Search for Fingerprints of Tetrahedral Symmetry

Transcription

1 Search for Fingerprints of Tetrahedral Symmetry in the Rare Earth and Actinide Regions D. Curien IPHC-DRS Strasbourg Part of the Tetranuc Project: open collaboration of more than 20 institutions both European and a non European Main collaborators for this work: J.Dudek,, K. Mazurek, F.Haas,, Q.D. Tuyen,, O. Stezowski, L. Riedinger, D. Hartley, R. Bark

2 Physics Motivation: bottom lines See J. Dudek talk on Tuesday Existence of high-rank symmetries predicted by the nuclear mean-field theory Here we consider two point group symmetries: Tetrahedral (pyramid like shape) 24 symmetry elements Octahedral (diamond like shape) 48 symmetry elements In quantum description the Hamiltonian has 48 and 96 symmetry elements almost the highest possible numbers (Triaxial nuclei = only 4 symmetry elements) D. Curien LEA-Catane October

3 Consequence: huge gaps In the Rare Earth region Observe big gaps at Z=56-58, 58, and N=90-94, 94, 112 D. Curien LEA-Catane October

4 Consequence: huge gaps In the Actinides region Observe big gaps at Z= 64, 70, 90-94,100 94,100 and N=112, D. Curien LEA-Catane October

5 New Magic Numbers: The tetrahedral nuclei are predicted around the following new shell closures: (Zt, Nt) ) = (32, 40, 56, 64, 70, 90, 136) Corresponding to the doubly magic nuclei: 32 Ge, 72 32Ge, 88 32Ge, 80 40Zr, 96 40Zr, Zr, Zr, Zr, Ba, Ba, Gd, Gd, Yb, Ba, 64Gd, 64Gd 70Yb 56Ba, 90Th D. Curien LEA-Catane October

6 Doubly-Magic Tetrahedral Nuclei D. Curien LEA-Catane October

7 Experimental Signs? Tetrahedral nuclei = oriented object Rotational bands There exist 4 types of octupole shapes: Y31, Y30, Y32, Y33 Tetrahedral symmetry Y32 with negative parity Tetrahedral symmetry implies Qt=0 Rotational bands without E2 s!! D. Curien LEA-Catane October

8 Do we have Candidates close to the Tetrahedral Magic Numbers? Z=62 Z=62 Z=64 Z=66 D. Curien LEA-Catane October

9 Obvious Experimental Observable: Branching Ratios B(E2)in/B(E1)out B(E1)out Exact symmetry : Q t = 0 but also D t =0 only E3 transition, no E1! but transition probability E1/E3 ~10 12! no E3! Partial symmetry breaking (zero-point motion, spin,.) Residual polarisation partly allowing E1 and E2 transitions We may expect a spin dependence of the branching ratios as compared with classical octupole D. Curien LEA-Catane October

10 Obvious Experimental Observable: B(E2)in/B(E1)out B(E1)out B(E 2 ) in/ B(E 1 ) out * 10 6 fm 2 D. Curien LEA-Catane October

11 Where in the Periodic Table? Uranium B(E3)(W.u.)=57 96 Zr Spiral2 156 Gd D. Curien LEA-Catane October

12 Search in the Rare Earth Region: approved proposals Site IPN-Orsay Oscar 156 Gd (D. Curien) JYFL-Jyv Jyväskylä 156 Gd Jurogam (J. Robin, D. Curien) ILL-Grenoble Gams 156 Gd (B. Lauss,, M. Jentshel,, D. Curien, J. Dudek ) LNL-Legnaro Legnaro GaSp 156 Gd (R. Singh, G. de Angelis D. Curien, J. Dudek) ithemba Afrodite 154 Gd (R. Bark) ANL Gammasphere 156 Dy (L. Riedinger) Main goals excitation function γ γ γ γ γ branching ratios new bands, new inter-band transitions forbidden E2 s level lifetimes quadrupole moments DSAM B(E1) matrix elements branching ratios Reaction Fusion Evaporation 154 Sm(α,2n) Fusion Evaporation 154 Sm(α,2n) Thermal neutrons 155 Gd(n,γ) ) E=TH Status Run 12/06 Analyzed J.Robin IPHC Run 10/07 Partially analysed at IPNL Q.D. Tuyen,, O. Stezowski Run 12/07 Analysed at ILL, results under investigation W. Urban Coulex 58 Ni Plan Dec 08-Jan 09? Coulex 84 Kr Plan for March 09 Fusion Evaporation 148 Nd( 12 C,4n) Scheduled for November 08 D. Curien LEA-Catane October

13 Why Gd 92? Negative parity band odd-spins spins: known as a vibrational octupole band with an even-spin partner Useful production reactions (low spin) in the literature: 155 Gd(n,γ) 156 Gd(n,n'γ) 154 Sm(α,2nγ) 150 Nd( 13 C,α3nγ) Coulomb Excitation 158 Gd(p,t) 154 Gd(t,p) J. Konijn et al., NPA 352 (1981) All All of them failed to see the E2 s s below Spin 9 D. Curien LEA-Catane October

14 Branching Ratios 50 (10) 16 (3) 6 (2) 7 (2) R=B(E2) in / B(E1) out x (7) NPB even member: R~270 from Iπ=10I =10 - to I=4 - D. Curien LEA-Catane October

15 156 Gd JYFL exp. Goals: γ γ γ (mutiplets) Forbidden E2 s Branching ratios 154 Sm(α,2n) 27 MeV Excitation function OSCAR-IPNO 154 Sm( First experiment with full TNT digital electronics First results: Zakopane 2008 Q. D. Tuyen (IPNL) D. Curien LEA-Catane October

16 Negative Parity Even-Spin Band D. Curien LEA-Catane October

17 Negative Parity Odd-Spin Band D. Curien LEA-Catane October

18 Direct comparison D. Curien LEA-Catane October

19 156 Gd partial level scheme from JYFL D. Curien LEA-Catane October

20 Branching Ratios X 100! D. Curien LEA-Catane October

21 Two comments: Lee Riedinger & Rob Bark, invited Professors at IPHC B(E2)/B(E1) branching ratios going towards zero; two possible reasons: Q2 0 0 for α 32 : no E2 Large B(E1;I I-1) 1)? At N=90 ( 152 Sm, 154 Sm) enhanced B(E1) values have been reported as large as W.u. for transition giving Q(npb)~2-7 x Q(gsb) ) (at 12 and 40 e.b.. ) is this SD??? The first 1-1 state could not be tetrahedral (which is good!) But is such a huge value compatible with octupole vibration either? er? D. Curien LEA-Catane October

22 Second Comment Rob Bark performed band mixing calculations mainly for 160Yb 160 Yb Even spin states Q~5 eb incompatible with a tetrahedral band Odd spin states chi-squared values obtained for the fit of branching ratios seem not to converge if one takes all band-states for the fit A A firm conclusion is not yet possible and would be in any case model dependent therefore therefore the conclusion at this level of the discussion is that we need to have a direct measurement of the absolute transition rates in these nuclei for as many as possible states in the tetrahedral candidates bands D. Curien LEA-Catane October

23 Actinide Region white : possible tetrahedral nuclei yellow : octupole nuclei green : coexistence of the two New Shape Coexistence? D. Curien LEA-Catane October

24 Branching Ratios B(E2)in/ in/b(e1)out * 10 6 state 220Th (90,130) 222Th (90,132) 224Th (90,134) 226Th (90,136) 228Th (90,138) 230Th (90,140) 232Th (90,142) (?) (?) (2)? ? 0.4(2) ?? 13-? 0.3(2) 0.5? 16?? (2) ?? (2)? (64) (3)?? 0? 182 (41) 2264 (470) ? D. Curien LEA-Catane October

25 Uranium Isotopes Case Octupole def. Octupole vib. or Tetrahedral shape Pb( 22 Ne,4n) P. Greenlees 208 Pb( B(E2)/B(E1) * Pa(p,2n) No E2 s! 232 Th(α,2n) Hindranced E1 s 3* Pa(p,4n) 230 Th(α,2n),2n) 236 U(d,pn) P. Zeyen et al. Z.Phys.A 2328,399 (1987): e-γ coincidence multi-coulex D. Ward D. Curien LEA-Catane October

26 Another last Comment: Question: are the E2 not seen because of enhanced E1 s s and/or a resolving power problem? 218 Ra B(E1, 11-)=4.10 )= W.u. If we use this value and supposed that the B(E2) are equivalent to the one in the gsb we find : For U I(E2)/I(E1)= 4.2% For Th I(E2)/I(E1)= 1.4% This is highly improbable, that one could not observed such transitions and, it is incoherent that the ratio is bigger for 230 U (no E2 reported)! D. Curien LEA-Catane October

27 Tetrahedral Fingerprints: Summary Rotational bands without E2 s s for pure symmetry Vanishing Q 2 Branching ratios are spin dependent (difference with usual octupole) First experimental results on 156 Gd: fully compatible with the above Emphasising the actinide region, where there exist signs of purer symmetry and shape coexistence D. Curien LEA-Catane October

28 Conclusion It is urgent to have direct measurement of the absolute values of quadrupole and dipole moments of the tetrahedral candidates through lifetime measurements (here lies the real smoking gun!) and compare them with the other negative parity bands in the various regions of interest D. Curien LEA-Catane October

29 List of main collaborators D. Curien, J. Dudek, J. Robin, Ch. Beck, S. Courtin, O. Dorvaux, G. Duchêne, T. Faul, B. Gall, F. Haas, F. Khalfallah, H. Molique, M. Rousseau, MD Salsac - IPHC, Strasbourg D. Guinet, N. Redon, Ch. Schmitt, O.Stezowski, Q.D. Tuyen,- IPN, Lyon P.T. Greenlees, P. Jones, R. Julin, S. Juutinen, S. Ketelhut, M. Nyman, P. Rahkila, J. Sorri, M. Leino, C. Scholey, J. Saren, U. Jakobsson, J. Uusitalo - JYFL, Jyvaskyla F. Azaiez, B. Berthier, D. Guillemaud-Mueller, M. Leblois, F. Ibrahim, C.Petrache, D. Verney - IPN, Orsay A. Astier, I. Deloncle, G. Georgiev- CSNSM, Orsay N.Dubray - CEA, Bruyères-le-Châtel R. A. Bark, J F. Sharpey-Schafer - ithemba, Cape-Town J. Gerl- GSI, Darmstadt B. Lauss, J. Jentschel, W. Urban -ILL,Grenoble D. Tonev - Bulgarian Academy of Sciences, Sofia L. Riedinger (and the US collaboration), N. Schunck - University of Tennessee D.J. Hartley - US Naval Academy, Annapolis P. Bednarczyk, B. Fornal, A. Maj, K. Mazurek, K. Zuber- IFJ-PAN, Krakow G. de Angelis, A. Gadea - INFN, Legnaro R.P. Singh, S. Muralithar, R. Kumar, A. Jhingan, J.J. Das, R. K. Bhowmik - IUAC, New Delhi 67 J. Dobaczewski, P. Olbratowski- Warsaw University A. Gozdz, A. Dobrowolski - University of Lublin Y. R. Shimizu - Kyushu University, Fukuoka Thank you! And more! D. Curien LEA-Catane October