Moving along Z=82, 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
Study shell structure along Z=82
What is the shell structure of neutron-rich Pb nuclei?
208Pb
N/Z~1.0– 1.6 N/Z~3.0
Evolution of shell structure: Measurement of the E(2+), E(4+) and B(E2)
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 N=126, Z=82 region
Indications of a weakening of Z=82 when approaching drip-line
Drip-line is far away and not at all possible to approach
TAG: long-living isomeric states
What can isomers tell us?!
ns Years
T1/2(isomer) is “long” decay is hindered
EXPERIMENTAL:
o mere existence!
o properties: E*, I(ħ), T1/2
o decay mode(s)
o intermediate levels: E*, Iħ, lifetimes,...
THEORY:
o single-particle energies
o configurations
o interaction strengths
o deformation
o ...
“FUTURE”:
o Coulomb excitation of isomers
o interaction cross section gives radii
o fusion-evaporation using isomeric beams
Search for exotic Pb isotopes “east of
208Pb”
Up to date information on heavy Pb following Pfutzner exp. @ GSI:
Populated up to 215Pb but g infos only on 212Pb
Predicted Presence of isomers involving high-j orbitals νg9/2, νi11/2, νj15/2
GSI
Neutron-rich lead
isotopes known up to
• 5x106 pps
• 2 HPGe detectors (Effγ=1%)
• 350 ions implanted
212Pb
M. Pfutzner PLB444 (1998) 32.
Beta-decay lifetimes
•Experimental β-decay data
needed around 208Pb to validate
theoretical models.
•Predictions might differ by
orders of magnitude.
•β-lifetimes needed for r-process
calculations.
•Last lifetime measured for
I.N. Borzov PRC67, 025802 (2003)
215Pb
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.
Production mechanism: relativistic fragmentation
pre-fragment
abrasion
Ablation
(evaporation)
o production of nuclei ranging from beam species to H
o vfragment ≈ vbeam
o short flight time chance to study short living nuclei
oCold fragmentation: fragments with N similar to the beam, but
with several protons stripped
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
Br-DE-Br method
212-218Pb
Experimental details:
91+
charge states of primary beam
(A/Q)212Pb == (A/Q)238U
Intermediate focal plane.
Mocadi simulation
(cross section not included)
92+
238U
90+
E215Pb= 650 MeV/u after S1 deg
212Pb
S2 position
2.5 g/cm2 Be
238U
1GeV/u
Deg. S1: 2.4 g/cm2
MONOCHROMATIC
Deg S2: 758 mg/cm2
Experimental details:
Active Stopper
9 DSSSD 5cm x 5cm
RISING
FRS detectors
stopped beam array
2TPC [XS4]
SCI41[TOF]
2x MUSIC [Z1,Z2]
Deg S4: ~2.4 g/cm2
SCI42 implantation
SCI43 veto
238U
1GeV/u
2TPC [XS2]
SCI21 [TOF]
15 Cluster (105 HPGe crystals)
εg = 9-14% (1.3-0.6 MeV)
Flash mult. = 4-5 crystals dead
RISING Stopped Beam set-up
24
12
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:
Beta-gamma correlation
for implanted nuclei:
Implantation trigger
Beta-decay trigger
Where is
215Pb
???
Fission fragments+high Z products  ID very complicated
Z
Heavy
products
Pb
Fission
fragments
A/Q
Charge state selection
215Pb_sett
DQ=-1
DQ=0
DQ=-2
Z
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.
DQ=+1
(Br)Ta-S2 – (Br)S2-S4
Br1 - Br2
x2
Bρ2 = (Bρ0 )2 ( 1+
)
D2
x4 - M  x2
Bρ4 = (Bρ0 )4 ( 1+
)
D4
Application of charge
state selection
Z
Z
DQ=0
Br1 - Br2
A/Q
Z
Pb
Clear ID plot, well resolved
A/Q
215Pb
setting with DQ=0, g information for:
Z
215Bi
212Pb
209Tl
206Hg
Z
minA
MaxA
80
206
210
81
209
213
82
212
217
83
215
219
219Bi
216Pb
213Tl
210Hg
A/Q
Isomers in ms range found in all measured even Pbs,
212-218Pb
Energy (keV)
Example of Eg vs. Time matrix to identify long-living isomers
214Pb
Time (25 ns)
185
Projection on Energy axis
339
834
Energy (keV)
Preliminary results: Pb chain
212Pb
6+ -> 4+: 160 keV
4+ -> 2+: 312 keV
6+
4+ -> 2+:
341 keV
4+:
->
170 keV
6+ -> 4+:
160 keV
4+ -> 2+:
401 keV
T1/2 = 5.0 (3) μs
2+ -> 0+: 805 keV
214Pb
T1/2 = 5.9 (1) μs
2+ -> 0+:
834 keV
216Pb
2+ -> 0+:
887 keV
T1/2=0.40 (1) μs
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 isotopes:350
6+ -> 4+:
160 keV
4+ -> 2+:
312 keV
T1/2 = 5.0 (3) μs
2+ -> 0+:
805 keV
The experimental levels and the seniority scheme
The 8+ isomer is a seniority isomer, involving neutrons in
the 2g9/2
210Pb
212Pb
216Pb
214Pb
8+
X
8+
X
The valence space in the Kuo-Herling interaction
208Pb
is a doubly-magic nucleus (Z=82, N=126).
For neutron-rich Lead isotopes, the N=6 major
shell is involved
S.p. energies
(MeV)
-1.40
-1.45
-1.90
-2.37
-2.51
PRC 43, 602 (1992)
N=184
-3.16
-3.94
N=126
Shells
3d3/2
2g7/2
4s1/2
3d5/2
1j15/2 N=7 major shell
1i11/2
2g9/2
Shell model calculations with K-H
Calculations with Antoine code and K-H interaction
218Pb
210Pb
th.
exp.
212Pb
th.
exp.
216Pb
214Pb
th.
exp
th.
exp.
th.
210Hg
isomer
208Hg
PRC 80, 061302(R)
Change in structure ?
210Hg
642 keV
546 keV
Energy (keV)
663 keV
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 ???)
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.Menegazzo, 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. Pf¨utzner, 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.