[en] Results of an exact treatment of the pairing transition from harmonic vibration to deformed rotation are compared to the results of the recently introduced ''critical point'' description. The exact results provide a microscopic justification for the assumptions used to find relevant analytic solutions of the collective pairing Hamiltonian. The energies and transition intensities from the two approaches are found to be in close agreement. Comparison to available data on the Pb isotopes suggests that ²⁰⁸Pb is very close to the critical-point of the pairing phase transition

[en] The STARS/LiBerACE collaboration has been exploring the surrogate technique with success in the actinide region. This method uses a direct reaction to measure the decay probability of the same compound nucleus produced via a neutron-induced channel. This paper serves as an overview of these activities. Using the STARS array at 88-inch Cyclotron at Lawrence Berkeley National Laboratory we have explored the following surrogate reactions: ²³⁴U(α, α(prime)f), ²³⁵U(³He, αf), ²³⁶U(α, α(prime)f), ²³⁸U (α,α(prime)f), ²³⁸U(³He,αf), ²³⁸U(³He, tf) surrogates for ²³³U(n,f), ²³³U(n,f), ²³⁵U(n,f), ²³⁷U(n,f), ²³⁶U(n,f), and ²³⁷Np(n,f), respectively

[en] This paper will briefly review the properties of M1 bands in the lead nuclei in the A∼200 mass region and discuss how these structures can be interpreted within the tilted axis cranking (TAC) model. Following this, the objectives are to compare the results of new Doppler shift attenuation method lifetimes for M1 (shears) bands in the light lead nuclei with the predictions of the TAC and Donau and Frauendorf models. Recent results for M1 bands in the A∼110 region will be shown and discussed in terms of the TAC model. In addition, the possibility of the existence of a new type of shears band based on E2 transitions will be presented. Lifetime measurements for such a structure in ¹¹⁰Cd will be discussed together with a comparison of the prediction of the TAC model. (c) 1999 American Institute of Physics

[en] This paper will briefly review the properties of M1 bands in the lead nuclei in the A∼200 mass region and discuss how these structures can be interpreted within the tilted axis cranking (TAC) model. Following this, the objectives are to compare the results of new Doppler shift attenuation method lifetimes for M1 (shears) bands in the light lead nuclei with the predictions of the TAC and Donau and Frauendorf models. Recent results for M1 bands in the A∼110 region will be shown and discussed in terms of the TAC model. In addition, the possibility of the existence of a new type of shears band based on E2 transitions will be presented. Lifetime measurements for such a structure in ¹¹⁰Cd will be discussed together with a comparison of the prediction of the TAC model

[en] The nucleus ⁶⁸Ge has been studied by gamma-ray spectroscopy following its population at high spin in the reaction ⁴⁰Ca(³²S,4p)⁶⁸Ge. The reaction channel was selected with the Microball array and gamma rays were detected with the Gammasphere array. The level scheme is very complex, reflecting the many different, and presumably mixed, excitation modes in this nucleus. Nevertheless, there appear to be some simplifications in the spin range above 18ℎ where we have identified a superdeformed band and several terminating bands. The results are compared with a cranked Nilsson-Strutinsky model without pairing

[en] High-spin states in the N=Z nucleus ⁷²Kr were populated using the ⁴⁰Ca(³⁶Ar,2p2n) reaction at a beam energy of 145 MeV. The yrast band has been observed up to a tentative spin of 20ℎ. Nonyrast rotational structures have also been observed for the first time. The alignment of g_9/2 protons and neutrons in the yrast band is observed to be significantly delayed relative to the heavier even-even Kr isotopes. Exact deformed cranked shell model calculations suggest that this could be due to the combined effect of isovector (T=1) and isoscalar (T=0) neutron-proton pairing correlations

[en] The intrinsic quadrupole moment Q₀ of superdeformed rotational bands in A∼150 nuclei depends on the associated single-particle configuration. We have derived an empirical formula based on the additivity of effective quadrupole moments of single-particle orbitals that describes existing measurements from ¹⁴²Sm to ¹⁵²Dy . To further test the formula, the predicted Q₀ moments for two superdeformed bands in ¹⁴⁶Gd of 14.05 eb were confronted with a new measurement yielding 13.9±0.4 eb and 13.9±0.3 eb , respectively. This excellent agreement provides empirical evidence of extreme single-particle behavior in highly deformed, collective systems

[en] The chiral-twin candidate bands recently observed in ¹³⁶Pm have been extended to high spins [I=(21)] using the Gammasphere γ-ray spectrometer and the Microball charged-particle detector array. A more-detailed spectroscopy of the bands was possible, where the rotational alignments and B(M1)/B(E2) ratios confirm that both sequences have the πh_11/2νh_11/2 configuration. Particle-rotor calculations of intraband and interband transition strength ratios of the chiral-twin bands are compared with experimental values for the first time. Good agreement was found between the predicted transition strength ratios and the experimental values, thus supporting the possible chiral nature of the πh_11/2νh_11/2 configuration in ¹³⁶Pm

[en] We present a semi-classical analysis of the B(M1) and B(E2) transition probabilities in the shears bands in ^193-199Pb as a function of the shears angle. This provides a semi-empirical confirmation of the shears mechanism proposed by S. Frauendorf using the Tilted-Axis-Cranking model. We interpret this as a consequence of a residual interaction between the proton and neutron blades, and it is shown that the main ingredient of this effective force can be described by a P₂(θ) term with a strength of 400-600 keV. Such an interaction can be mediated through the core by particle-vibration coupling. The competition between the shears and the rotation of the core is investigated within the framework of a classical solution of two particles-plus-rotor model

[en] We have studied the unresolved γ-ray spectrum of three ytterbium nuclei produced in heavy-ion fusion reactions and measured the two damping widths, Γ_μ, the spreading (compound-damping) width, and Γ_rot, the rotational damping width, together with P_nar, the probability that the population enters and leaves a given state via the same component of the wave function. Our results for Γ_μ and Γ_rot are in good agreement with theoretical expectations, as well as with those of other groups using different methods. Our results for P_nar lead to a new method for quantitative evaluation of the full order-to-chaos transition in these Yb nuclei. Finally, we provide the first evidence for the motional narrowing of Γ_rot in nuclei. The basic physics occurring in this region is level mixing, which our simulation technique reproduces surprisingly well