The/~-decay of a~ has been investigated at the fission-product separators JOSEF and LOHENGRIN. The half-life of a~ has been determined to 2.5(1)s and a level scheme for t~ has been established from 7 ray singles as well as X-7 and 7-7 coincidence measurements. Conversion coefficients for transitions in l~ and level half-lives between 10 ps and 2 ns have been determined. Three rotational bands are identified among the low-lying levels with band heads at 0 keV, 206 keV and 208 keV. The bands are probably based on the Nilsson configurations [422 5/2+], [301 3/2-] and [303 5/2-], respectively. The deformation has been determined to /?q=0.40(4) and 0.41(8) for the ground state band and the band based on the 206 keV level from the half-lives of the first and second excited members of these bands. This shows that the rapid onset of deformation at N = 60 which is typical for the A = 100 region of neutron-rich nuclei, takes also place in the Nb isotopes. Nilsson model calculations describe the experimental data well, especially the several determined transition probabilities including those for E1 transitions from the 206 and 208 keV band heads to the ground state.

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The half-lives of 8 low lying levels of aO3Nb have been determined at the fission-product separator JOSEF. A t~--777-7 triple--coincidence method was used which consists in a measurement of the time delay between the feeding of the levels through the t~-decay of ~03Zr and their 7 decay, in coinddenee with a tagging 7 ray, ~th plastic, BaF~ and Ge detectors, respectiveiy. Most of the investigated levels are members of the three known rotational bands based on the ground state and the 164 and 248 keV levels. The deformation Bq=0.31(3) of ~O"Nb could be deduced from the half-life data. The half-lives are well reproduced through calculations in the frame of the Nilsson model.

The half-lives of low-lying levels of the neutron-rich isotopes tO2Mo and 104Mo have been measured at the fission product separator JOSEF with the g-7-7 triple coincidence technique. Values oft1/2 = 126(.~)ps and 0.72(4)ns have been obtained for the 21" levels at 296 keV and 192 keV in lO2Mo and m4Mo, respectively. Deformation parameters of t~q = 0.28(1) and 0.31(1), respectively, are deduced, which are smaller than those of the isotones of Sr and Zr. The interacting boson model accounts well for the trend of 2* energies in the Mo isotopes but slightly overpredicts the B(E2) values at saturation. A smooth dependence of r(21*J vs. E(2t § is found for the A ~ 100 region in agreement with the hydrodynamical model.

Abstract. The lifetimes of the three lowest excited members of the rotational ground-state band of 1~ at 102.6, 241.1 and 433.2 keV as well as of the levels at 353.8 and 456.1 keV have been determined through a measurement of the delayed coincidences between/~ particles from the decay of the parent l~ and the 7 rays which depopulate the levels. A plastic and a BaF 2 detector have been used for the /~ and 7 rays, respectively, in these experiments at the fission product separator JOSEF. The results for all three band members evince that this nucleus has a sizeable deformation, with a value of/3q=0.34(1). This value is larger than that of the neighbouring eveneven Mo isotopes which indicates an odd-even effect. On the other hand it confirms the observation that the Mo nuclei are in general less strongly deformed than their Sr and Zr isotones. Particle-rotor model calculations with the Nilsson parameters ~c = 0.084, j~ = 0.28 provide a good description of the ground-state band and of a side band based on the 346.5 keV level, including the new lifetimes. This substantiates the Nilsson orbital assignments of [41113/2 + and [532]5/2-for these two bands, respectively, where the sideband is strongly affected by a [54113/2-admixture. There is evidence for an additional band based on the 641.1 keV level for which the assignment [42213/2 + is proposed.