DOI 10.1140/epja/i2004-10047-3 Eur. Phys. J. A 22, 455–459 (2004) T HE EUROPEAN P HYSICAL JOURNAL A Beta-decay of 71 Co and 73 Co M. Sawicka 1, a , I. Matea 2 , H. Grawe 3 , R. Grzywacz 1,4 , M. Pf¨ utzner 1 , M. Lewitowicz 2 , J.M. Daugas 5 , B.A. Brown 6 , A. Lisetskiy 6 , F. Becker 3 , G. B´ elier 5 , C. Bingham 7 , R. Borcea 8 , E. Bouchez 9 , A. Buta 8 , E. Dragulescu 8 , G. de France 2 , G. Georgiev 2 , J. Giovinazzo 10 , F. Hammache 11 , F. Ibrahim 11 , P. Mayet 12 , V. M´ eot 5 , F. Negoita 8 , F. De Oliveira-Santos 2 , O. Perru 12 , O. Roig 5 , K. Rykaczewski 4 , M.G. Saint-Laurent 2 , J.E. Sauvestre 5 , O. Sorlin 11 , M. Stanoiu 2 , I. Stefan 8 , C. Stodel 2 , Ch. Theisen 9 , D. Verney 2 , and J. ˙ Zylicz 1 1 IEP, Warsaw University, ul. Ho˙ za 69, PL-00681 Warsaw, Poland 2 GANIL B.P. 5027, F-14076 Caen Cedex 5, France 3 Gesellschaft f¨ ur Schwerionenforschung mbH, Darmstadt, D-64291 Darmstadt, Germany 4 ORNL, Physics Division, Oak Ridge, TN 37830, USA 5 CEA Bruy´ eres-le-Chˆ atel DIF/DPTA/SPN, B.P. 12, F-91680 Bruy` eres-le-Chˆ atel, France 6 Department of Physics and Astronomy and NSCL, Michigan State University, East Lansing, MI 48824-1321, USA 7 Department of Physics and Astronomy, University of Tennesse, Knoxville, TN 37996, USA 8 IFIN, P.O. Box MG6, RO-76900 Bucharest-Magurele, Romania 9 CEA Saclay, DSM/DAPNIA/SPhN, F-91191 Gif-sur-Yvette Cedex, France 10 CENBG, B.P. 120, F-33175 Gradignan Cedex, France 11 IPN, F-91406 Orsay Cedex, France 12 University of Leuven, IKS, Celestijnenlaan 200 D, B-3001 Leuven, Belgium Received: 28 April 2004 / Revised version: 1 July 2004 / Published online: 16 November 2004 – c  Societ` a Italiana di Fisica / Springer-Verlag 2004 Communicated by C. Signorini Abstract. A decay spectroscopy study of the neutron-rich cobalt isotopes has been performed using frag- mentation of a 86 Kr 36+ beam and the new LISE2000 spectrometer at GANIL. For 71 Co and 73 Co, the β-delayed γ radiation has been observed for the first time, and the half-lives were found to be 79(5) ms and 41(4) ms, respectively. Features of the decay are discussed qualitatively in terms of nuclear models. PACS. 25.70.Mn Projectile and target fragmentation – 21.10.Tg Lifetimes – 23.20.Lv γ transitions and level energies – 27.50.+e 59 ≤ A ≤ 89 The structure of neutron-rich nuclei has attracted growing interest with the development of modern tech- niques enabling access to the exotic regions of the nuclides chart. According to theoretical studies, large neutron ex- cess in nuclear systems can affect the nucleon-nucleon in- teraction and result in a rearrangement of traditional shell gaps and magic numbers [1–4]. To verify these predictions beyond the neutron-rich 0p and (1s,0d) nuclei, experiments have been performed close to the N = 40 and N = 50 neutron shells. The medium-mass neutron-rich nuclei around Z = 28 have been treated as a test case for the evolution of the nuclear structure above N = 40 [5–11]. One way to obtain infor- mation on nuclear structure is the study of microsecond isomers. Such isomers have been identified and studied in 69,70 Ni [7] and 76 Ni [12]. However, for the intermedi- ate nickel isotopes 71–75 Ni, no evidence of isomerism has a e-mail: sawicka@fuw.edu.pl been found (although this may be due to shorter half-lives that are beyond the range of sensitivity of the method). On the other hand, a search for γ transitions in 70 Ni and 72 Ni following the β-decay of 70 Co and 72 Co has been suc- cessful [13]. This has encouraged the search for γ -rays in the β-decay of 71 Co and 73 Co. These two odd-A cobalt isotopes have been identified in earlier experiments using the fragment separator FRS at GSI. The isotope 71 Co was found among the fragmentation products of a 500 MeV/u 86 Kr beam [14]. In another experiment [15], the spatial and time correlation between fragments and the subse- quent β-particles were recorded resulting in a half-life for 71 Co of 0.21(4) s. We note, however, that in a recent experiment focused on the β-decay of neutron-rich 21 Sc- 27 Co produced by the fragmentation reaction of a 76 Ge 30+ beam, a different half-life value of 97(2) ms was reported for 71 Co [16]. The isotope 73 Co was identified for the first time following the projectile fission of a 750 MeV/u 238 U