1. Georgi Georgiev CSNSM, Orsay, France
Nuclear structure studies at the r-process path Coulomb excitation of odd-A neutron-rich Rb isotopes at REX-ISOLDE.
Georgi Georgiev
CSNSM, Orsay, France

2. Overview
Physics motivation – sudden onset of deformation in the neutron-rich N=60 region
Experimental approach
Radioactive beams (post-accelerated)
Coulomb excitation – few general remarks
The Coulex setup at REX-ISOLDE
Experimental results:
93Rb and 95Rb spherical features below N=60
97Rb and 99Rb – a sudden onset of deformation right above N=60
Summary and outlook

3. N=60 - from spherical to strongly deformed structures
Zr (Z=40) and Sr (Z=38)
 clear shape change across N=60
9/2+[404], 1/2-[550], 3/2-[541], 3/2+[411]
W. Urban et al., Eur. Phys. J. A22, 241 (2004)‏
S. Naimi et al., PRL 105, (2010)
Ground States
Kr (Z=36)  NO onset of deformation
Mass and laser measurements show that the Kr isotopes do not undergo a rapid shape change of their ground states from N=58 to 60

4. What is going on with the proton side? Rb’s (Z = 37)
3/2-[312], 3/2-[301], 3/2+[431]
C. Thibault et al., Phys. Rev. C23, 2720 (1981)
Rb's ground states
r-process path – according to one of the scenarios it can pass exactly through the 93,95,97,99Rb nuclei
 Study of the excited states in the Rb isotopes from below (Coulomb excitation) – a key for the understanding of the nuclear structure around N=60

5. ISOLDE@CERN 1.4 GeV protons from PS-Booster 30-60 keV X1+  GPS or HRS
EBIS charge breeding to A/q ~ 4
REX
up to 3 MeV/u
MINIBALL 8 triple Ge clusters 6-fold segmented
REX trap cooling and bunching

6. Coulomb excitation – few remarks
Coulomb excitation – purely electromagnetic interaction involved – full analytical description
a – half distance approach a=b/2; ksi – adiabaticity parameter = a*deltaE/hbar.v ≤1; f – total excitation cross-section function
K. Alder et al., RMP 28 (56) 432

7. Secondary target 1-2 mg/cm2
Miniball setup
Secondary target 1-2 mg/cm2
Particle detection
beam dump
Ge detector
Ionization chamber
Beam from
REX-ISOLDE
@ 2.9 MeV/u
CD Detector
(16° – 53°)   
Reaction Chamber & Miniball cluster array
REX beam

8. 93,95,97, 99Rb – excited states known before
Isomeric decays
???
b - decay

9. Experimental results 93,95Rb – work in progress!
Strong resemblance between the level schemes (and transition probabilities) in 93Rb and 95Rb

10. Experimental results 97,99Rb – work in progress!
“All new” excited states in 97Rb and 99Rb
A “collaps” of the structure after N=60
Considerable contamination for 99Rb (50 ms) from products decaying in-flight
Still to be done before B(E2) could be extracted:
angular distribution of gamma-rays for spin-determination
existence of gamma-gamma coincidences ?

11. Summary and outlook
The Coulomb-excitation is a powerful tool for nuclear structure studies far from stability giving independent and complementary information to the other approaches
The structure of the Rubidium isotopes is one of the keys ingredients needed to clarify the nature of the sudden onset of deformation around the neutron-rich N=60 region, giving an insight to the possible proton involvements into the process.
A considerable number of levels populated by the Coulomb excitation in the supposedly spherical 93Rb and 95Rb – a starting point for the understanding of the deformed to 97Rb and 99Rb.
First unambiguous identification of excited states in 97Rb and 99Rb.
Additional experimental sensitivity (e.g. polarized beams?) needed for the very exotic cases where no much experimental information is available.

12. and REX-ISOLDE and Miniball collaborations
CSNSM, Orsay, France - C. Sotty
LPSC, Grenoble, France – G.S. Simpson
CEA/DIF/DPTA/SPN, Arpajon, France - J.M. Daugas, R. Chevrier, P. Morel
IKP Köln, Germany - A. Blazhev, K. Geibel, P. Reiter, M. Seidlitz, B. Siebeck, N. Warr
IKS, Leuven, Belgium - N. Bree, J. Diriken, B. Lannoo
University of Camerino, Italy - S. Das Gupta
University of Liverpool, UK - L. Gaffney
Warsaw University, Poland - K. Hadynska-Klek, P. Napiorkowski, M. Zielinska
TU Darmstadt, Germany - T. Kroell, M. Scheck
ISOLDE, CERN, Geneva, Switzerland - J. Pakarinen, F. Wenander
ANU, Canberra, Australia - A. Stuchbery
and REX-ISOLDE and Miniball collaborations

14. Simulation trying to reproduce IC
99Rb – pushing the limits
99Rb case T1/2 = 50 ms; trapping time ~80 ms; breading time ~80 ms
Ionization Chamber
β-decay
Shift of components
Simulation trying to reproduce IC
Different T1/2

15. Experiment vs. theory

16. Mean-field theory results for odd-mass Rb’s
R. Rodriguez-Guzman, P. Sarriguren, L. M. Robledo, PRC82, R (2010)
A<89 – spherical
91,93,95Rb – oblate plus shape coexistence
A>99 – strongly prolate deformed

17. 97Rb preliminary results
97Rb on 2.83MeV/u :
known transitions
unknown transitions
target excitation
contaminants (97Sr,97Y …)
97Rb (T1/2 = 170 ms) 5E5 pps (4E5/µCu)
First observation of excited states in 97Rb
type
unknown
97Sr
target excitation (Pt)
γ-transition
68 keV
103 keV
124 keV
167 keV
191 keV
227 keV
241 keV
356 keV
97Rb on 2.83MeV/u :