K. Sistemich

The deformation of the neutron-rich isotopes102Mo and104Mo

The half-lives of low-lying levels of the neutron-rich isotopes102Mo and104Mo have been measured at the fission product separator Josef with the ß-γ-γ triple coincidence technique. Values of t1/2= 126(4)ps and 0.72(4)ns have been obtained for the 21+ levels at 296 keV and 192 keV in102Mo and104Mo, respectively. Deformation parameters of ßq=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 τ 21+ vs. E 21+ is found for the A ∼ 100 region in agreement with the hydrodynamical model.

The deformedN=60 nucleus 41 101 Nb

The β− decay of101Zr has been investigated at the fission-product separators JOSEF and LOHENGRIN. The half-life of101Zr has been determined to 2.5(1) s and a level scheme for101Nb has been established fromγ ray singles as well as X/3-γ and γ—γ coincidence measurements. Conversion coefficients for transitions in101Nb 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 βq=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.

The deformedN=60 nucleus41101Nb

The β− decay of101Zr has been investigated at the fission-product separators JOSEF and LOHENGRIN. The half-life of101Zr has been determined to 2.5(1) s and a level scheme for101Nb has been established fromγ ray singles as well as X/3-γ and γ—γ coincidence measurements. Conversion coefficients for transitions in101Nb 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 βq=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.

Lifetime measurements and particle-rotor calculations for 42 105 Mo63

The half-lives of seven levels in105Mo have been determined with high precision at the fission-product separator Josef. Theβ-γ-γ triple coincidence method with plastic and BaF2 scintillators was used for a determination of the time between theβq feeding of a level of interest and itsγ depopulation. A value ofβq=0.38 (1) for the deformation parameter has been deduced from the half-lives of the first and second excited states. The results of Particle-Rotor model-calculations with the Nilsson parametersκ=0.084 andμ=0.28 support the assignment of the Nilsson orbital [532] 5/2− to the ground state of105Mo.

The deformation of103Nb

Tie half-lives of 8 low lying levels of103Nb have been determined at the fission-product separator JOSEF. A B-γ-γ triple-coincidence method was used which consists in a measurement of the time delay between the feeding of the levels through the B− decay of103Zr and their γ decay, in coincidence with a tagging γ ray, with plastic, BaF2 and Ge detectors, respectively. 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) of103Nb could be deduced from the half-life data. The half-lives are well reproduced through calculations in the frame of the Nilsson model.

Fast deuteron production in proton-nucleus interactions

Deuteron formation in proton-nucleus reactions due to the coalescence mechanism with final-state interaction between nucleons is discussed in order to establish the kinematical region of the validity of this approach. The experimental data on fast deuteron production in proton-nucleus collisions at bombarding energies above 3 GeV are analyzed. It is shown that most experimental results were obtained for deuteron momenta below the critical limit of the coalescence model. It is discussed to which extent further experimental studies could help to identify contributions from the coalescence mechanism and to get better understanding of the fast deuteron-production process.

The structure of the deformed nucleus 42 103 Mo61

The lifetimes of the three lowest excited members of the rotational ground-state band of103Mo 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 betweenβ particles from the decay of the parent103Nb and theγ rays which depopulate the levels. A plastic and a BaF2 detector have been used for theβ andγ 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 ofβq=0.34(1). This value is larger than that of the neighbouring even-even 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 parametersκ=0.084,μ=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 [411]3/2+ and [532]5/2− for these two bands, respectively, where the sideband is strongly affected by a [541]3/2− admixture. There is evidence for an additional band based on the 641.1 keV level for which the assignment [422]3/2+ is proposed.

On theβ− decay of the 2s isomer in98Y and the existence of an intruder band in98Zr

The γ radiation from the ß− decay of the high-spin isomer in98Y has been studied at the separator JOSEF. A main goal was to get insight into the spins of levels in98Zr which have been interpreted as members of an intruder band based on the 02+ state at 854 keV. The measuredγ-γ angular-correlations lead to the assignment of spins of 4 and 3 ℏ to the 2491 and 1844 keV levels, respectively, and contradict their interpretation as the 6+ and 4+ states of the proposed band. For the 2.0s isomer of98Y, the ß{−}-decay properties suggest I=5 with a major configuration [πg9/2,νs1/2(g27/2)0].

Structure of the Triplet of low-lying states in101Mo

The properties of the triplet of low-lying states in101Mo have been studied through spectroscopy of theγ radiation following thermal neutron capture in100Mo and β− decay of101Nb and through a measurement of the proton angular distributions in the100Mo(d,p) reaction with 14 MeV deuteron energy. The half-lives of the 13.5 keV state and the 57.0 keV 5/2+ state have been measured as 226(7) and 133(7)ns, respectively. These values and the quadrupole/dipole mixing ratios of the 13.5 keV and 43.5 keV transitions yield spin and parity 3/2+ for the 13.5 keV level. The E2 components in the 13.5 (3/2+ →1/2+) and 43.5 keV (5/2+→3/2+) transitions are ≦ 8·10−4 and 54(9)%, respectively. The possibility of an additional state near to the 57.0 keV level is discussed. IBFM/PTQM calculations, taking into consideration the transitional character of the100Mo boson core, account for the electromagnetic-transition and transfer-reaction pattern of the triplet of states.

Level Lifetimes in the PS Region: Spherical and Deformed Structures at A∼100

Level lifetimes in the range between about 10 ps and 1 ns have been determined for the heavy isotopes of Zr, Nb, and Mo at the fission‐product separator JOSEF. The results allow the identification of spherical configurations for the lighter isotopes and the determination of the deformation of the heavier ones. It could be shown that a rapid onset of deformation takes place at N=60 in the Nb chain since a deformation of βq=0.40(5) and 0.37(8) is deduced for 101Nb and 103Nb, respectively, while spherical shell‐model and particle‐phonon coupled states exist in 99Nb. Hence, the situation in the Nb chain is similar to that in neighbors with smaller Z. The closed shell character of 96Zr is confirmed by the half‐life of 127(10) ps of the 81+ level at 4390 keV. This half‐life is in accordance with the expectation for a π(g9/2)2 configuration and an effective charge eeff = 1.65(6)e.The energy of the state evinces a strong single particle gap at Z=40.