NN2012, San Antonio, May 27 - June 1, 2012 High-seniority states in spherical nuclei: Triple pair breaking in tin isotopes Alain Astier, CSNSM Orsay, France Introduction Experimental conditions Fusion-fission reactions Detection (fragments and γ ’s) & Data analysis Sn results and discussion New high-spin states identified New isomeric states revealed → B(E2)’s Shell-Model calculations Up to 3 neutron pairs broken in the ν h 11/2 orbit Conclusion & Perspectives Outline:

J.A. Pinston and G. Genevey, J. Phys. G: Nucl. Part. Phys. 30 (2004) R57-R82 Introduction: Nuclear structure of the A<132 Sn isotopes at high spin d 5/2 g 7/ h 11/ 2 d 3/2 s 1/2 64 High-spin structure of the n-rich Sn isotopes: - Very low number of involved configurations - Spherical configurations (Z=50) - Ideal for testing the two-body matrix elements of the residual interactions Higher spins in Sn isotopes expected by breaking more neutron pairs Several experimental challenges: - Production of these (moderately)-rich Sn isotopes at high-spin - Detection of the de-excitation of the high-spin states through long-lived isomers [(h 11/2 ) -3 (d 3/2 ) -1 ] recently reported in 128 Sn [ S. Pietri et al., PRC 83, (2011) ] Isomers in the A<130 Sn isotopes: [(h 11/2 ) 2 ] and 7 - [(h 11/2 ) 1 (d 3/2 ) 1 ] in the even isotopes - 23/2 + [(h 11/2 ) 2 (d 3/2 ) 1 ] and 27/2 - [(h 11/2 ) 3 ] in the odd isotopes

Experimental conditions 18 O (85 MeV) Pb → 226 Th* → FF1 * + FF2 * ~150 nuclei produced Symmetric fission around Z=45 Energy of 85 MeV for 18 O beam chosen in order to induce a “cold” fission σ f ~ 100 mb, I beam ~ 5 pnA,  ~ 10 4 fissions/s in the target Thick Pb target (100 mg/cm 2 )  low-lying γ -rays emitted by stopped nuclei ~ 4x10 9 events with fold ≥ 3 High spin states of neutron-rich isotopes are populated by fusion-fission: EUROBALL 239 Ge (+ BGO shield)  ph ~ 8.5%

78 Ni 132 Sn 100 Sn Fission fragments 18 O Pb 12 C U → 50 Sn + 40 Zr → 50 Sn + 48 Cd

Experimental tools 1) γ-coincidences: unambiguous individual identification and assignment thanks to coincidences between complementary fragments Gates: 2 γ’s of 104 Mo Ex : reaction 18 O Pb → 226 Th* : 115, 116, 117, 118, 119 Cd partners of 104 Mo Counts Energy (keV)

Experimental tools 2) Angular correlations E1E1 E2E2 I0I0 I1I1 I2I2 E1E1 E2E2 Ge 1 Ge 2  Multipolarities of (E1,E2) transitions determined from W(  ) 3) Time measurements T 1/2 E1E1 E2E2 I0I0 I1I1 I2I2 T 1/2 can be extracted from the time spectrum Tac(E 2 )-Tac(E 1 ) Useful range for isomers: 10 ns < T 1/2 < 300 ns

High-spin states in Sn isotopes 118 Cd – 1190 keV 118 Cd – 118 Cd (Sn partners in the 12 C+ 238 U data set) → Lots of unknown (and ~prompt!) transitions, belonging to the Sn complementary fragments → New cascades established in several Sn isotopes Shown here: in 120 Sn

High-spin states in Sn isotopes Confirmation from the coincidence with the Zr isotopes in the ( 18 O+ 208 Pb) data set 124 Sn → 126 Sn → 124 Sn → 123 Sn → 122 Sn → 121 Sn → 120 Sn

High-spin states in Sn isotopes Angular correlations (O+Pb reaction) Q - D Q - Q Q - D Q - Q

New level schemes (odd Sn’s)

New level schemes (even Sn’s)

Euroball-SAPhIR experiment (thin target) SAPhIR: 32 photovoltaic cells,  ff ~ 45% inside the Euroball target chamber Fission fragments stopped in SAPhIR: isomeric decay at rest (no Doppler!) Useful range for lifetime measurements: 50 ns < T 1/2 < 1 μs Study of the 12 C+ 238 U reaction, using a thin (0.14 mg/cm 2 ) 12 C target

SAPhIR events Detection of delayed γ rays by Euroball in the range [50 ns - 1 μs] Gate 241 keV Gate 253 keV Gate 264 keV → new isomer (15 - ) in 120 Sn and 122 Sn! ( 120 Sn) ( 122 Sn) ( 120 Sn+ 122 Sn) delayed γ-γ matrix from delayed γ-γ-t events

Odd-A Sn isotopesEven-A Sn isotopes Summary of the new information obtained in Sn isotopes at high spin How can we interpret these states?

Theoretical knowledge: the π( h 11/2 ) n case evenodd Unfortunately no experimental data available for states with seniority ν =4,5,6… For n=6 (i.e. π h 11/2 midoccupation): 16 + → is forbidden (same seniority) 16 + → is allowed!  T 1/2 (16 +, n=6) < T 1/2 (16 +, n=4) Residual interactions taken from the experimental spectrum of 148 Dy (n=2). Seniority is conserved. From R.D. Lawson, Z. Phys. A 303, 51 (1981)

Shell-model results ANTOINE code, see K. Sieja et al., PRC 79, (2009) for calculation details ( 6 holes in 132 Sn )( 7 holes in 132 Sn ) h2h2 h4h4 h6h6 h3h3 h5h5 h2d1h2d1 h4d1h4d1 h6d1h6d1 h1d1h1d1 h3d1h3d1 h5d1h5d1 h 4 d 2 + h 4 d 1 s 1

Odd-A Sn isotopesEven-A Sn isotopes Experimental ν( h 11/2 ) n states with n=1,2,3,4,5,6 in the A<132 Sn isotopes h2h2 h4h4 h6 ?h6 ? h3h3 h5h5 h2d1h2d1 h4d1h4d1 h6d1h6d1 h1d1h1d1 h3d1h3d1 h5d1h5d1 The 18 + state (from h 6 ) is not yrast: h 4 d 2 or h 4 d 1 s 1 are “cheaper”! → 18 + (h 6 ) not observed Blocking of the odd neutron in d 3/2 → Seniority-6 state (h 6 ) observed!

B(E2)’s and ν h 11/2 filling 10 + : ( h 11/2 ) 2 27/2 - : ( h 11/2 ) : ( h 11/2 ) 3 ( d 3/2 ) 1 Same behaviors of the 3 series Half-filling at A=123 (N=73)  Confirmation of the 15 - configuration

Conclusions & Perspectives Fusion-fission  an efficient γ-spectrometer: a powerful tool to produce and study high-spin states of neutron-rich isotopes Soon: high spins of Te isotopes same neutrons, “only” 2 protons more…  Tins are all the same!  Shell-Model works very well  Major improvement of 8 Sn level schemes  Seniority-6 states identified for the first time Perspectives End 2012: campaigns thermal neutrons on 235 U/ 241 Pu thick targets The SAPhIR-Euroball coupling, with the thin-target reaction, was very useful Reference: “High-spin states with seniority ν =4,5, and 6 in Sn” A.A. et al., Phys. Rev. C 85, (2012) (20 th paper reporting results of the O+Pb experiment!!!)

Collaboration A. Astier, M.-G. Porquet, I. Deloncle, CSNSM, IN2P3-CNRS/Université Paris-Sud, Orsay, France Ch. Theisen, M. Houry, R. Lucas CEA Saclay, IRFU/SPhN, Gif-sur-Yvette Cedex, France D. Verney IPNO, IN2P3-CNRS/Université Paris-Sud, Orsay, France F. Azaiez, D. Curien, O. Dorvaux, G. Duchêne, B.J.P. Gall, M. Rousseau IPHC, IN2P3-CNRS/Université Louis Pasteur, Strasbourg Cedex 2, France G. Barreau CENBG, IN2P3-CNRS/Université Bordeaux I, Gradignan, France N. Redon, O. Stézowski IPNL, IN2P3-CNRS/Université Claude Bernard, Villeurbanne Cedex, France

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