SPECTROSCOPY OF 257RF

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SPECTROSCOPY OF 257RF

• Andreas Heinz
• A. W. Wright Nuclear Structure Laboratory
• Yale University

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Where is the next doubly-magic nucleus?
http//randbereiche.de/unbihexium-310.shtml and
J. Qian
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At the Time the Superheavy Element puzzle
Hot fusion (48Ca actinide targets)
Cold fusion (Pb, Bi targets)
Elements 114, 116 produced in hot fusion with 1
pb.
Usual method of identification does not work.
Ansatz use A/q as an additional parameter.
a decay chains terminated by spontaneous fission
257Rf
257Rf as test case.
Workshop on Nuclear Structure Physics near the
Coulomb Barrier
June 18, 2009
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257Rf
Hessberger et al., Z. Phys. A 359, 415 (1997)
Z 104 No in-beam spectroscopy up to now. N
153 1 neutron out of the N152 subshell
closure. Decays almost entirely by alpha
decay. One known isomer
A complex alpha spectrum
Not all alpha lines are correlated are they all
from 257Rf? ? use A/q of the FMA at Argonne!
1st generation alpha
257Rf from a decay of 265Hs
Correlated 1st generation alpha
Workshop on Nuclear Structure Physics near the
Coulomb Barrier
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June 18, 2009
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Experiment
Setup 50Ti 170Er ? 220Th
Experiment 50Ti 208Pb ? 258Rf

• Compound Nucleus 258Rf (Z 104)
• Beam Energy 233 MeV
• Average beam intensity 115 pnA
• Beam time 115.2 hours

I. Dragojevic et al., PRC 78, 024605 (2008)
Production cross section F.P.
Hessberger s(257Rf) 10(1) nb S. Hofmann
s(257Rf) 16 nb I. Dragojevic s(257Rf)
40 nb
NPA 734 2004 (93-100)
Workshop on Nuclear Structure Physics near the
Coulomb Barrier
June 18, 2009
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Experimental Setup FMA
DSSD
Ge
Ge
A/q dispersion
Ge
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Results Alpha Decay
mass information
? Two masses are clearly separated in the setup
reaction (216,217Th). ? Only one mass for
alpha-correlated ERs ? all alphas from 257Rf.
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Results Spontaneous Fission
random
Mass of a-correlated ERs

• Two fission groups
• From 257Rf
• From 256Rf

Mass of SF-correlated ERs
SF branch ratio of 257Rf Somerville et al., PRC
31, 1801 (1985) 0.14(9)
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Alpha Decay
Similar T1/2
Not observed in 261Sg 257Rf
!
a

• Two alpha groups
• 8.91-9.20 MeV from an isomeric state (75 keV) in
  257Rf
• 8.75-8.91 MeV Purely from the ground state of
  257Rf ?

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Alpha Decay Spectrum
This work
257Rf
a
ICE
Spectra are distorted by summing of alpha and
conversion electron signals!
253No
G. D. Jones, NIM A 488, 471 (2002)
DSSD
a
Recoil
ICE
Workshop on Nuclear Structure Physics near the
Coulomb Barrier
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June 18, 2009
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Alpha Decay Measured with a Catcher
He jet alpha X-ray coincidences ? minimize
electron summing.
Bemis et al., PRL 31, 647 (1973)
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N152
152
Deformed subshell closure at N 152
Neutron
A. Sobiczewski, K. Pomorski, Prog. Part. Nucl.
Phys. 58, 292 (2007)

• The shell gap size can be extracted from the
  energy difference between the 9/2-734 and the
  1/2620 configurations.

R. R. Chasman et al., RMP 49, 833 (1977)
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Configurations
What are the configurations for the ground state
and the 75-keV isomeric state? - A look at N153
isotones
Candidates 1/2620, 3/2622, 7/2613,
11/2-725
Known daughter states 5/2622, 9/2-734
Spin flip rules out 3/2622
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Decay Properties
B. Streicher et al., Acta Physica Polonica 38,
1561 (2007) Ea(8778 keV) ? E?(167 keV) A.
Lopez-Martens et al. Eur. Phys. J. A 32, 245
(2007) 5/2622 ? 9/2-734 167 keV in
253No Two strong transitions in isotones at
N153 (249Cm, 251Cf, 253Fm) 1/2620 ?
5/2622 1/2620 ? 1/2620 From
systematics Ground state is expected to have
1/2620
a hindrance factor
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Configuration of the 75-keV Isomer

• Configuration of the 75-keV isomeric state
• 7/2613 or 11/2-725?
• 11/2-725 is more favored
• Low hindrance factor
• 11/2-725 ? 9/2-734
• is favored to 7/2613 ? 9/2-734
• Intensity ratio
• 11/2-725 ? 9/2-734 11/2-725 ? 11/2-
  (9/2-734)
• Experimental 1 0.73
• Theoretical 1 0.5
• Long half-life
• Experimental (4 s)
• Theoretical f? (T1/2)exp/(T1/2)WU1/?
• assume f?10 100 we get for 7/2613 ?
  T1/2 6-600 ns
• we get for 11/2-725 ? T1/2 3-300 s

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Evolution of the N152 Shell gap
No strong alpha transition 1/2620 ? 1/2620
(in contrast to isotones) due to the high
excitation energy of the 1/2620 state in 253No
(low Q value).
Microsc.-macrosc.
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Deformation Energy in the Microscopic-Macroscopic
Model
The deformation energy is at a maximum near 254No
(Z 102, N152). ? important for stability!
A. Sobiczewski, K. Pomorski, Prog. Part. Nucl.
Phys. 58, 292 (2007)
Workshop on Nuclear Structure Physics near the
Coulomb Barrier
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June 18, 2009
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Deformation Energy Multipole Decomposition
A. Sobiczewski, K. Pomorski, Prog. Part. Nucl.
Phys. 58, 292 (2007)
Workshop on Nuclear Structure Physics near the
Coulomb Barrier
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June 18, 2009
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Deformation Energy Multipole Decomposition
Contribution of ß6 to the deformation energy.
A. Sobiczewski, K. Pomorski, Prog. Part. Nucl.
Phys. 58, 292 (2007)
Workshop on Nuclear Structure Physics near the
Coulomb Barrier
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June 18, 2009
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Size of the N152 Shell Gap
ß6 increases the size of the N152 shell gap!
We expect an increase of the shell gap size up to
nobelium.
A. Sobiczewski, K. Pomorski, Prog. Part. Nucl.
Phys. 58, 292 (2007)
Workshop on Nuclear Structure Physics near the
Coulomb Barrier
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June 18, 2009
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(High-K) Isomers in 257Rf
Isomeric
Electron E vs.lnT
Isomeric
? or ICE
Ground/Isomeric
ER
Electron-alpha correlation
a
Ground
Daughter
? High ICE (sum) energy ? Group A is correlated
with high-E alphas Group B is not. ER-ICE-a-a
correlation found for Group A. ER-ICE-a
correlation found for Group B.
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High-K Isomers in 257Rf

• Macroscopic-Microscopic model
• universal parameterization of the Woods-Saxon
  potential
• Lipkin-Nogami prescription for pairing
• Built a 3-qp high-K isomer on a 8- 2-qp (proton)
  high-K isomer (which has been observed in 254No).

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ICE Group A
High conversion electron energy ( 380
keV) Correlated with high E alphas ? 11/2-725
isomeric state in the decay path
Speculation 3 qp high-K configuration (2 protons
1 neutron)
Lowest 2qp states by calculation 8-9/2624X7/2
-514 1102 keV 5-9/2624X1/2-521 1090
keV
Most likely configuration of the observed isomer
K-forbiddenness f?20 - 38
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Gamma Rays Coincident with ICE Group A
Rf X rays Ka1 133 keV Ka2 126 keV
Expected number of Counts at these
energies Random 7x10-3 X rays 0.2
Gamma coincident with ICE Group A
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ICE Group B

• Reminder
• High ICE energies ( 350 keV)
• Not correlated with high energy alphas ?
  11/2-725 isomeric state not in the decay path.
• No ER-ICE-a-a correlation probability for that
  is 10-3
• Most likely
• 257Rf ? 257Lr (EC)

Experimental partial half-life 37 s Estimated
g.s. ? g.s. partial half-life 360 s ? 90 of
the intensity populates excited states.
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Summary

• SF of 257Rf and 256Rf were observed and clearly
  separated thanks to A/Q provided by the FMA.
• Alpha decays from 257Rf and its EC-daughter 257Lr
  were observed and used to construct a partial
  decay scheme.
• The N152 shell gap increases in magnitude up to
  Z102.
• Two delayed-ICE groups were observed for the
  first time.
• Group A clearly indicates a new isomeric state
  in 257Rf possibly with a high-K 3-qp
  configuration.
• Group B most likely due to EC decay of 257Rf to
  excited states in 257Lr followed by ICE emission.
• J. Qian et al., PRC (2009) in press.

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Acknowledgements

• Wright Nuclear Structure Laboratory, Yale
  University
• J. Qian, Ryan Winkler, R. Casperson, A.B.
  Garnsworthy, M. Bunce, G. Henning, A. Schmidt, R.
  Terry, V. Werner, J. Vinson, E. Williams
• Argonne National Laboratory
• Teng Lek Khoo, Robert V.F. Janssens, D. Peterson,
  D. Seweryniak, I. Ahmad, B.B. Back, M.P.
  Carpenter, J.P. Greene, A.A. Hecht, C.L. Jiang,
  F.G. Kondev, T. Lauritsen, C.J. Lister, A.
  Robinson, R. Scott, R. Vondrasek, X. Wang,
• S. Zhu
• Japan Atomic Energy Agency
• Masato Asai

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The setup reaction 50Ti 170Er 220Th
216,216mTh and 217, 217mTh were used for the
calibration.

• Energy calibration based on the setup reaction
  avoids
• Energy loss in the DSSD dead layer.
• Differences in energy due to emission angle
• Efficiency of DSSD
• eDSSD 44(5)

Fix three parameters for the distribution of
alpha energy peaks (e, s, t ) at 8 MeV
Skewed normal distribution
Nucl. Instr. And Meth. In Phys. Res. A 413 (1998)
357-366
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A/Q Calibration Using 216,217Th
Set-up reaction 50Ti(170Er,xn)220-xTh
Projection on A/Q
Alpha-Energy
A/Q Channels
A/Q Channels
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FMA A/Q resolution 257Rf
257Rf
309.28 Q20
203.87 Q19
FMA setup calculation Distance between two
neighbor masses gt 8 channels. Only one mass
(257) was produced!
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Results of 257Rf

• 142 Events were observed in 5280 minutes
• Estimated beam dose 1.94x1017 particles
• Preliminary estimated cross section 7 nb, which
  is close to GSIs 10(1) nb

Alpha decay spectra

• Complex spectrum
• Extremely low statistics!

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Gamma Rays from 253No and 249Fm
142 full energy 257Rf alpha decay events, 9 among
which have coincident gamma events (20 µs time
window).
49 full energy 253No alpha decay events, 11 among
which have coincident gamma events (20 µs time
window).
Preliminary
Very preliminary decay scheme Only consistency
of gamma energies and alpha energies is
considered.
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Test of theories

• Search for the heaviest nuclei
• Berkeley, GSI, Dubna, RIKEN
• Elements 112, 113, 114, 115, 118
• FRIB in the future
• Spectroscopy of high-Z nuclei benchmarks
  theories!
• 254No studied
• 257Rf this work
• Decay Spectroscopy
• In-beam spectroscopy is difficult because of
  radiation damage of Ge detectors

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Motivation

• K isomer predicted by MM model
• K projection of J on symmetric axis of prolate
  deformed nucleus
• High-J orbital in the region of the heaviest
  nuclei ? High-K states of multi-qp
  configuration
• High-K states can probe single particle
  energies
• Predicted high-K 3qp configuration in 257Rf.
  Does it really exist and can we find it?

K selection rule
K-forbidenness
where
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• 50Ti (208Pb, n) 257Rf
• Observations
• Alpha correlated ERs
• --- One mass 257Rf
• Alpha decays from 257Rf (257Lr)
• --- 257Rf ground state and 75-keV isomeric state
• Spontaneous fission
• --- SF branch ratio of 257Rf
• Internal Conversion electrons
• --- two groups with different features
• Gamma coincident with ICE group A
• Question to ask
• What are the configurations of the ground state
  and the 75-keV isomeric state?
• What are the origins of ICE groups?

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Discussion rotational energy
Deformation alignment ltjgt2-K2 is small
Rotation Alignment ltjgt2-K2 may not be small
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Microscopic Approach
Mean field Spherical Spherical harmonic
oscillator Deformed non-spherical harmonic
oscillator, Woods-Saxon and so on Residual
Interaction Pairing
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Deformed Nuclei and rotation
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Nilsson model for deformed nuclei
Nilsson expression of configuration OpNnz?
P. Moller et al., 1997
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Calibration of A/q using setup reaction 50Ti
(170Er, xn) 220-xTh
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A/Q from Th isotopes to Rf isotopes
Measured distances of peak centers in the setup
reaction gt 8 channels
Deduce mass distribution of Rf
Good mass separation ability (Precise
measurement) 95 confidence of the peak center
center (-6.3, 6.3) (Thin distribution) 95
confidence of the volatility of the peak
distribution (3.85, 6.48) Based on 216Th
charge state 23
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FMA A/Q resolution 257Rf
309.28 Q20
203.87 Q19
FMA setup calculation Distance between two
neighbor masses gt 8 channels. So, only one mass
(257) was produced!
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How does the PGAC work?
Charged particles
MACY design (top view)
beam
window
Cathode
X
Anode
Y
window
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PGAC read-out with delay lines
Delay between two neighboring lines 1 ns Total
vertical delay 93 ns Total
horizontal delay 203 ns
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EC capture and partial half life
Partial half life (total half life)/(branch
ratio)