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Collinear laser spectroscopy was performed on Ga (Z ¼ 31) isotopes at ISOLDE, CERN. A gas-filled linear Paul trap (ISCOOL) was used to extend measurements towards very neutron-rich isotopes (N ¼ 36-50 Nuclear structure has for some time been described by the single-particle (SP) states of nucleons in the shell model. The evolution and reordering of these levels along isotopic chains is explored at radioactive ion beam facilities to provide information on the nature of the nucleonnucleon interaction. Key to these studies is the determination of the value of the nuclear spin of each state, which provides a means of level identification. Whereas the spin may sometimes be inferred from nuclear decay and -spectroscopy data, laser spectroscopy [1,2] permits a measurement of the nuclear spin, in addition to the state's magnetic dipole and electric quadrupole moments. The latter two observables are very sensitive to the wave function and thus to the SP shell evolution. The sensitivity of the laser technique has been critically enhanced using bunched beams from a gas-filled linear rf quadrupole known as an ion beam cooler [3]. In this Letter we report the application of ISCOOL [4]-an ion beam cooler recently installed at ISOLDE-for collinear laser spectroscopy on Ga isotopes from stable to the magic N ¼ 50 shell gap, located 15 isotopes away from stability. For the first time g.s. spins have been measured, revealing sudden changes not observed in earlier experiments.The Ga isotopes have three protons outside the Z ¼ 28 shell gap. In a normal shell-model ordering, the three protons would occupy the p 3=2 level, leading to a g.s. spin I ¼ 3=2 for all odd-A Ga isotopes. However, in the Cu isotones with two protons fewer, it has been demonstrated that the proton SP ordering changes when neutrons start occupying the g 9=2 orbital around N ¼ 40 [5][6][7][8][9][10][11][12][13][14][15]. An inversion of the p 3=2 and f 5=2 SP levels was established recently in 75 Cu at N ¼ 46 [11], where the 5=2 À g.s. is near degenerate with a 3=2 À and 1=2 À state [11]. In this Letter we establish the g.s. spins and structure of the odd-A Ga isotopes from N ¼ 36 up to the N ¼ 50 shell closure, and we investigate the systematics of the 1=2 À , 3=2 À and 5=2 À levels.Fission fragments were produced in a thick UC x target (45 g=cm 2 ) using 1.4 GeV protons at an average current of $2 A. A proton-neutron converter [16] was used to suppress the Rb production. The Ga yield was selectively enhanced by a factor of 100 using the Resonant Ionization Laser-Ion Source [17], extracted and accelerated to 30 keV and mass selected. The ions were cooled and bunched by the newly-installed ISCOOL [4] and delivered to the collinear laser spectroscopy setup [18]. The ion beam was

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We report the first confirmation of the predicted inversion between the 2p 3=2 and 1f 5=2 nuclear states in the g 9=2 midshell. This was achieved at the ISOLDE facility, by using a combination of insource laser spectroscopy and collinear laser spectroscopy on the ground states of 71;73;75 Cu, which measured the nuclear spin and magnetic moments. Much of the current effort in nuclear physics is focused on determining how the nuclear shell structure is changing in neutron-rich nuclei. This has been triggered by the observation of unexpected phenomena in several neutronrich isotopes, since radioactive ion beams of such nuclei became available more than three decades ago. In the lighter elements (e.g., He, Li, Be), neutron halos and skins were observed. Around the neutron-rich 32 Mg region an ''island of inversion'' was discovered. In the neutron-rich region towards doubly magic 78 Ni, a sudden drop in the position of the first excited 5=2 À state in 71;73 Cu isotopes was observed more than a decade ago [1]. The lowering of the 5=2 À energy from above 1 MeV in 69 Cu to 166 keV in 73 Cu suggested that this state might become the ground state in 75 Cu. The migration of this level, associated with the occupation of the 1f 5=2 single-particle orbital, was attributed to a strong attractive monopole interaction that becomes active when neutrons occupy the 1g 9=2 orbital [2]. Such monopole interactions exist also in near-stable nuclei, but their impact on the evolution of shell structure and shell gaps in far-from-stability nuclei remained unnoticed until recently [3]. Also in other neutron-rich regions dramatic monopole shifts were observed when valence neutrons and protons are occupying orbits having their orbital and spin angular momentum, respectively, aligned and antialigned. It is now understood that one of the physics mechanisms driving these monopole shifts is the tensor part of the residual nucleon-nucleon interaction [4]. A steep lowering of the 1=2 À level from about 1 MeV in 69 Cu down to 135 keV in 73 Cu has also been observed [5,6]. Thus this level is also a potential ground-state candidate in 75 Cu. While most shell-model interactions do reproduce a lowering of the 5=2 À level and predict an inversion with the normal 3=2 À ground state somewhere between 73 Cu and 79 Cu [4,[7][8][9][10], none of them reproduce the lowering of the 1=2 À state. Some significant physics mechanism is either omitted or seriously underestimated in each of the recently developed shell-model interactions. Therefore, experimental establishment of ground-and excited-state nuclear spins and the properties of their wave function (through spectroscopic factors, magnetic moments, transition moments, etc.) is a crucial step in PRL 103,