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

IntroductionExperimental conditions

Fusion-fission reactionsDetection (fragments and γ’s) & Data analysis

Sn results and discussionNew high-spin states identifiedNew isomeric states revealed → B(E2)’sShell-Model calculationsUp to 3 neutron pairs broken in the νh11/2 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<132Sn isotopes at high spin

d5/2

g7/2

82

50

h11/

2d3/2s1/2 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 pairsSeveral 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

- 15- [(h11/2)-3 (d3/2)-1] recently reported in 128Sn [S. Pietri et al., PRC83, 044328 (2011)]

Isomers in the A<130Sn isotopes: - 10+ [(h11/2)2] and 7- [(h11/2)1 (d3/2)1] in the even isotopes

- 23/2+ [(h11/2)2 (d3/2)1] and 27/2- [(h11/2)3] in the odd isotopes

Experimental conditions

18O (85 MeV) + 208Pb → 226Th* → FF1* + FF2* + <4-6n>

8

82

90

~150 nuclei producedSymmetric fission around Z=45

Energy of 85 MeV for 18O beam chosen in order to induce a “cold” fissionσf ~ 100 mb, Ibeam ~ 5 pnA, ~ 104 fissions/s in the target

Thick Pb target (100 mg/cm2) low-lying γ-rays emitted by stopped nuclei ~ 4x109 events with fold ≥ 3

High spin states of neutron-rich isotopes are populated by fusion-fission:

EUROBALL239 Ge (+BGO shield)

ph ~ 8.5%

78Ni

132Sn100Sn

Fission fragments

18O + 208Pb

12C + 238U

→ 50Sn +40Zr

→ 50Sn +48Cd

Experimental tools

1) γ-coincidences: unambiguous individual identification and assignment thanks to coincidences between complementary fragments

Gates: 2 γ’s of 104Mo

Ex : reaction 18O + 208Pb → 226Th* : 115,116,117,118,119Cd partners of 104Mo

8

82

90 48

42

Cou

nts

Energy (keV)

Experimental tools

2) Angular correlations

E1

E2

I0

I1

I2

E1

E2

Ge1

Ge2

Multipolarities of (E1,E2) transitions determined from W()

3) Time measurements

T1/2

E1

E2

I0

I1

I2

T1/2 can be extracted from the time spectrum

Tac(E2)-Tac(E1)

Useful range for isomers: 10 ns < T1/2 < 300 ns

High-spin states in Sn isotopes

118Cd – 1190 keV

118Cd – 118Cd(Sn partners in the12C+238U data set)

1190 - 241

→ Lots of unknown (and ~prompt!) transitions, belonging to the Sn complementary fragments

→ New cascades established in several Sn isotopes

Shown here:1190-557-241-782-639 in 120Sn

High-spin states in Sn isotopes

Confirmation from the coincidence with the Zr isotopes in the (18O+208Pb) data set

124Sn

→ 126Sn

→ 124Sn

→ 123Sn

→ 122Sn

→ 121Sn

→ 120Sn

High-spin states in Sn isotopes

Angular correlations(O+Pb reaction)

Q - DQ - Q

Q - QQ - DQ - QQ - QQ - QQ - 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 < T1/2 < 1 μs

Study of the 12C+238U reaction, using a thin (0.14 mg/cm2) 12C 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 120Sn and 122Sn!

(120Sn)

(122Sn)

(120Sn+122Sn)

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(h11/2)n case

even odd

Unfortunately no experimental data available for states with seniority ν=4,5,6…

For n=6 (i.e. πh11/2 midoccupation):

16+ → 14+1 is forbidden (same seniority)

16+ → 14+2 is allowed!

T1/2 (16+, n=6) < T1/2 (16+, n=4)

Residual interactions taken from the experimental spectrum of 148Dy (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., PRC79, 064310 (2009) for calculation details

(6 holes in 132Sn) (7 holes in 132Sn )

h2

h4

h6

h3

h5

h2d1

h4d1

h6d1

h1d1

h3d1

h5d1

h4d2

+ h4d1s1

Odd-A Sn isotopesEven-A Sn isotopes

Experimental ν(h11/2)n states with n=1,2,3,4,5,6 in the A<132Sn isotopes

h2

h4

h6 ?

h3

h5

h2d1

h4d1

h6d1

h1d1

h3d1

h5d1

The 18+ state (from h6) is not yrast:h4d2 or h4d1s1 are “cheaper”!→ 18+ (h6) not observed

Blocking of the odd neutron in d3/2

→ Seniority-6 state (h6) observed!

B(E2)’s and νh11/2 filling

10+ : (h11/2)2

27/2-: (h11/2)3

15-: (h11/2)3 (d3/2)1

Same behaviors of the 3 seriesHalf-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: EXOGAM@ILL campaigns thermal neutrons on 235U/241Pu 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 119-126Sn” A.A. et al., Phys. Rev. C85, 054316 (2012)(20th paper reporting results of the O+Pb experiment!!!)

Collaboration

A. Astier, M.-G. Porquet , I. Deloncle, CSNSM, IN2P3-CNRS/Université Paris-Sud, 91405 Orsay, France

Ch. Theisen, M. Houry, R. LucasCEA Saclay, IRFU/SPhN, 91191 Gif-sur-Yvette Cedex, France

D. VerneyIPNO, IN2P3-CNRS/Université Paris-Sud, 91406 Orsay, France

F. Azaiez, D. Curien, O. Dorvaux, G. Duchêne, B.J.P. Gall, M. RousseauIPHC, IN2P3-CNRS/Université Louis Pasteur, 67037 Strasbourg Cedex 2, France

G. Barreau CENBG, IN2P3-CNRS/Université Bordeaux I, 33175 Gradignan, France

N. Redon, O. Stézowski IPNL, IN2P3-CNRS/Université Claude Bernard, 69622 Villeurbanne Cedex, France

Thank you for your attention!