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

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

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

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  1. 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

  2. 208Pb N/Z~1.0– 1.6 N/Z~3.0 Study shell structure along Z=82 What is the shell structure of neutron-rich Pb nuclei? Evolution of shell structure: Measurement of the E(2+), E(4+) and B(E2)

  3. Search for exotic Pb isotopes “east of 208Pb” 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

  4. What can isomers tell us?! ns Years T1/2(isomer) is “long” decay is hindered EXPERIMENTAL: • mere existence! • properties: E*, I(ħ), T1/2 • decay mode(s) • intermediate levels: E*, Iħ, lifetimes,... THEORY: • single-particle energies • configurations • interaction strengths • deformation • ... • “FUTURE”: • Coulomb excitation of isomers • interaction cross section gives radii • fusion-evaporation using isomeric beams

  5. 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 • 5x106 pps • 2 HPGe detectors (Effγ=1%) • 350 ions implanted Neutron-rich lead isotopes known up to 212Pb M. Pfutzner PLB444 (1998) 32.

  6. 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 215Pb I.N. Borzov PRC67, 025802 (2003)

  7. 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.

  8. Ablation (evaporation) abrasion 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

  9. 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

  10. 91+ 2.5 g/cm2 Be 92+ 238U 90+ 238U 1GeV/u 212Pb Deg. S1: 2.4 g/cm2 S2 position MONOCHROMATIC Deg S2: 758 mg/cm2 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

  11. Active Stopper 9 DSSSD 5cm x 5cm 2TPC [XS4] SCI41[TOF] 2x MUSIC [Z1,Z2] Deg S4: ~2.4 g/cm2 2TPC [XS2] SCI21 [TOF] 15 Cluster (105 HPGe crystals) εg = 9-14% (1.3-0.6 MeV) Flash mult. = 4-5 crystals dead Experimental details: RISING stopped beam array FRS detectors SCI42 implantation SCI43 veto 238U 1GeV/u

  12. RISING Stopped Beam set-up 24 12

  13. start Veto 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

  14. Z Heavy products Pb Fission fragments A/Q Where is 215Pb ??? Fission fragments+high Z products  ID very complicated

  15. 215Pb_sett DQ=-1 DQ=0 DQ=-2 Z DQ=+1 Br1 - Br2 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

  16. DQ=0 Z Br1 - Br2 Pb Application of charge state selection Z A/Q Z Clear ID plot, well resolved A/Q

  17. 206Hg 209Tl 212Pb 215Bi 210Hg 213Tl 216Pb 219Bi 215Pb setting with DQ=0, g information for: Z A/Q

  18. 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)

  19. Preliminary results: Pb chain T1/2 = 5.0 (3) μs 212Pb 6+ -> 4+: 160 keV 4+ -> 2+: 312 keV 2+ -> 0+: 805 keV T1/2 = 5.9 (1) μs 214Pb 4+ -> 2+: 341 keV 6+ -> 4+: 170 keV 2+ -> 0+: 834 keV T1/2=0.40 (1) μs 216Pb 6+ -> 4+: 160 keV 4+ -> 2+: 401 keV 2+ -> 0+: 887 keV

  20. 6+ -> 4+: 160 keV T1/2 = 5.0 (3) μs 4+ -> 2+: 312 keV 2+ -> 0+: 805 keV 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 PRELIMINARY

  21. X 8+ 8+ X The experimental levels and the seniority scheme The 8+ isomer is a seniority isomer, involving neutrons in the 2g9/2 216Pb 212Pb 214Pb 210Pb

  22. S.p. energies (MeV) N=184 Shells -1.40 3d3/2 -1.45 2g7/2 4s1/2 -1.90 3d5/2 -2.37 -2.51 1j15/2 N=7 major shell -3.16 1i11/2 2g9/2 -3.94 N=126 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 PRC 43, 602 (1992)

  23. Shell model calculations with K-H Calculations with Antoine code and K-H interaction 218Pb 216Pb 210Pb 212Pb 214Pb th. exp. th. exp. th. exp th. exp. th.

  24. 210Hg isomer 208Hg PRC 80, 061302(R) Change in structure ? Preliminar! 210Hg 642 keV 663 keV 546 keV Energy (keV)

  25. 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 ???)

  26. 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.

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