Moving along Z=82, beyond the doubly-magic 208Pb nucleus

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Moving along Z82, beyond the doubly-magic 208Pb
nucleus

• G.Benzoni
• INFN sezione di Milano

• Outline
• physics motivations
• reaction mechanism
• experimental details
• data analysis
• Selection of nuclei of interest
• preliminary results and comparison with shell
  model calculations

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Study shell structure along Z82
What is the shell structure of neutron-rich Pb
nuclei?
Evolution of shell structure Measurement of the
E(2), E(4) and B(E2)
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Search for exotic Pb isotopes east of 208Pb
207Bi 208Bi 209Bi 210Bi 211Bi 212Bi 213Bi 214Bi 215Bi
206Pb 207Pb 208Pb 209Pb 210Pb 211Pb 212Pb 213Pb 214Pb
205Tl 206Tl 207Tl 208Tl 209Tl 210Tl
204Hg 205Hg 206Hg


stable
Excited states
Isomeric states
Need to test stability of shell structure N126,
Z82 region Indications of a weakening of Z82
when approaching drip-line Drip-line is far away
and not at all possible to approach
TAG long-living isomeric states
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What can isomers tell us?!
ns? Years

• T1/2(isomer) is long decay is hindered
• EXPERIMENTAL
• mere existence!
• properties E, I(h), T1/2
• decay mode(s)
• intermediate levels E, Ih, lifetimes,...
• THEORY
• single-particle energies
• configurations
• interaction strengths
• deformation
• ...

• FUTURE
• Coulomb excitation of isomers
• interaction cross section gives radii
• fusion-evaporation using isomeric beams

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Search for exotic Pb isotopes east of 208Pb
Up to date information on heavy Pb following
Pfutzner exp. _at_ GSI Populated up to 215Pb but g
infos only on 212Pb Predicted Presence of
isomers involving high-j orbitals ?g9/2, ?i11/2,
?j15/2
GSI

• 5x106 pps
• 2 HPGe detectors (Eff?1)
• 350 ions implanted

Neutron-rich lead isotopes known up to 212Pb
M. Pfutzner PLB444 (1998) 32.
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Beta-decay lifetimes

• Experimental ß-decay data needed around 208Pb to
  validate theoretical models.
• Predictions might differ by orders of magnitude.
• ß-lifetimes needed for r-process calculations.
• Last lifetime measured for 215Pb

I.N. Borzov PRC67, 025802 (2003)
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Experimental approach
238U fragmentation at 1 GeV/u allows to reach
heavy Pb isotopes with reasonable cross section
(212Pb up to 220Pb).
Epax
The GSI UNILAC-SIS accelerator system combined
with the FRS and RISING setup provide a UNIQUE
worldwide facility to populate and study the
neutron-rich lead isotopes.
Abrabla
M. Pfutzner PLB444 (1998) 32.
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Production mechanism relativistic fragmentation
pre-fragment

• production of nuclei ranging from beam species
  to H
• vfragment vbeam
• short flight time? chance to study short living
  nuclei
• Cold fragmentation fragments with N similar to
  the beam, but with several protons stripped

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Experimental details
Experiment performed at the end of September
2009 5 days data taking Beam current 1-2 x 109
pps (238U 1GeV/u) 3 FRS settings 205Pb ID
confirmation 215-217Pb production settings
populated nuclei ranging from 212-218Pb
GSI-FRS
Br-DE-Br method
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Experimental details
charge states of primary beam (A/Q)212Pb
(A/Q)238U
Intermediate focal plane. Mocadi simulation
(cross section not included)
E215Pb 650 MeV/u after S1 deg
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Experimental details
FRS detectors
RISING stopped beam array
SCI42 implantation SCI43 veto
238U 1GeV/u
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RISING Stopped Beam set-up
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12
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The active stopper
Active stopper 3x3 DSSSD 5cm x 5cm Logarithmic
preamplifiers
position implantation-decay correlation
energy (implantation/decay)
Implant GeV scale Beta decay MeV scale
Implantation-gamma correlation for
isomers Implantation trigger
Beta-gamma correlation for implanted
nuclei Beta-decay trigger
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Where is 215Pb ???
Fission fragmentshigh Z products ? ID very
complicated
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Charge state selection

• Formation of many charge states owing to
  interactions with materials
• Isotope identification is complicated
• Need to disentangle nuclei that change their
  charge state after S2 deg.
• (Br)Ta-S2 (Br)S2-S4

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Application of charge state selection
Z
A/Q
Z
Clear ID plot, well resolved
A/Q
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215Pb setting with DQ0, g information for
Z
Z minA MaxA
80 206 210
81 209 213
82 212 217
83 215 219
A/Q
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Isomers in ms range found in all measured even
Pbs, 212-218Pb
Example of Eg vs. Time matrix to identify
long-living isomers
214Pb
Energy (keV)
Projection on Energy axis
PRELIMINARY
185
339
Time (25 ns)
834
Energy (keV)
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Preliminary results Pb chain
T1/2 5.0 (3) µs
212Pb
6 -gt 4 160 keV
4 -gt 2 312 keV
2 -gt 0 805 keV
T1/2 5.9 (1) µs
214Pb
4 -gt 2 341 keV
6 -gt 4 170 keV
2 -gt 0 834 keV
T1/20.40 (1) µs
216Pb
6 -gt 4 160 keV
4 -gt 2 401 keV
2 -gt 0 887 keV
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212Pb comparison btw us and M.Pfutzner...
narrower slits to cut primary beam and heavier
nuclei (increased beam intensity) Higher g
efficieny (10)
Number of isotopes 100
Number of isotopes350
PRELIMINARY
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The experimental levels and the seniority scheme
The 8 isomer is a seniority isomer, involving
neutrons in the 2g9/2
216Pb
214Pb
212Pb
210Pb
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The seniority scheme
Nucleons in a valence jn configuration behave
according to a seniority scheme the states can
be labelled by their seniority ?
For even-even nuclei, the 0 ground state has
seniority ? 0, while the 2, 4, 6, 8
states have ? 2
In a pure seniority scheme, the relative level
energies do not depend on the number of particles
in the shell j
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The valence space in the Kuo-Herling interaction
208Pb is a doubly-magic nucleus (Z82,
N126). For neutron-rich Lead isotopes, the N6
major shell is involved
PRC 43, 602 (1992)
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Shell model calculations with K-H
Calculations with Antoine code and K-H
interaction
218Pb
216Pb
210Pb
212Pb
214Pb
th.
exp
th.
exp.
th.
th.
exp.
th.
exp.
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210Hg isomer
208Hg
PRC 80, 061302(R)
Change in structure ?
Preliminar!
210Hg
642 keV
663 keV
546 keV
Energy (keV)
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• Resume
• recente exp. to study heay Pb isotopes with 238U
  fragmentation reactions
• preliminary results on Pb chain up to 216Pb
• Preliminary comparison with shell model
  calculations confirming 8 isomer is a seniority
  isomers

• Still to do
• study other charge states
• detailed study of isomers in species (Hg, Tl,
  Bi,Po)
• study short/long living isomers with other TDC
  signals
• extract isomeric ratios
• study beta decay
• ......... (suggestions ???)

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Collaboration 70 people and 18 institutions
G. Benzoni, J.J. Valiente-Dobon, A. Gottardo, R.
Nicolini. A. Bracco, F.C.L. Crespi,F. Camera,
A. Corsi, S. Leoni, B. Million, O. Wieland, G.de
Angelis, D.R. Napoli, E. Sahin,S.Lunardi,R.Menegaz
zo, D. Mengoni, F. Recchia, P. Boutachkov, L.
Cortes, C. Domingo-Prado,F. Farinon, H. Geissel,
J. Gerl,N. Goel, M. Gorska, J. Grebosz, E.
Gregor, T.Haberman,I. Kojouharov, N. Kurz, C.
Nociforo, S. Pietri, A. Prochazka, W.Prokopowicz,
H. Schaffner,A. Sharma, H. Weick,
H-J.Wollersheim, A.M. Bruce, A.M. Denis Bacelar,
A. Algora,A. Gadea, M. Pfutzner, Zs. Podolyak,
N. Al-Dahan, N. Alkhomashi, M. Bowry, M. Bunce,A.
Deo, G.F. Farrelly, M.W. Reed, P.H. Regan, T.P.D.
Swan, P.M. Walker, K. Eppinger,S. Klupp, K.
Steger, J. Alcantara Nunez, Y. Ayyad, J.
Benlliure, E. Casarejos,R. Janik,B. Sitar, P.
Strmen, I. Szarka, M. Doncel, S.Mandal, D. Siwal,
F. Naqvi,T. Pissulla,D. Rudolph,R.
Hoischen,P.R.P. Allegro, R.V.Ribas,Zs. Dombradi
Universita degli Studi e INFN sezione di Milano,
Milano, I INFN-LNL, Legnaro (Pd), I
Universita di Padova e INFN sezione di Padova,
Padova, I University of the West of Scotland,
Paisley, UK GSI, Darmstadt, D Univ. Of
Brighton, Brighton, UK IFIC, Valencia, E
University of Warsaw, Warsaw, Pl Universiy of
Surrey, Guildford, UK TU Munich, Munich, D
University of Santiago de Compostela, Santiago
de Compostela, E Comenius University,
Bratislava, Sk Univ. Of Salamanca, Salamanca,
E Univ. of Delhi, Delhi, IND IKP Koeln,
Koeln, D Lund University, Lund, S Univ. Of Sao
Paulo, Sao Paulo, Br ATOMKI, Debrecen, H.
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Main ingredients of isomer analysis
Eg, Tg ? digital electronics for Ge det.