Michael Dworschak, GSI

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First direct Penning trap mass measurements on
transuranium elements with SHIPTRAP

• Michael Dworschak, GSI
• for the SHIPTRAP collaboration

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Outline

• Introduction
• SHIPTRAP setup
• Direct mass measurements for Z gt 100
• Conclusions

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Regions of interest at SHIPTRAP
82
Heavy Elements
126
proton emitters
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NZ, rp-process
- Strong repulsive force due to large number of
protons - Stability of SHE only due to shell
effects - For nobelium T(SF) gt T(alpha)
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20
50
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8
20
8
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Spontaneous Fission

? disrupting force
? backdriving force
E / MeV

? scission point
a
2
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Shell corrections in the region of heavy elements
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Deformation in the region of heavy elements
P.Möller et al. At. Data and Nucl. Data Tab. 59,
185 (1995)
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The Recoil Separator SHIP

velocity filter
5 MeV/u
0.1-1 MeV/u
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The SHIPTRAP set-up
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Principle of Penning Traps
Cyclotron frequency
q/m

• PENNING trap
• Strong homogeneous
• magnetic field
• Weak electric 3D
• quadrupole field

end cap
ring electrode
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Ion Motion in a Penning Trap

• Motion of an ion is the superposition of three
  characteristic
• harmonic motions
• axial motion (frequency fz)
• magnetron motion (frequency f)
• modified cyclotron motion (frequency f)
• In an ideal Penning trap the frequencies of the
  radial motions obey the relation

Typical frequencies q e, m 100 u, B 7
T ? f- 1 kHz f 1 MHz
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TOF Resonance Mass Spectrometry
Time-of-flight resonance technique
Scan of excitation frequency
Injection of ions into the trap and excitation of
radius r-
Excitation near fc ? coupling of radial motions,
conv.
1 m
Ejection along the magnetic field lines ? radial
energy converted to axial energy
Time-of-flight (TOF) measurement
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TOF Resonance Mass Spectrometry
Time-of-flight resonance technique
1 m
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TOF Resonance Mass Spectrometry
Time-of-flight resonance technique
1 m
Resolving power
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Mass determination
Relation between cyclotron frequency and mass due
to
Fluctuations of magnetic field due to temperature
and pressure changes -gt Calibration needed
Determine atomic mass from frequency ratio with
a well-known reference mass
Normally 85Rb or 133Cs are chosen as reference
masses
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SHIPTRAP Performance
A 147
Mass resolving power of m/dm 100,000 in
purification trap ? separation of isobars
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Direct Mass Measurements above Z 100
Requirements

• energy matching of reaction products to trap's
  energy scale
• high efficiency to deal with very low production
  rates
• 1 atom/s _at_ Z102 (s ? mb)
• 1 atom/week _at_ Z112 (s ? pb)
• high cleanliness for low background
• stable and reliable operation over extended time

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Production of nobelium isotopes
Fusion-evaporation
4.5MeV/u about 1013 particles / s
200 keV/u about 1 particle / s
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Production of nobelium isotopes
Fusion probability increasing with beam energy
Survival probability of compound nucleus
decreasing with beam energy
208Pb(48Ca,1-3n)253-255No
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Direct Mass Measurements of 252-254No
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Direct Mass Measurements of 252-254No

• August 2008
• 206-208Pb(48Ca,2n)252-254No
• doubly-charged nobelium ions extracted
• low production rates
• -gt about 4 h for each resonance
• 133Cs used as reference mass (same q/m ratio)

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Direct Mass Measurements of 252-254No
Results in agreement with previous AME
values ME uncertainties in the order of 10-30
keV
First direct mass measurements in the region Z gt
100
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Principle of mass determination with a decay
energies
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Definitions in the Atomic Mass Evaluation
Primary data determined by at least two
independent measurements
Secondary data determined by only one measurement
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Combining the results from a decays and Penning
trap

• Difficulties
• decays not between ground states
• "broken" a-chains
• energy summing with conversion electrons

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Combining the results from a decays and Penning
trap

• Difficulties
• decays not between ground states
• "broken" a-chains
• energy summing with conversion electrons

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Combining the results from a decays and Penning
trap
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Link to island of stability

• Combine new, directly measured masses and
  a-decay spectroscopy
• Determine the masses of short-lived higher-Z
  nuclides

To be determined a-decay of 262Sg (15)
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Direct Mass Measurements of 255Lr
Extend direct mass measurements to higher Z
255Lr

• April 2009
• 209Bi(48Ca,2n)255Lr
• rate of incoming particles for 255Lr only 0.3
  ions/s
• singly and doubly-charged ions extracted

255Lr nuclide with lowest rate ever measured in a
Penning trap
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The Route to SHE

• increase sensitivity and efficiency
• (non-destructive) detection system with
  single-ion sensitivity
• new cryogenic gas cell
• improve primary beam
• access to more neutron-rich nuclides
• hot-fusion reactions with actinide targets
• connection to gas-filled separator TASCA

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Summary and Outlook

• First direct mass measurements of nobelium
  isotopes have been performed with SHIPTRAP
• Using results from direct mass measurements more
  primary nuclides could be obtained
• Nobelium isotopes linked to superheavy elements
  by a-decay chains
• Next step go to higher-Z nuclides
• In the long-term future Penning traps can
  contribute to identify long-lived SHE

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SHIPTRAP Collaborators
M. Dworschak, D. Ackermann, K. Blaum, M. Block,
C. Droese, S. Eliseev, E. Haettner, F. Herfurth,
F. P. Heßberger, S. Hofmann, J. Ketter, J.
Ketelaer, H.-J. Kluge, G. Marx, M. Mazzocco, Yu.
Novikov, W. R. Plaß, A. Popeko, D. Rodríguez, C.
Scheidenberger, L. Schweikhard, P. Thirolf, G.
Vorobjev, C. Weber
Thank you for your attention !
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Penning trap basics
Relevant for mass measurements
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Cyclotron frequency measurement
off res.
on res.
m r w
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Super Heavy Elements
GSI elements Z 107-112
112
Rg
Ds
Mt
Hs
Bh

• how heavy can the elements be?
• location of the island of stability?
• structure of SHE?
• stability due to shell effects
• ? accurate binding energies needed

252-254No
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TOF Cyclotron Resonance Curve
TOF as a function of the excitation frequency
off res.
on res.
Resolving power
Determine atomic mass from frequency ratio with
a well-known reference mass.