Collective nuclear states with mixed proton-neutron symmetry

1
Collective nuclear states with mixed
proton-neutron symmetry

• Norbert Pietralla
• Institut für Kernphysik
• Technische Universität Darmstadt

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Collective nuclear states with mixed
proton-neutron symmetry

• Motivation
• interest in effective pn-valence-shell
  interaction Vpn
• Proton-neutron mixed-symmetry states
• definition and signatures
• specific examples and recent developments
• future experimental challenges
• Summary

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Generic Aspects of Nuclear Structure
Heavy Atomic nucleus
Two-fluid quantum system

• many-body system

• COLLECTIVITY

• consists of two equivalent entities
  (protons-neutrons)

• ISOSPIN SYMMETRY

• quantum system

• SHELL STRUCTURE

Study mixed symmetry (MS) structures !
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Model pn-symmetry in valence shell
Contain pn-antisymmetric boson pairs
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Simple Example Harmonic Oscillator
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Experimental signature for MSSsM1 transitions
Lp
Ln
gp ? 1 ?N, g? ? 0 ?N
M1 forbidden between FSSs!
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MSSs at the analytical Limits
M1 phenomena in the nuclear valence shell
Np N? 1
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Scissors mode in deformed nuclei (Darmstadt, 1983)
AnimationRob Casperson (Yale)
Scissors mode classically current loop gt
M1 magnetic dipole excitation electron
scattering photon scattering
Bohle et al., NPA 458, 205 (1986).
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Bremsstrahlung Photons U.Kneissl,N.Pietralla,A.Zi
lges, J.Phys.G 32, R217 (2006).
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M1 phenomena in the nuclear valence shell
Collectivity of the Scissors Mode
Richter, Kneissl, von Brentano et al.
Measure of quadrupole collectivity
Stuttgart-Darmstadt-Köln
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1
N. Pietralla et al., PRC 58, 184 (1998)
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Mixed symmetry phonon structure at N 52
Orce et. al (2006)
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Identification of multi-phonon MSSs
E2 strength from g.s.
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First observation of vibrational MS structure in
odd-mass nuclei
Doppler-shift lifetimes from (n,n?) at Lexington
94Mo
93Nb
2ms
1196
2
871
0
B(M1)93Nb 0.32(11)0.62(24)mN2 B(M1)94Mo
0.56(5) mN2
J.N. Orce et al, PRL 97, 062504 (2006).
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Systematic SM calculations from NN interaction
with Vlow-k
J. D. Holt et al, (submitted)
Yale-Rutgers collaboration
Inverse kinematics Coulomb excitation
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Form Factors of MSSs at S-DALINAC
M1 integral ? one value
Cross sections differential ? a function (1-dim
more structure information)
Electron Spectrometer
130 MeV in-house electron LINAC well suited for
nuclear structure studies
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Electron scattering on vibrational multi-phonon
MSSs
Darmstadt data with O.Burda, P.von
Neumann-Cosel A.Richter et al.
94Mo(e,e)
iThembaLabs data with H.Fujita P.von
Neumann-Cosel et al.
94Mo(p,p)

• Ideal for one-phonon states
• Sensitive to 14 lowest 2 states

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94Mo(e,e) Form FactorsTest of quantum
mechanical wavefunction
IBM-2 and SM Vlow-k in good agreement with
experiment. (J.Holt, later this
week) combination with hadronic probe
forward-angle (p,p) at ithembaLabs allows for
isospin-sensitivity.
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Need for data on exotic nuclei
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Inverse kinematics CoulEx to MSSs
Coulomb excitation
138Ce
138Ce
Detector array e.g. Gammasphere at Argonne Natl
Lab
138Ce _at_ ANL
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Facility and Experiments _at_ ANL

• ATLAS Superconducting LINAC
• Heavy ions to 82 of Coulomb barrier
• Gammasphere
• 4p detector array with 110 HPGe detectors
• Experiments
• 136,138Ce, 124,126,128,130,132,134Xe
• Extract M1 transition strengths for solid
  identification of MSSs

with G.Rainovski, T.Ahn and the ANL group
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CoulEx of Mixed-Symmetry States
Observe 21,2,3,4,5,6 up to 2.7 MeV
G.Rainovski, NP et al., Phys. Rev. Lett. 96
122501 (2006).
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Angular distributions
a2(24), a4(24)
52 d0.184 - 42 97 M1
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The population cross-sections
Radiation character from angular distribution
CLX code (H. Ower J. Gerl H. Scheit) Based
on Multiple step Coulex theory (Winter de Boer)
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Transition Strengths
E2
M1
E2
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Two-Level Mixing
0.18
0.06
0.12
a2 B(M124?21)/ B(M1tot) 0.12/0.18 0.7
VF-mix 44(3) keV
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MSS Investigation _at_ Argonne (Missia) program
overview
Sm
Previous literature
Ce
Published Under analysis Expt accepted proposed
Ba
Xe
Chart from http//www.nndc.bnl.gov
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Direct evidence for p-n interaction Vpn in the
valence shell
2 evolution in N80 isotones
2ms
21
Proton Number Z

• Energy splitting increases with growing valence
  shell (increasing Np)

Application to Radioactive Beam Accelerators
(e.g. CERN, FAIR, )
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What RIB intensity needed for studying MSSs?
15 h at 6.109 pps!
1/25
107pps for 2 weeks
2ms
1/250
106pps for 2 weeks
2ms
To identify excited 2 state (beyond the 21) in
vibrational nucleus (B(E2)1Wu) with Gammasphere
for 2 weeks beam time we need 105pps. For
complete spectroscopy 106-107 pps will be needed!
1/2500
105pps for 2 weeks
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Conclusion

• Interest in effective p-n interaction in the
  valence shell
• MSSs provide sensitive tests for Vpn
• Vast amount of data has appeared and a consistent
  picture emerges
• (e,e) and Coulex as experimental probe
  very well suited
• Applicable to RIBs

2ms
Vpn (NpNn)1/2
21
Proton Number Z
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From NSAC-charge Nuclear Physics with the Rare
Isotope Accelerator

• Themes and challenges of Modern Science
• Complexity out of simplicity
• How the world, with all its apparent complexity
  and diversity can be constructed out of a few
  elementary building blocks and their interactions
• Simplicity out of complexity
• How the world of complex systems can display such
  astonishing regularity and simplicity
• Understanding the nature of the physical universe
• Manipulating nature for the benefit of mankind

Nuclei Two-fluid, many-body, strongly-interacting
, quantal systems provide wonderful laboratories
for frontier research in all four areas