Experimental measurement of the deformation through the electromagnetic probe Shape coexistence in exotic Kr isotopes. E. Clément CNRS/GANIL Kazimierz 2010 Shape coexistence in exotic Kr Shape coexistence in the proper sense only if (i) The energies of the states are similar, but separated by a barrier, so that mixing between the different components of the wave functions is weak and the states retain their character. Single particule level scheme (MeV) (ii) The shapes involved are clearly distinguishable 74Kr Shape coexistence in n-deficient Kr : an experimentalist view What can we measure experimentally ? Establish the shape isomer : 0+2 Collectivity in such nuclei : level scheme and B(E2) . Shape (oblate - prolate ?) : Q0 Wave function mixing ? : r²(E0) Shape isomer : systematic of 0+2 states prolate oblate 6+ 6+ 791 4+ 2+ 1233 611 0+ 2+ 710 0+ 0+ 918 4+ 558 0+ 456 858 824 4+ 612 Transition strenght : r²(E0).10-3 2+ 2+ 768 671 6+ 6+ 52 72Kr 74Kr 72(6) 84(18) 2+ 508 0+ 0+ 346 770 424 76Kr 79(11) 664 562 2+ 1017 455 0+ 78Kr 47(13) E. Bouchez et al. Phys. Rev. Lett., 90 (2003) • 0+ 4+ Shape inversion Maximum mixing of wave function in 74Kr Collectivity measurement : the B(E2) Recoil Distance Doppler Shift Measure the B(E2) through the lifetime of the state ( ≈ ps ! ) Target and stopper at a distance d During the desexcitation of the nuclei, g are emitted : In flight Shifted by the Doppler effect Stopped E0 Collectivity measurement : the B(E2) • The collectivity of the shape-coexisting states are highly pertubated by the mixing Weak mixing ≈ quantum rotor 74Kr GSB Strong mixing perturbation of the collectivity Collectivity measurement : safe coulomb excitation 1er order: Reorientation effect 2nd order: 2+ 2+ a(1) a(1) a(2) a(2) 0+ 0+ d __ d = __ Ruth Pif dif d a (2) a I f M ( E 2) I j (2) I f M ( E 2) I f I j M ( E 2) I i j a (1 ) I f M ( E 2) I i B(E2) Q0 I f M ( E 2) I i Static quadrupole moment sensitivity minimisation du 2 : 74Kr 0 . 70 0 .30 1 . 02 0 .21 21 M ( E 2 ) 21 0 . 33 I g ( 41 ) 41 M ( E 2 ) 41 1 Ig (2 ) 0 . 59 Negative matrix element (positive quadrupole moment Q0) prolate deformation 74Kr 2 Ig (2 ) I g ( 21 ) 0 . 28 2 2 M ( E 2 ) 2 2 0 . 33 0 .23 Positive matrix element (Negative quadrupole moment Q0) oblate Deformation Radioactive beams experiment at GANIL • • • 78Kr The 74,76Kr RIB are produced by fragmentation of a 78Kr beam on a thick carbon target. Radioactive nuclei are extracted and ionized Post-accelaration of the RIB 1 MeV/u 1 10 MeV/u 4.7 MeV/u 1.5104 pps 1 3 70 MeV/u 1012 pps 2 74Kr 6104 pps Safe Coulomb excitation g detection Pb Particle detection E. Clément et al. PRC 75, 054313 (2007) Very well known technique for stable nuclei but for radioactive one … The differential Coulomb excitation cross section is sensitive to transitionnal and diagonal E2 matrix elements GOSIA code Safe Coulomb excitation results 74Kr 13 E2 transitional matrix elements 76Kr 16 E2 transitional matrix elements In 74Kr and 76Kr, a prolate ground state coexists with an : describe the coupling between states oblateTransition excited probability configuration 5 E2 diagonal matrix element 5 E2 diagonal matrix element Spectroscopic quadrupole moment : intrinsic properties of the nucleus E. Bouchez PhD 2003 E. Clément PhD 2006 E. Clément et al. PRC 75, 054313 (2007) Configurations mixing Shape coexistence in a two-state mixing model Pure states Perturbed states Extract mixing and shape parameters from set of experimental matrix elements. Configurations mixing Shape coexistence in a two-state mixing model Pure states Perturbed states Extract mixing and shape parameters from set of experimental matrix elements. • Energy perturbation of 0+2 states 76Kr 74Kr 72Kr E. Bouchez et al. Phys. Rev. Lett 90 (2003) cos2θ0 0.73(1) 0.48(1) 0.10(1) 0.69(4) 0.48(2) * • Full set of matrix elements : E. Clément et al. Phys. Rev. C 75, 054313 (2007) oExcited Vampir approach: A. Petrovici et al., Nucl. Phys. A 665, 333 (00) * 0.6 0.5 Model describes mixing of 0+ states well, but ambiguities remain for higher-lying states. Two-band mixing of prolate and oblate configurations is too simple. Vampir calculations A. Petrovici et al., Nucl. Phys. A 665, 333 (00) Beyond … Several theoretical approaches, such as shell-model methods, selfconsistent triaxial mean-field models or beyond-mean-field models predict shape coexistence at low excitation energy in the light krypton isotopes. The transition from a prolate ground-state shape in 76Kr and 74Kr to oblate in 72Kr has only been reproduced in the so-called excited VAMPIR approach, This approach has only limited predictive power since the shell-model interaction is locally derived for a given mass region. On the other hand, no self-consistent mean-field (and beyond) calculation has reproduced this feature of the light krypton isotopes so far. Shape coexistence in mean-field models • In-band reduced transition probability and spectroscopic quadrupole moments GCM-HFB (Gogny-D1S) E. Clément et al., PRC 75, 054313 (2007) M. Girod et al. Physics Letters B 676 (2009) 39–43 GCM-HFB (SLy6) M. Bender, P. Bonche et P.H. Heenen, Phys. Rev. C 74, 024312 (2006) Shape coexistence in mean-field models (2) Skyrme HFB+GCM method Skyrme SLy6 force density dependent pairing interaction g Restricted to axial symmetry : no K=2 states Inversion of oblate and prolate states Collectivity of the prolate rotational band is correctly reproduced B(E2) values e2fm4 Interband B(E2) are under estimated E. Clément et al., PRC 75, 054313 (2007) Shape coexistence in mean-field models (3) Gogny HFB+GCM with Gaussian overlap approximation Gogny D1S force Axial and triaxial degrees of freedom E. Clément et al., PRC 75, 054313 (2007) Shape coexistence in mean-field models (3) Gogny The agreement is remarkable for excitation energy and matrix elements K=0 prolate rotational ground state band K=2 gamma vibrational band Strong mixing of K=0 and K=2 components for 2+3 and 2+2 states Grouping the non-yrast states above 0+2 state in band structures is not straightforward 2+3 oblate rotational state E. Clément et al., PRC 75, 054313 (2007) g g Shape coexistence in mean-field models (3) Gogny M. Girod et al. Physics Letters B 676 (2009) 39–43 Potential energy surface using the Gogny GCM+GOA appraoch Shape coexistence in mean-field models (3) Gogny M. Girod et al. Physics Letters B 676 (2009) 39–43 Is the triaxiality the key ? Difference #1: effective interaction very similar single-particle energies → no big differences on the meanfield level axial quadrupole deformation q0 (exact GCM formalism) • Good agreement for in-band B(E2) • Wrong ordering of states: oblate shape from76Kr to72Kr • K=2 outside model space M. Bender and P. –H. Heenen Phys. Rev. C 78, 024309 (2008) ↔ triaxial quadrupole deformation q0, q2 Euler angles Ω=(θ1,θ2,θ3) → 5-dimensional collective Hamiltonian (Gaussian overlap approximation) • Excellent agreement for Ex, B(E2), and Qs • Inversion of ground state shape from prolate in 76Kr to oblate in 72Kr • Assignment of prolate, oblate, and K=2 states • When triaxiality is “off” same results than the “old” Skyrme Triaxiality seems to be the key to describe prolate-oblate shape coexistence in this region Do the GCM (+GOA) approach and the triaxiality key work everywhere ? In the n-rich side ? 2+1 The n-rich Sr (Z=38), Zr (Z=40) isotopes present one of the most impressive deformation change in the nuclear chart Systematic of the 2+ energy (Raman’s formula : b2~0.17 0.4) + 2n Low lying 0+ states were observed E(0+) [keV] 0+2 + 2n Shape transition at N=60 E [MeV] HFB Gogny D1S b2 Both deformations should coexist at low energy • Shape coexistence between highly deformed and quasi-spherical shapes Shape transition at N=60 C. Y. Wu et al. PRC 70 (2004) W. Urban et al Nucl. Phys. A 689 (2001) N=58 N=60 Shape transition at N=60 : Coulomb excitation B(E2↓) < 625 e²fm4 < 152 e²fm4 399 ( -39 67) e²fm4 < 22 e²fm4 Qs = -6 (9) efm² 462 (11) e²fm4 The Electric spectroscopic Q0 is null as its B(E2) is rather large Quasi vibrator character ??. No quadrupole ? but it doesn’t exclude octupole or something else ?? The large B(E2) might indicate a large contribution of the protons E. Clément et al., IS451 collaboration Gogny calculations Qualitatively good agreement The abrupt change not reproduced Very low energy of the 0+2 state is not reproduced overestimate the mixing ? Highly dominated by K=2 configuration 94Sr 96Sr 98Sr 100Sr Conclusion We have studied the shape coexistence in the ndeficient Kr isotopes Beyond the mean field calculations reproduce the experimental results when the triaxiality degree of freedom is available Same calculations seem to not reproduce the shape transition at N=60. What is missing ? P. Möller et al Phys. Rev. Lett 103, 212501 (2009) Shape transition at N=60 40 1g7/2 1g 2d5/2 1g9/2 2p 1f 2p1/2 1f5/2 2p3/2 1g7/2 3s1/2 50 40 ll ll ll llll 28 2d5/2 0+ ll ll ll ll ll 50 Beyond N=60, the tensor force participates to the lowering 0+2 state and to the high collectivity of 2+1 state. 2 0+ 1 p K. Sieja et al PRC 79, 064310 (2009) n But in the current valence space, need higher effective charge to reproduce the known B(E2) Shape coexistence in mean-field models (3) Gogny Coulomb excitation analysis : GOSIA* *D. Cline, C.Y. Wu, T. Czosnyka; Univ. of Rochester Lifetimes are the most important constraint because directly connected to the transitional matrix element B(E2) 74Kr 5 lifetime known from the literature 2+ Lifetime incompatible with our coulex data 4+ Lifetime measurement A. Görgen, E. Clément et al., EPJA 26 (2005) 2+ 4+ Shape coexistence in Se isotopes Similar j(1) in 68Se & 70Se : • 70Se oblate near ground state • Prolate at higher spin G. Rainovski et al., J.Phys.G 28, 2617 (2002) Shape coexistence in mean-field models (6) Gogny Qs from Gogny configuration mixing calculation Good agreement of B(E2) Shape change in the GSB in 70,72Se 70,72Se behaviors differ from neighboring Kr and Ge Isotopes 68Se more “classical” compare to Kr and Ge Clear evidence for neutron orbital playing an important role in the shape transition Established sign for extruder or intruder orbital Search for isomer in odd neutron Sr and Zr A. Jokinen WOG workshop Leuven 2009 W. Urban, Eur. Phys. J. A 22, 241-252 (2004) h11/2 g7/2 2d5/2 g9/2 from core 9/2+ isomer identified ng9/2[404] extruder neutron orbital from 78Ni core Create the N=60 deformed gap pg9/2 <> nh11/2 influence ? Neutron excitation from d5/2 to h11/2 Octupole correlation ?