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

G.BenzoniINFN 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

N/Z~1.0– 1.6

N/Z~3.0

What is the shell structure of neutron-rich Pb nuclei?

Evolution of shell structure: Measurement of the E(2+), E(4+) and B(E2)

208Pb

Study shell structure along Z=82

207Bi 208Bi 209Bi 210Bi 211Bi 212Bi 213Bi 214Bi 215Bi

206Pb 207Pb 208208PbPb 209Pb 210Pb 211Pb 212Pb 213Pb 214Pb

205Tl 206Tl 207Tl 208Tl 209Tl 210Tl

204Hg 205Hg 206Hg

stable

Excited states

Isomeric states

Search for exotic Pb isotopes “east of 208Pb”

Need to test stability of shell structure N=126, Z=82 regionIndications of a weakening of Z=82 when approaching drip-lineDrip-line is far away and not at all possible to approachTAG: long-living isomeric states

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 ...

What can isomers tell us?!

“FUTURE”:o Coulomb excitation of isomers

o interaction cross section gives radiio fusion-evaporation using isomeric beams

ns Years

Search for exotic Pb isotopes “east of 208Pb”

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

• 5x106 pps• 2 HPGe detectors (Effγ=1%)• 350 ions implanted

GSI

Up to date information on heavy Pb following Pfutzner exp. @ GSI:Populated up to 215Pb but infos only on 212Pb

Predicted Presence of isomers involving high-j orbitals νg9/2, νi11/2, νj15/2

•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 215Pb

Beta-decay lifetimes

I.N. Borzov PRC67, 025802 (2003)

238U fragmentation at 1 GeV/u allows to reach heavy Pb isotopes with reasonable

cross section (212Pb up to 220Pb).

M. Pfutzner PLB444 (1998) 32.

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.

Experimental approach:

Epax

Abrabla

Production mechanism: relativistic fragmentation

o production of nuclei ranging from beam species to Ho vfragment ≈ vbeam

o short flight time chance to study short living nucleioCold fragmentation: fragments with N similar to the beam, but with several protons stripped

abrasion Ablation (evaporation

)

pre-fragment

Experimental details:

Experiment performed at the end of September 20095 days data takingBeam current 1-2 x 109 pps (238U 1GeV/u)3 FRS settings: 205Pb : ID confirmation215-217Pb: “production” settings:

populated nuclei ranging from 212-218Pb

BE-B method

Experimental details:

2.5 g/cm2 Be

Deg. S1: 2.4 g/cm2

MONOCHROMATICDeg S2: 758 mg/cm2

238U1GeV/u

238U

212Pb

S2 position

92+

91+

90+

Intermediate focal plane.Mocadi simulation (cross section not included)

charge states of primary beam

(A/Q)212Pb == (A/Q)238U

E215Pb= 650 MeV/u after S1 deg

Experimental details:

2TPC [XS2]SCI21 [TOF]

2TPC [XS4]SCI41[TOF]2x MUSIC [Z1,Z2]

15 Cluster (105 HPGe crystals) ε = 9-14% (1.3-0.6 MeV) Flash mult. = 4-5 crystals dead

Active Stopper 9 DSSSD 5cm x

5cm

238U1GeV/u

FRS detectorsRISING stopped beam array

SCI42 implantationSCI43 veto

Deg S4: ~2.4 g/cm2

24 12

RISING Stopped Beam set-up

The active stopper

– position

– implantation-decay correlation

– energy (implantation/decay)

Active stopper: 3x3 DSSSD5cm x 5cm Logarithmic preamplifiers

Vetostart

Implantation-gamma correlation for isomers:

Implantation trigger

Beta-gamma correlation for implanted nuclei:

Beta-decay trigger

Implant GeV scaleBeta decay MeV scale

Fission fragments+high Z products ID very complicated

Where is 215Pb ???

Fission fragments

Heavy products

Pb

A/Q

Z

215Pb_sett

Z Q=+1

Q=-1

Q=-2

Q=0

Formation of many charge states owing to interactions with materialsIsotope identification is complicated Need to disentangle nuclei that change their charge state after S2 deg.

(B)Ta-S2 – (B)S2-S4

)D

x+()(Bρ=Bρ

2

2202 1

)D

xMx+()(Bρ=Bρ

4

24404 1

Charge state selection

Pb

Clear ID plot, well resolved

Application of charge state selection

Z

Q=0

A/Q

A/Q

Z

Z

215Pb setting with Q=0, information for:

206Hg 210Hg209Tl 213Tl

212Pb 216Pb215Bi 219Bi

Z minA MaxA

80 206 210

81 209 213

82 212 217

83 215 219A/Q

Z

Isomers in s range found in all measured even Pbs, 212-218Pb

Time (25 ns)

En

erg

y (

keV

)

214Pb

Example of E vs. Time matrix to identify long-living isomers

Energy (keV)

Projection on Energy axis

185 339

834

PRELIMIN

AR

Y

Preliminary results: Pb chain

214Pb

212Pb

216Pb

6+ -> 4+: 160 keV

4+ -> 2+: 312 keV 2+ -> 0+: 805 keV

T1/2 = 5.0 (3) μs

6+ -> 4+: 170 keV

4+ -> 2+: 341 keV

2+ -> 0+: 834 keV

T1/2 = 5.9 (1) μs

6+ -> 4+: 160 keV 2+ ->

0+: 887 keV

4+ -> 2+: 401 keV

T1/2=0.40 (1) μs

6+ -> 4+: 160 keV

4+ -> 2+: 312 keV 2+ -> 0+:

805 keV

T1/2 = 5.0 (3) μs

212Pb comparison btw us and M.Pfutzner...

narrower slits to cut primary beam and heavier nuclei (increased beam intensity)Higher efficieny (~10%)

Number of isotopes:350

Number of isotopes ~ 100

PRELIMIN

AR

Y

8+ X8+ X

The experimental levels and the seniority scheme

The 8+ isomer is a seniority isomer, involving neutrons in the 2g9/2

210Pb 214Pb212Pb 216Pb

The valence space in the Kuo-Herling interaction

PRC 43, 602 (1992)

208Pb is a doubly-magic nucleus (Z=82, N=126).

For neutron-rich Lead isotopes, the N=6 major shell is involved

2g9/2

1i11/2

1j15/2

3d5/2

3d3/2

4s1/2

2g7/2

-3.94

-2.37-2.51

-3.16

-1.45-1.90

-1.40

S.p. energies (MeV) Shells

N=126

N=184

N=7 major shell

Shell model calculations with K-H

th. expth. exp.

Calculations with Antoine code and K-H interaction

th. th.exp.

210Pb

th.exp.

212Pb216Pb

214Pb

218Pb

210Hg isomer

PRC 80, 061302(R)

208Hg

210Hg 642 keV

663 keV546 keV

Change in structure ?Change in structure ?

Prel

imin

ar!

Energy (keV)

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 ???)

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

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.