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-Mixing in 178Hf

The 48.6 keV, K=5- [t1/2 =141 year] isomer in 242mAm

Nuclear Structure Implications

Implications for controlled energy release

*Work supported by AFOSR and NSF

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

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

Ge detectorCHICO

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 4π Time resolution: 500 ps

Mass resolution Δm/m = 5%

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

Coulomb excitation of K isomers in 178HfRochester— 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. Seweryniak;

LBNL—A. O. Macchiavelli, K. Vetter; GSI—J. Gerl, Ch. Schlegel, H. J. Wollersheim;

Warsaw—P. Napiorkowski, J. Srebrny;

Yale—J. 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) K=8- 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) 242501Physical Review Letters 96 (2006) 042505Physical Review C (2006) Submitted

Coulomb excitation pathways to High-K isomer bands in 178Hf

Stimulated depopulation of the K=16+ isomer in 178Hf

Summary for 178Hf

• Populated the K= 6+,8-, 14-, and 16+ isomeric bands at 10-4 probability and measured Eλ 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 K=16+ isomer in 178Hf.

• No evidence of a state required to mediate photo depopulation of the K=16+ isomer claimed by Collins et al.

The 242mAm, 48.6keV, K =5-, (t1/2=141 y) isomer

Motivation:

1. Measure coupling between K=5- isomer band and low-K bands

Experiment:

1. 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

2. 242mAm(40Ar,40Ar)242mAm at 170 MeV using the ATLAS Linac at (Argonne)

3. Detect back-scattered Ar (CHICO) in coincidence with one photon in Gammasphere (101 Ge) + 5 LEPS detectors. Am recoils stopped in target

4. Target activity 1.6 milliCi

Study of the 242mAm, 48.6keV, K =5-, (t1/2=141 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

242mAm Coulomb excitation -ray spectrum

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

242Am Level SchemeNew levels are shown in bold.

Unconnected transitions were not observed.

K = 0- K = 3- K = 5- K = 6-

Gamma yields for the K=5- and K=6- following Coulomb excitation of 242Am

In-band transitions

Gamma yields for the K=5- and K=6- following Coulomb excitation of 242Am

Interband transitions

Coulomb excitation of the mixed K=5- and K=6- bands in 242Am

Assumptions:

• Strongly-deformed axially-symmetric rotor model

• ΔK=1 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 <K=6-|E2|K=5-> = -0.180 e.b

• gK-gR equals +0,080 and +0.100 for intrinsic K=5- and K=6- bands

• Intrinsic <K=6-|M1|K=5-> = -0.280 nm,

Neutron-proton multiplets in 242AmNew levels are shown in bold.

Previously known levels from Salicio et al., Phys. Rev. C 37, 2371 (1988).

π[523]5/2- ± ν[631]1/2+ π[523]5/2- ± ν[622]5/2+ π[523]5/2- ± ν[624]7/2+

Known K=3- DecaysNew levels are shown in bold.

Transitions with thin arrows from Salicio et al.Unconnected levels were not observed.

1 2

K-allowed

K-forbidden transitions to K=0- band have comparable strength to K-allowed transitions to the K=5- band

Explanation K=2- / K=3- Coriolis mixing

Nuclear Structure and Band Mixing242Am

• Complete K=1 Coriolis mixing of K=5- and K= 6- bands due to level degeneracy

• K=2- and K=3- bands Coriolis mixed: decay by comparable E2 strengths to both ground K=0- and isomeric K=5- bands ~1 s.p.u.

• Detailed knowledge of the K=1 mixed wave functions, Coriolis interaction strength, and intrinsic E2 plus M1 properties.

178Hf

• Measured E2 and E3 coupling of K=0+, 2+ bands to K=4+,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 (I~12)

• Results consistent with Coriolis mixing

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

Isomer Depopulation242mAm K=5- Isomer

• Heavy-ion E2 excitation of K=3- band observed ~1% at IK=3=11-

• The Coriolis mixed K=3- and K=2- bands could mediate depopulation of the K=5- isomer to K=0- ground band

• Measured properties sufficient to predict reliable depopulation cross sections for the K=5- isomer.

178m2Hf K=16+ Isomer

• No useful state found to mediate photo-depopulation

• Calculated heavy-ion Coulomb depopulation (E2, E3) to ground and K=14- 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

This work was supported by:

• Air Force Office of Scientific Research

• National Science Foundation

• U.S. Department of Energy

Acknowledgements

END