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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A new method of optical pumping in an ion beam cooler buncher has been developed to selectively enhance ionic metastable state populations. The technique permits the study of elements previously inaccessible to laser spectroscopy and has been applied here to the study of Nb. Model independent meansquare charge radii and nuclear moments have been studied for 90;90 m;91;91 m;92;93;99;101;103 Nb to cover the region of the N ¼ 50 shell closure and N % 60 sudden onset of deformation. The increase in mean-square charge radius is observed to be less than that for Y, with a substantial degree of softness observed before and after N ¼ 60. Collinear laser spectroscopy has for many years been used to provide the (sole) model-independent probe of charge distributions of radioactive nuclei [1]. Efficient optical measurements are typically limited to transitions from atomic (or ionic) ground states or naturally populated low-lying metastable states. However, elements around Z ¼ 40 such as Y, Nb, and Mo present a variety of challenges to such spectroscopic approaches. In this Letter, we report a new, general technique of wide applicability, in which radio ions are deliberately prepared in metastable states by a rapid and efficient method of optical pumping. The technique is exploited here in the spectroscopic study of the A ¼ 100 region.The well-known onset of deformation in the neutronrich A % 100 region has been of long standing interest and the subject of recent mass [2,3], -ray spectroscopic [4,5], and optical measurements [6,7]. Collinear laser spectroscopy of Y isotopes was recently performed [6] in order to extract quadrupole moments in the region for comparison with charge-radii systematics. It was seen that with increasing neutron number from the N ¼ 50 shell closure, the nuclear deformation becomes increasingly oblate and increasingly soft. At N ¼ 60, the transition to a strongly deformed rigid prolate shape occurs, but prior to this, although the nuclear deformation is increasing with N, a proportionate increase in softness is also observed. Mass measurements [2,3] have indicated that the shape change, which is most pronounced for 39 Y, weakens with increasing Z, becoming almost undetectable in the 42 Mo chain. Further detailed model-independent optical analysis of the mean-square charge radii, nuclear moments and values of spin has so far, however, been impossible in the region because of experimental atomic physics constraints.Production of radioactive beams in this region, which are of a refractory nature, is presently limited to relatively small yields, and efficient spectroscopy is therefore required. Such elements are produced for optical study at the JYFL IGISOL facility, University of Jyväskylä, Finland [8]. Following mass selection at this on-line separator, the emittance of the ion beam is reduced in an ion beam cooler [7], which is a gas-filled rf quadrupole, held at a potential just below the $30 kV of the ion source. If desired, a trapping potential can accumulate the ions at the end of the device for ...

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Collinear laser spectroscopy was performed on the 80 Ga isotope at ISOLDE, CERN. A low-lying isomeric state with a half-life much greater than 200 ms was discovered. The nuclear spins and moments of the ground and isomeric states and the isomer shift are discussed. Probable spins and parities are assigned to both long-lived states (3 − and 6 − ) deduced from a comparison of the measured moments to shell-model calculations.