High-seniority states in spherical nuclei: Triple pair breaking in tin isotopes
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Transcript of High-seniority states in spherical nuclei: Triple pair breaking in tin isotopes
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!