 1 s.p.u.
• Detailed knowledge of the ?K1 mixed wave
  functions, Coriolis interaction strength, and
  intrinsic E2 plus M1 properties.

• 178Hf
• Measured E2 and E3 coupling of K0, 2 bands to
  K4,6,8-,16 isomer bands
• Discovered complete breakdown of K at high spin
  in nominal low-K bands whereas K is well
  conserved for high-K bands
• K-forbidden transition strengths few
  single-particle units at high spin (I12)
• Results consistent with Coriolis mixing

Breadth and scope of these results provide a
stringent test of models of nuclear structure for
collective nuclei.
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Isomer Depopulation

• 242mAm K5- Isomer
• Heavy-ion E2 excitation of K3- band observed
  1 at IK311-
• The Coriolis mixed K3- and K2- bands could
  mediate depopulation of the K5- isomer to K0-
  ground band
• Measured properties sufficient to predict
  reliable depopulation cross sections for the K5-
  isomer.

• 178m2Hf K16 Isomer
• No useful state found to mediate
  photo-depopulation
• Calculated heavy-ion Coulomb depopulation (E2,
  E3) to ground and K14- bands are ?1 effects.
• Depopulation cross sections are small.

Studies of the fundamental properties of K
isomers have determined the configurations and
residual interactions needed to make reliable
theoretical predictions of isomer depopulation
mechanisms
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Acknowledgements

• This work was supported by
• Air Force Office of Scientific Research
• National Science Foundation
• U.S. Department of Energy

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END