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

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

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, 032502 (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

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

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

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

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

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

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

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 ?

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.

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

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

Experiment vs. theory

Mean-field theory results for odd-mass Rb’s R. Rodriguez-Guzman, P. Sarriguren, L. M. Robledo, PRC82, 061302R (2010) A<89 – spherical 91,93,95Rb – oblate plus shape coexistence A>99 – strongly prolate deformed

97Rb preliminary results 97Rb on 60Ni @ 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 196Pt @ 2.83MeV/u :