Moving along Z=82, beyond the doubly-magic 208 Pb nucleus. G.Benzoni INFN sezione di Milano

Transcription

1 Moving along Z=82, beyond the doubly-magic 208 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 Study shell structure along Z=82 What is the shell structure of neutron-rich nuclei? 208 N/Z~ N/Z~3.0 Evolution of shell structure: Measurement of the E(2 + ), E(4 + ) and B(E2)

3 Search for exotic isotopes east of Bi 208 Bi 209 Bi 210 Bi 211 Bi 212 Bi 213 Bi 214 Bi 215 Bi Tl 206 Tl 207 Tl 208 Tl 209 Tl 210 Tl 204 Hg 205 Hg 206 Hg 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?! T 1/2 (isomer) is long decay is hindered ns Years 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 FUTURE : o Coulomb excitation of isomers o interaction cross section gives radii o fusion-evaporation using isomeric beams o...

5 Search for exotic isotopes east of 208 Up to date information on heavy following Pfutzner GSI: Populated up to 215 but γ infos only on 212 Predicted Presence of isomers involving high-j orbitals νg9/2, νi11/2, νj15/2 GSI Neutron-rich lead isotopes known up to 212 5x10 6 pps 2 HPGe detectors (Eff γ =1%) 350 ions implanted M. Pfutzner PLB444 (1998) 32.

6 Beta-decay lifetimes Experimental β-decay data needed around 208 to validate theoretical models. Predictions might differ by orders of magnitude. β-lifetimes needed for r-process calculations. Last lifetime measured for 215 I.N. Borzov PRC67, (2003)

7 Experimental approach: 238 U fragmentation at 1 GeV/u allows to reach heavy isotopes with reasonable cross section ( 212 up to 220 ). 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. Cross section (mb b) Epax Abrabla M. Pfutzner PLB444 (1998) 32. A

8 Production mechanism: relativistic fragmentation pre-fragment abrasion Ablation (evaporation) o production of nuclei ranging from beam species to H o v fragment v beam o short flight time chance to study short living nuclei ocold fragmentation: fragments with N similar to the beam, but with several protons stripped

9 Experimental details: Experiment performed at the end of September days data taking Beam current 1-2 x 10 9 pps (238U 1GeV/u) 3 FRS settings: 205 : ID confirmation : production settings: populated nuclei ranging from Bρ E-Bρ method

10 Experimental details: 91+ charge states of primary beam (A/Q) 212 == (A/Q) 238 U U 212 Intermediate focal plane. Mocadi simulation (cross section not included) E 215 = 650 MeV/u after S1 deg S2 position 2.5 g/cm 2 Be 238 U 1GeV/u Deg. S1: 2.4 g/cm 2 MONOCHROMATIC Deg S2: 758 mg/cm 2

11 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 238 U 1GeV/u SCI42 implantation SCI43 veto 2TPC [XS2] SCI21 [TOF] 15 Cluster (105 HPGe crystals) εγ = 9-14% ( MeV) Flash mult. = 4-5 crystals dead

12 RISING Stopped Beam set-up 24 12

13 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 Where is 215??? Fission fragments+high Z products ID very complicated Z Heavy products Fission fragments A/Q

15 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. (Bρ) Ta-S2 (Bρ) S2-S4 Z 215_sett Q=-2 Q=-1 Q=+1 Q=0 Βρ1 Βρ2 Bρ =(Bρ ) ( Bρ4 =(Bρ 0 ) 4( 1+ x x2 + D ) M D x 2 )

16 Z Application of charge state selection Q=0 Z Βρ1 Βρ2 A/Q Clear ID plot, well resolved Z A/Q

17 215 setting with Q=0, γ information for: Z 215 Bi 219 Bi Tl 213 Hg 210 Tl Hg Z mina MaxA A/Q

18 Isomers in µs range found in all measured even s, Example of Eγ vs. Time matrix to identify long-living isomers Energy (kev) 214 Projection on Energy axis Time (25 ns) Energy (kev)

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

20 212 comparison btw us and M.Pfutzner... narrower slits to cut primary beam and heavier nuclei (increased beam intensity) Higher γ efficieny (~10%) Number of isotopes:350 Number of isotopes ~ > 4 + : 160 kev T 1/2 = 5.0 (3) µs 4 + -> 2 + : 312 kev 2 + -> 0 + : 805 kev

21 The experimental levels and the seniority scheme The 8 + isomer is a seniority isomer, involving neutrons in the 2g 9/ X X

22 The seniority scheme Nucleons in a valence j n configuration behave according to a seniority scheme: the states can be labelled by their seniority ν For even-even nuclei, the 0 + ground state has seniority ν = 0, while the 2 +, 4 +, 6 +, 8 + states have ν = ν = ν = (2g 9/2 ) (2g 9/2 ) (2g 9/2 ) (2g 9/2 ) 8 In a pure seniority scheme, the relative level energies do not depend on the number of particles in the shell j

23 The valence space in the Kuo-Herling interaction 208 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) N=184 Shells 3d 3/2 2g 7/2 4s 1/2 3d 5/2 1j 15/2 1i 11/2 PRC 43, 602 (1992) N=7 major shell N=126 2g 9/2

24 Shell model calculations with K-H Calculations with Antoine code and K-H interaction th. exp. th. exp. th. exp th. exp. th.

25 210 Hg isomer 208 Hg PRC 80, (R) Change in structure? 210 Hg 642 kev 546 kev 663 kev Energy (kev)

26 Resume: recente exp. to study heay isotopes with 238 U fragmentation reactions preliminary results on chain up to 216 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???)

27 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.

28 Main ingredients of isomer analysis: A q = Bρ 1 βγ Need to measure: ToF X S2 X S4 Scintillators + TPC de dx Z 2 Ionization chambers Eγ, Tγ digital electronics for Ge det.