Emmanuel Clément IN2P3/GANIL – Caen France Shape coexistence in neutron-rich Sr isotopes: Coulomb excitation of 98,96Sr Emmanuel Clément IN2P3/GANIL – Caen France Andréas Görgen CEA-Saclay Irfu/SPhN France

Evidence for shape competition around N=60 2+1  + 2n 0+2 + 2n 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) Low lying 0+ states were observed Level scheme, dr², and identification of isomers are measured since decade but no direct measurement of the deformation in excited states was reported so far 2+1  + 2n 0+2 + 2n E(0+) [keV]

Evidence for shape competition around N=60 Systematic of 2+1 state The nuclear deformation is one of the fundamental property of a nucleus The n-rich Sr, Zr isotopes present one of the most impressive deformation change in the nuclear chart Experimental values like level scheme, dr², and identification of isomers are measured since decade in this nuclei but no direct measurement of the deformation in excited states was reported so far Remain extremely challenging for theoretical model Systematic of 2+1 state Observation of 0+2 state B(E2; 0+1 2+1) measurement Quadrupole moment

Evidence for shape competition around N=60 0+2 0+1 p n 40 The 2+1 state in Zr is described predominantly by n excitation In Sr isotopes, 2+1 might be created by p and n excitation  higher B(E2) 0+2 state created by 2p-2h excitation across Z=40 1g7/2 ll 3s1/2 1g7/2 ll 1g 2d5/2 2d5/2 ll ll ll 50 0+2 50 1g9/2 2p1/2 40 2p 1f5/2 ll ll ll Beyond N=60, the tensor force participates to the lowering 0+2 state and to the high collectivity of 2+1 state. Same mechanism than N=28, 20 for 42Si and 32Mg 0+1 ll ll 1f 2p3/2 28 But in the current valence space, need higher effective charge to reproduce the known B(E2)  p and n orbital from the core or higher shells. Q0 ?? p n K. Sieja et al PRC 79, 064310 (2009)

Evidence for shape competition around N=60 h11/2 g9/2 from core g7/2 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 Jokinen WOG workshop Leuven 2009 S. Naimi & ISOLTRAP , Phys. Rev. Lett. 105, 032502 (2010) W. Urban, Eur. Phys. J. A 22, 241-252 (2004) 2d5/2 g7/2 h11/2 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 ?

the mean field approach Shape coexistence in the mean field approach P. Möller et al Phys. Rev. Lett 103, 212501 (2009) Large area of competing minima Prolate, oblate or spherical states within small energy range In Sr and Zr : possible coexistence between nearly spherical and deformed minima ? shape coexistence

the mean field approach Shape coexistence in the mean field approach P. Möller et al Phys. Rev. Lett 103, 212501 (2009) 94Sr 96Sr 98Sr 100Sr

the mean field approach Shape coexistence in the mean field approach HFB Gogny D1S E [MeV] Both deformations should coexist at low energy Shape coexistence between almost same deformation between prolate & oblate (axial calculation) Shape coexistence between highly deformed and quasi-spherical shapes All theoretical calculations predict an onset of quadrupole deformation at the neutron number N=60 but differ for excitation energy, collectivity and deformation Important constraints for theories : E (2+1), E (0+2), r²(E0), B(E2), Q0

Evidence for shape competition around N=60 – Experimentally t = 7(4) ps  Nearly spherical ground state Highly deformed rotational band b ≈ 0.4

Evidence for shape competition around N=60 – Experimentally Recent results : C. Y. Wu et al. PRC 70 (2004) W. Urban et al Nucl. Phys. A 689 (2001) Lifetime compatible with b = 0.25  gradual shape change ? N=58 N=60 The measurement of transition strength and intrinsic quadrupole moments are sensitive probe to understand the complex shape coexistence in Sr isotopes  Coulomb excitation

Evidence for shape competition around N=60 – Experimentally Two experiments were performed In 2007; 96Sr ; Molecular extraction In 2010; 98Sr; Atomic extraction + in-trap b-decay

IS451 Beam production 2007 run First Sr beam at REX-ISOLDE We choose the molecular extraction as 96Sr19F+ to not suffer from the Rb contamination HRS A= 115 UCx Target 96Rb ( Trap HS ) …. off‏ 7 to 1.3 105 96Sr19F+/C Separator A/Q = 4.17 Molecular extraction EBIS Pure 96Sr23+ REX-beam 2.87 MeV/u 1 to 0.5 ~104 pps PSB Proton beam 115In 12

After several 98Rb period IS451 Beam production 2010 run We choose the atomic extraction as 98Sr+ and relied on the in-trap b-decay of the short –lived 98Rb (T1/2 = 96 ms ) for the purification UCx Target HRS A= 98 98Rb (>95 % ) + 98Sr (< 5%) Trap After several 98Rb period Atomic extraction EBIS PSB 96Rb (<5 % ) + 98Sr (>95%) ~6.104 pps Proton beam 13

Results so far …. +2 neutron ~ 20 W.u Qs < +34 efm²  b < 0.14 New in 98Sr (from g-g) New in 98Rb (from g-g) ~ 20 W.u Qs < +34 efm²  b < 0.14