DOI 10.1140/epja/i2005-10285-9 Eur. Phys. J. A 28, 307–312 (2006) T HE EUROPEAN P HYSICAL JOURNAL A Study of the N = 50 major shell effect toward 78 Ni at PARRNe ? : Evidence of a weak-coupling structure in 83 32 Ge 51 O. Perru 1, a , D. Verney 2, b , F. Ibrahim 1 , O. Bajeat 1 , C. Bourgeois 1 , F. Clapier 1 , E. Cottereau 1 , C. Donzaud 1 , S. Du 1 , M. Ducourtieux 1 , S. Essabaa 1 , S. Gal` es 1 , D. Guillemaud-Mueller 1 , O. Hubert 1 , C. Lau 1 , H. Lefort 1 , F. Le Blanc 1 , A.C. Mueller 1 , J. Obert 1 , N. Pauwels 1 , J.C. Potier 1 , F. Pougheon 1 , J. Proust 1 , B. Roussi` ere 1 , J. Sauvage 1 , and O. Sorlin 1, c 1 Institut de Physique Nucl´ eaire, 15 rue Georges Clemenceau, 91406 Orsay, France 2 GANIL, Bld Henri Becquerel, 14076 Caen Cedex 5, France Received: 27 December 2005 / Revised: 20 April 2006 / Published online: 3 July 2006 – c  Societ` a Italiana di Fisica / Springer-Verlag 2006 Communicated by C. Signorini Abstract. The γ-ray de-excitations following the β-decay of 83 Ga and the β-n decay of 84 Ga have been studied. The radioactive species were produced using the PARRNe on-line mass-separator installed at the IPN Orsay Tandem accelerator. Two γ-lines were attributed to 83 Ge with the aid of β-γ and γ-γ coincidences. The Z identification of the γ-lines was provided by time analysis of a buid-up/decay cycle. The excited levels of 83 Ge can be explained by the coupling of the single neutron state ν 2d 5/2 to the first 2 + excitation of the 82 Ge core. PACS. 29.30.-h Spectrometers and spectroscopic techniques – 21.10.-k Properties of nuclei; nuclear energy levels – 23.20.Lv gamma transitions and level energies – 27.50.+e 59 ≤ A ≤ 89 1 Introduction Recently considerable evidence has been pointed out for the existence of shell gap reinforcements and disappear- ances far off stability, leading in certain cases to the state- ment that some well-known magic numbers would van- ish while new ones would raise. Among the Mayer and Jensen “historical” magic numbers, 50 is known to retain its magic character for protons (Sn isotopes) from N = 50 to N = 82 [1]. On the other hand, probing the stiffness of the 50 neutron shell gap from Z = 50 down to Z = 28 still represents a vivid and extremely active field of investiga- tion in present nuclear-structure research (see [2–4] and references therein). The conclusions of refs. [2] and [3] on a possible N = 50 shell effect weakening are contradic- tory. To illustrate why this question is addressed, let us consider for instance one of the most direct (but certainly ? Production d’Atomes Radioactifs Riches en Neutrons. a Present address: CEA/DIF/DPTA/SPN/L2EN, BP 12, 91680 Bruy` eres-Le-Chˆ atel, France. b Permanent address: Institut de Physique Nucl´ eaire, 15 rue Georges Clemenceau, 91406 Orsay, France; e-mail: verney@ipno.in2p3.fr. c Permanent address: GANIL, Bld Henri Becquerel, 14076 Caen Cedex 5, France. not unique) pieces of evidence that a shell closure domi- nates the nuclear structure: the evolution in energy of the first 2 + excited state of the even-even nuclei at the cross- ing of the corresponding magic number of nucleons. As can be seen in fig. 1 the N = 50 shell closure has definitely a strong influence, but the constant decrease of the 2 + 1 en- ergy for the N = 50 isotones from Z = 40 to Z = 32, i.e. 42 44 46 48 50 52 54 56 N 0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 E(2 + 1 ) [keV] Zn Z=30 Ge Z=32 Se Z=34 Kr Z=36 Sr Z=38 Zr Z=40 Fig. 1. Systematics of the experimentally observed E2 + 1 in the stable and neutron-rich nuclei near the N = 50 shell closure.