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Coulomb Excitation of the K p =5 - 242m Am Isomer (t 1/2 =141 y)

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CCFP 2006

A. B. Hayes and D. Cline

University of Rochester, Dept. of Physics and Astronomy, Rochester, NY U.S.A.

J. J. Carroll, D. Gohlke, R. Propri, and R. Wheeler

Youngstown State University, Youngstown, OH U.S.A.

S. A. Karamian

Joint Institute for Nuclear Research, Dubna, Russia

J. A. Becker, R. A. Macri, K. J. Moody, and C. Y. Wu

Lawrence Livermore National Laboratory, Livermore, CA U.S.A.

M. P. Carpenter, J. P. Greene, A. A. Hecht, R. V. F. Janssens, T. Lauritsen, C. J. Lister, D. Seweryniak, X. Wang, and S. Zhu

Argonne National Laboratory, Argonne, IL U.S.A.

A. O. Macchiavelli

Lawrence Berkeley National Laboratory, Berkeley, CA U.S.A.

- K Quantum number
- Previous K-isomer work
- Present 242mAm experiment
- Gamma decay data
- Electromagnetic matrix elements

- Interpretation
- Conclusions

- K is the projection of the total spin I on the nuclear symmetry axis
- K-selection rule: K
- For axially symmetric nucleus
- is the multipole order of EM transition

- Degree of forbiddenness = K -
- Transition is “-times” forbidden

U. Rochester—A. B. Hayes, D. Cline, C. Y. Wu, H. Hua, M. W. Simon, R. Teng ANL—R. V. F. Janssens, C. J. Lister, E. F. Moore, R. C. Pardo, D. Sewereniak LBNL—A. O. Macchiavelli, K. Vetter GSI—J. Gerl, Ch. Schlegel, H. J. Wollersheim Warsaw University—P. Napiorkowski, J. Srebrny WNSL, Yale University—J. Ai, H. Amro, C. Beausang, R. F. Casten, A. A. Hecht, A. Heinz, R. Hughes, D. A. Meyer

- 178Hf(136Xe,136Xe)178Hf Coulomb Excitation @ 650 MeV with Gammasphere and CHICO at ATLAS (Argonne)
- Ta(178Hf,178Hf)Ta at 73%—86% Coulomb barrier at ATLAS Measured 178m2Hf K=16+ 31 year isomer activity (Yale)
Extended 178Hf level scheme, measured matrix elements coupling low-K bands to K=4+,6+,8- bands

Fit a set of GSB16+ band matrix elements from combined data of experiments 1 and 2

Previous Rochester Work: 178Hf Level SchemeNew levels and isomer states in bold

Interpretation of 178Hf Data

Publications PRL 89, 242501 (2002)

PRL 96, 042505 (2006)

High-K Bands

Constant moments of inertia

Align at much higher spin

Highly hindered transitions between high-spin, high-K states

K is good in high-K bands

Low-K Bands

Rapid loss of hindrance with increasing spin

Up-bends in the moments of inertia suggest mixing

Total breakdown of

K-conservation at

I≈12 in low-K bands

Results are consistent with collective alignment effects.

Expect similar behavior in other deformed nuclei.

K-Mixing in Low-K Wave Functions of 178Hf

- Probes of four ranges of K in the GSB
GSB→4+: 2≤K≤6

GSB→6+: 4≤K≤8

GSB→8-: 5≤K≤11

GSB→16+: 14≤K≤18

- Wave functions are completely mixed for IGSB>12, Igamma>12
- B(Eλ) values saturate at ~1 W.u.

Coulomb Excitation of the Kp=5-242mAm Isomer (t1/2=141 y)

- Motivation
- Coulomb excitation of a pure isomer target— ~104 times greater sensitivity to matrix elements coupled to the isomer band
- Measure coupling between K=5- isomer band and low-K bands
- Are the same spin-dependent K-mixing effects observed as in 178Hf?

- Experiment
- First Coulomb excitation of a 98% pure isomer target
- 242mAm(40Ar,40Ar)242mAm at 170 MeV using the ATLAS Linac at (Argonne) 500 g/cm21.6 mCi242mAm on 5mg/cm2 Ni
- Detect back-scattered Ar (CHICO) in coincidence with one photon Gammasphere (101 Ge) + 5 LEPS detectors

Gammasphere Single -ray Spectrum

Level SchemeNew levels are shown in bold.Unconnected levels were not observed.

Level SchemeNew levels are shown in bold.Unconnected levels were not observed.

Measured Partial WidthsIncluding internal conversion

Level SchemeNew levels are shown in bold.Unconnected levels were not observed.

243Am(D,T)242Am Grotdal et al., Physica Scripta 14, 263 (1976)

Unidentified 99 keV and 171 keV states

Level SchemeNew levels are shown in bold.Unconnected levels were not observed.

Level Energy vs. Spin I

Moment of Inertia vs. Rotational Frequency

Level SchemeNew levels are shown in bold.Unconnected levels were not observed.

Measured -ray Yields and Fit Results

Measured -ray Yields and Fit Results

Reduced Transition Probabilities

Known StructuresNew levels are shown in bold.Previously known levels from Salicio et al., Phys. Rev. C 37, 2371 (1988).

Known K=3- DecaysNew levels are shown in bold.Transitions with thin arrows from Salicio et al.Unconnected levels were not observed.

- K-forbidden transitions to K=0- band have comparable strength to K-allowed transitions to the K=5- band
- K=2- / K=3- Coriolis mixing

1 2

K-allowed

Band Mixing

- 178Hf
- E2 and E3 coupling of K=0+,2+ bands to K=4+,6+,8-,16+ bands
- Transition probabilities ~ few single-particle units at high spin (I~12)
- Mixing of K=2—14 components at high spin in nominal low-K wave functions, consistent with Coriolis effects
- High-K wave functions comparatively pure

- 242Am
- K=5- states coupled to K=6- states by E2 ~20single-particle units
- K=3- — K=0- E2 (=1) coupling comparable in strength to K=3- — K=5- E2 (allowed) ~1 s.p.u.
- Consistent with K=1 mixing (K=5 with K=6 and K=2 with K=3)
- Coriolis interaction strength over-predicts mixing by a factor of ~3
- Strong first-order mixing—even at low spin—complicates measurement of (K>1) effects

Depopulation

- 178m2Hf K=16+ Isomer
- Calculated heavy-ion Coulomb depopulation (E2) to ground state is 1% effect
- Calculated heavy-ion Coulomb depopulation (E1) through intermediate 15- state 0.1% effect
- No useful intermediate states found for photo-depopulation

- 242mAm K=5- Isomer
- Heavy-ion E2 excitation of K=3- band observed ~1% at IK=3=11-
- K=3- band -decay branches to K=0- comparable in strength to K=3- to K=5-
- Search continues for states coupled to isomer

Summary

- A 98% pure 242mAm K=5- isomer target Coulomb excited to I~18
- 30 states added including discovery of a new K=6- band
- Unexpectedly strong K=5- to K=6- coupling
- Populated states which are coupled to the K=0- ground band
- Strong K=1 mixing effects
- Iterative fit of matrix elements in progress
- K=5- isomer state coupled to I,K=1,0- ground state through K=3- band—consistent with mixing with K=2- band
- Coupling between K=3-,5- and K=0- bands is subject of further experimental and theoretical investigation

This work was supported by:

- Air Force Office of Scientific Research
- National Science Foundation
- U.S. Department of Energy

Measured Gyromagnetic Ratiosfrom measured partial widths

Known Rotational Bands in 242AmSalicio et al., Phys. Rev. C 37, 2371 (1988)

Low-Energy Photon Spectrometers