1. 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

2. 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]

3. 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

4. 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, (2009)

5. 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, (2010)
W. Urban, Eur. Phys. J. A 22, (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 ?

6. the mean field approach
Shape coexistence in
the mean field approach
P. Möller et al Phys. Rev. Lett
103, (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

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

8. 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

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

10. 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

11. 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

12. 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
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

13. 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

14. 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