Nuclear structure studies at the rprocess path Coulomb

Nuclear structure studies at the r-process path Coulomb excitation of odd-A neutron-rich Rb isotopes at REX-ISOLDE. 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: • 93 Rb and 95 Rb spherical features below N=60 • 97 Rb and 99 Rb – 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 Kr (Z=36) NO onset of deformation S. Naimi et al. , PRL 105, 032502 (2010) W. Urban et al. , Eur. Phys. J. A 22, 241 (2004) Ground States 9/2+[404], 1/2 -[550], 3/2 -[541], 3/2+[411] 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) C. Thibault et al. , Phys. Rev. C 23, 2720 (1981) Rb's ground states r-process path – according to one of the scenarios it can pass exactly through the 93, 95, 97, 99 Rb nuclei 3/2 -[312], 3/2 -[301], 3/2+[431] 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 Ge. V protons from PS-Booster 30 -60 ke. V X 1+ GPS or HRS EBIS charge breeding to A/q ~ 4 REX up to 3 Me. V/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 K. Alder et al. , RMP 28 (56) 432

Miniball setup Secondary target 1 -2 mg/cm 2 beam dump Ge detector Particle detection Ionization chamber Beam from REX-ISOLDE @ 2. 9 Me. V/u CD Detector (16° – 53°) X RE am be Reaction Chamber & Miniball cluster array

93, 95, 97, 99 Rb – excited states known before Isomeric decays y b ca e d - ? ? ?

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

Experimental results 97, 99 Rb – work in progress! • “All new” excited states in 97 Rb and 99 Rb • A “collaps” of the structure after N=60 • Considerable contamination for 99 Rb (50 ms) from products decaying in-flight Still to be done before B(E 2) 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 93 Rb and 95 Rb – a starting point for the understanding of the deformed to 97 Rb and 99 Rb. • First unambiguous identification of excited states in 97 Rb and 99 Rb. • Additional experimental sensitivity (e. g. polarized beams? ) needed for the very exotic cases where no much experimental information is available.

Collaboration • 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

99 Rb – pushing the limits 99 Rb case T 1/2 = 50 ms; trapping time ~80 ms; breading time ~80 ms Ionization Chamber Shift of components β-decay Different T 1/2 Simulation trying to reproduce IC

Experiment vs. theory

Mean-field theory results for odd-mass Rb’s R. Rodriguez-Guzman, P. Sarriguren, L. M. Robledo, PRC 82, 061302 R (2010) • A<89 – spherical • 91, 93, 95 Rb – oblate plus shape coexistence • A>99 – strongly prolate deformed

97 Rb 97 Rb preliminary results on 60 Ni @ 2. 83 Me. V/u : 97 Rb known transitions (T 1/2 = 170 ms) 5 E 5 pps (4 E 5/µCu) unknown transitions target excitation contaminants (97 Sr, 97 Y …) First observation of excited states in 97 Rb on 196 Pt @ 2. 83 Me. V/u : 97 Rb γ-transition type 68 ke. V unknown 103 ke. V unknown 124 ke. V unknown 167 ke. V 97 Sr 191 ke. V unknown 227 ke. V unknown 241 ke. V unknown 356 ke. V target excitation (Pt)