Spectroscopy of superheavy nuclei: status Teng Lek Khoo Argonne National Laboratory

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Spectroscopy of superheavy nuclei statusTeng
Lek KhooArgonne National Laboratory

• Overview
• Recent experiments (in-beam others)
• Physics learned
• Aims for in-beam experiments with Gretina-BGS

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Fission barrier from shell energy
EE(LD) E(shell) E(pair) Tsf(exp)/Tsf
(LD)gt1013
Superheavy nuclei are at the limits of Coulomb
stability would fission instantaneously,
but shell-correction energy lowers the ground
state, thereby creating a barrier against fission.
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Heavy shell-stabilized nuclei

• Opportunities to study nuclei at the limits of
• Charge
• Spin
• Excitation Energy
• What are the limits?

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Initial states for ? decay to g.s.
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Imax(254No) gt Imax(220Th)!
Bf(254No) gt Bf(220Th)
gt5 MeV
8 MeV
20
I (hbar)
0
10
30
208Pb(48Ca,2n)
Structure at high spin possible
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Limit in Z ? gt118?
Th-Lr
from Oganessian
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Esp from spectroscopy

• Traditional method for investigating SHN
  synthesize ever heavier nuclei.
• Spectroscopy of shell-stabilized nuclei gives
  direct information on single-particle energies
  Esp , hence, shell gaps.

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Importance of single-particle energies Esp

• Gaps in Esp ? shell energy Eshell ? superheavy
  nuclei.
• Esp necessary for a quantitative description of
  SHN.
• Task define Esp use them to test theory.

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Questions

• Where (in N,Z) are the magic gaps for superheavy
  nuclei (SHN)?
• Where is the island of stability?

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Where is the island (in Z,N)?
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Where are the magic gaps for superheavy nuclei?
Predicted magic gaps from different models
Macroscopic/Microscopic (MM), Skyrme (SHF)
Relativistic mean field (RMF).
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Questions

• Where (in N,Z) are the magic gaps for superheavy
  nuclei (SHN)?
• Where is the island of stability?
• How accurate are the best nuclear models when
  extrapolated to the limits of stability?
• Can single-particles energies Esp of SHN provide
  decisive test of theory?

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• Can spectroscopy of SHN can address the big
  questions?

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Data on structure of SHN

• Solid existing data Th Es (Z90-99) Ahmad et
  al.
• Recent new data Fm Rf (Z100-104)
• Future data Rf Hs (Z104-106)
• Far future Z gt 106
• Some new researchers in area
• tend to be unaware of 1,
• often dont realize that model should describe
  all cases (1-4),
• make predictions, but without first testing
  theory with known data.
• Principle. For a prediction to be credible, the
  theory must first be validated it must at least
  correctly describe what is known.

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How does, e.g., nobelium probe the
single-particle levels of
Aim structure of nuclei with largest ZMove
Fermi level towards proton magic gap

• the heavier nuclei (say with Z114)?
• higher-lying proton orbitals?
• Deformation (and rotation) drive down their
  energies.

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Proton and neutron single-particle energies
(Woods-Saxon potential)
protons
neutrons
R. Chasman et al., Rev. Mod. Phys. 49, 833 (1977)
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Rotation decreases E(qp) especially for high-j
orbitals
Courtesy S. K. Tandel
k17/2 from above N184 gap
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Woods-Saxon single particle energies near 254No
Gaps degeneracies ? rise fall of E2qp
11/2725
-6.19
-6.24
7/2613
i13/2
-6.32
3/2622
-6.44
1/2620
h9/2

Fermi level
f5/2
N152
9/2734
-7.59
N150
-7.97
7/2624
-8.10
5/2622
E (MeV)
protons
neutrons
-4.93
5/2642
Single-particle energies from Woods-Saxon
potential with universal parameters. Pairing
Lipkin-Nogami prescription. Blocking of 2 orbits
included. Pair strength chosen to reproduce ?(5)
from measured ground-state masses Gp24/A,
Gn17.8/A
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• Systematics necessary.

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ATLAS at Argonne National Laboratory
Gretina-BGS
BGS
sisom 0.1-0.3 µb s/sfission 10-6
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DSSD of FMA
Time spatial correlations
e
254No
a
Timescale of Events
25MeV
a
Energy (MeV)
0.5-8MeV
Electrons 0.1 0.5 MeV
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Recent studies of SHN

• In beam
• Bands in 248,250Fm, 252, 254No 251Md, 253No,
  255Lr
• Entry distributions 253,254No Bf gt 5 MeV,
  max 32
• Reactions (DIC, transfer) Pu, Cm
• Decay
• High-K isomers N150 (244Pu, 246Cm, 250Fm,
  252No), 254No, 255Lr, 256,257Rf, (244Cm, 256Fm)
• ? decay 255Lr, Md

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In-beam measurements with fusion reactionsNiche
area for Gretina/BGS

• Rotational bands in even-even odd-even nuclei
• Ground state bands, 2-qp bands, high-K bands
  (tagged on isomers), vibrational bands 1- 3-
  qp bands.
• J(1,2) -- high-j particle alignments, pairing
  s.p.e. gaps.
• Bandhead energies ? test single-particle
  energies.
• M1/E2 branching ratios ? (gK gR)?
  configurations.

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Odd-A nuclei 251Md Chatillon et al., PRL 98,
132503 (2007)
M1 (gK gR)2
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N150 isotones
1.8 sa
1.1 sa
109 ms
1.9 sb
Nakatsukasa
Z94-102, one framework
Robinson et al aTandel et al, bGreenlees et al.
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Tandel et al., PRL Herzberg et al., Nature
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Isomers

• Isomers due to K hindrance.
• Lessons
• (a) K good quantum number.
• (b) Nuclei axially symmetric.
• (c) Isomers provide a sensitive tool for
  identifying 2-qp states.

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?
?
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?
?
?
Esp
Proton Esp, valid to Z102 (E2qp 254No) Z103
(E1qp 255Lr)
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Comments on self-consistent m-f models

• Parameters in interactions determined based on
  bulk nuclear properties (binding energy, radii)
  of doubly-magic nuclei, with little input from
  Esp.
• Success
• Correct orbitals (mostly) around Fermi level
• Accuracy within 0.5 MeV for many levels.

• However
• Discrepancies in Esp of up to 0.7 MeV (SLy4, D1S)
  or 1.1 MeV (NL1).
• Gaps (1.2 1.7 MeV) for deformed nuclei in wrong
  locations.
• For accurate predictions of magic gaps for SHN
• a. Accuracy not sufficient,
• b. Improved interactions required.

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Universal Woods-Saxon

• Accurate Esp (lt0.3 MeV).
• Noteworthy because of huge extrapolation (25)
  from 208Pb to Z102.
• If it continues to apply for Z gt 102, 103
• magic gaps at Z114 (2.2 MeV) and (N184?).
• If not, deviations would signal self-consistency
  effects predicted by DFTs.

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X
X
?
Where are the magic gaps? Macroscopic/Microscopic
(MM), Skyrme (SHF) Relativistic (RMF) mean
field.
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SHN excellent examples of mean-field motion

• Description in terms motion in an axially
  deformed mean field accurate for Pu Lr (Z 94 -
  103)
• ?2 nearly constant (within lt 0.02).
• Higher order deformation parameters important
  essential ?2 ?4 ?6 ?8
• Deformed shell gaps reflected in data
• Z 100, N 152

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Isomer ratio 30?
Jeppesen et al., PR C 79, 031303(R) (2009)
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Preliminary Results 256Rf, search for 2qp isomer
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Mystery absence of 2-qp isomer in 256104Rf

• Isomer ratio 3 ? not likely a 2-qp isomer,
  which usually has an isomer fraction 30.
• No clear evidence of K? 8- isomer predicted at
  1 MeV.

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Comments

• In difficult cases (low ?, short ?, high
  e-conversion), confirmation important.
• Which result is right (if either)?
• Neither result can be explained with a model of
  an axially symmetric nucleus.
• Sudden breakdown of model, e.g. of K quantum
  number, at Z104?
• Or a more mundane effect?

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Conclusions

• Data on SHN decisively test theory.
• Many open questions.
• Universal Woods-Saxon
• energies accurate (within 0.3 MeV)
• suggest a shell gap (2.2 MeV) at Z114.
• Self-consistent mean-field theories
• virtue self-consistency,
• but effective interactions must be improved.
• SHN survive to large spin ? spectroscopy.
• Gretina-BGS ? exciting and unique opportunities
  to study SHN at high spin.