Odd-odd Systems Research Articles

Candidates for long-lived high-Kground states in superheavy nuclei

On the basis of systematic calculations for 1364 heavy and superheavy nuclei, including odd-systems, we have found a few candidates for high-K ground states in superheavy nuclei. The macroscopic-microscopic model based on the deformed Woods-Saxon single particle potential which we use offers a reasonable description of SH systems, including known: nuclear masses, $Q_{\alpha}$-values, fission barriers, ground state deformations, super- and hyper-deformed minima in the heaviest nuclei. %For odd and odd-odd systems, both ways of including pairing correlations, % blocking and the quasi-particle method, have been applied. Exceptionally untypical high-K intruder contents of the g.s. found for some nuclei accompanied by a sizable excitation of the parent configuration in daughter suggest a dramatic hindrance of the $\alpha$-decay. Multidimensional hyper-cube configuration - constrained calculations of the Potential Energy Surfaces (PES's) for one especially promising candidate, $^{272}$ Mt, shows a $\backsimeq$ 6 MeV increase in the fission barrier above the configuration- unconstrained barrier. There is a possibility, that one such high-K ground- or low-lying state may be the longest lived superheavy isotope.

Predictions for superheavy elements beyondZ=126

We study energy landscapes for selected superheavy even-even systems with $Z=128--148$ using both the microscopic-macroscopic Woods-Saxon and Skyrme SLy6 Hartree-Fock plus BCS models. Including nonaxial shapes proves crucial, as many apparent minima obtained when studying only axially symmetric shapes turn out to be saddle points, and triaxiality can drastically reduce fission barriers. Although both models often predict similar equilibria, only the SLy6 model predicts moderate stability in the $N\ensuremath{\gtrsim}228$ region, with barriers up to 4.5 MeV. A rough estimate suggests fission half-lives in the $\ensuremath{\mu}$s range; this would mean $\ensuremath{\sim}$ms halflives of odd and odd-odd systems. Nuclei from the $N\ensuremath{\approx}180$ region with predicted $\ensuremath{\approx}$4 MeV fission barriers have short $\ensuremath{\alpha}$ half-lives. There is no sign of stability for intermediate neutron numbers. As the SLy6 interaction is known to overestimate fission barriers, no long-lived nuclei are to be expected in the whole studied region. The shell corrections obtained in two models suggest a probable doubly magic system $Z=164$, $N=308$, with the half-life of 100 s estimated in the Woods-Saxon model.

Shell model study of pairing correlations

A systematic study of the pairing correlations as a function of temperature and angular momentum was performed in the sd-shell region using the spherical shell model approach. Pairing correlations were derived for even-even, even-odd, and odd-odd systems near $N=Z$ and also for the asymmetric case of $N=Z+4$. The results indicate that the pairing content and the behavior of pair correlations is similar in even-even and odd-mass nuclei. For the odd-odd $N=Z$ system, angular momentum $I=0$ state is an isospin, $t=1$ neutron-proton paired configuration. Further, these $t=1$ correlations are shown to be dramatically reduced for the asymmetric case of $N=Z+4$. The shell model results obtained are qualitatively explained within a simplified degenerate model.

Influence of the spin-orbit couplingon nuclear superfluidity along the N = Z line

We show that the spin-orbit potential of the nuclear mean field destroys isoscalar superfluid correlations in self-conjugate nuclei. Using group theory and boson mapping techniques on a Hamiltonian including single particle splittings and a SOST (8) pairing interaction, we give analytical expressions for the spin-orbit dependence of some N = Z properties such as the relative position of T = 0 and T = 1 states in odd-odd systems or double binding-energy differences of even-even nuclei.

Isoscalar and isovector pair correlations in an isospin-invariant pairing plus quadrupole model

Isovector and isoscalar correlations are investigated using an isospin-invariant pairing plus quadrupole model with inclusion of an isoscalar pairing force. An exact shell model calculation is carried out on the ${(g}_{9/2}{)}_{p}{(g}_{9/2}{)}_{n}$ shell, as a schematic example. We study isoscalar $(\ensuremath{\tau}=0)$ and isovector $(\ensuremath{\tau}=1)$ pair correlations in the ground state and excited states, and examine the interplay between isoscalar and isovector correlations. Under the single ${g}_{9/2}$ shell approximation, it is found that, for the $N=Z$ even-even system, the number of $\ensuremath{\tau}=1, J=8$ pairs increases drastically in a state with total spin $I=8$ along the yrast line. For $N=Z$ odd-odd systems, there is a crossing of an even-spin ground band by an odd-spin band at $I=1$ along the yrast line. The $\ensuremath{\tau}=0, J=9$ proton-neutron correlations increase suddenly at $I=7.$ This can be interpreted as the alignment of a proton-neutron pair. Finally, we examine the $J=0$ pair correlations in ground states of even-even and odd-odd systems.

Discrete line spectroscopy of Ga with AYEBALL

The excited states of the self-conjugate, odd-odd system, Ga have been investigated following the fusion-evaporation reaction at a beam energy of 65 MeV. Identification of candidate transitions in this nucleus were made using the AYEBALL -ray array following the selection of the recoil products with the Argonne fragment mass analyser (FMA) and a split anode ionization chamber. Spin assignments have been made on the basis of measured directional correlations from oriented states (DCO ratios). The resulting level scheme is presented and candidates for T = 1 and T = 0 structures suggested.

Cranking model with proton-neutron correlations

A cranking Hartree-Fock-BCS HFB model with T=0 and T=1 proton-neutron correlations is proposed and discussed. Numerical calculations are carried out on a single (g{sub 9/2}){sub p}(g{sub 9/2}){sub n} shell as a first practical try. It is found that the T=0 proton-neutron pairing correlation is crucial for a crossing of an even-spin ground band by an odd-spin band, a so-called T=1 and T=0 band crossing in an N=Z odd-odd system. A conventional spin alignment of a ground state band in an even-even system is also examined. A delay of this spin alignment due to a T=1, J=0 proton-neutron pairing correlation is found for nuclei with N=Z. {copyright} {ital 1998} {ital The American Physical Society}

COUNTING NUCLEON PAIRS

A simple method is used to count average numbers of different kinds of nucleon pairs within the Sp(4) seniority-like model which describesT=1 pairing modes of protons and neutrons. These numbers as well as the total strengths of pair-transfer processes clearly indicate that pn configurations are preferred in theNp=Nn odd-odd systems.

Microscopic approach to the U(6/4 x 4)

Relations between the ${\mathrm{Spin}}^{\mathrm{BFF}}$(6) symmetry Hamiltonian for odd-odd systems and the interacting boson-fermion-fermion model (IBFFM) Hamiltonian are investigated. An exact equivalence is established by slightly revising the exchange term of the IBFFM Hamiltonian. The ${\mathrm{Spin}}^{\mathrm{BFF}}$(6) symmetry is realized approximately in IBFFM by extending the quasiparticle configuration space for odd fermions. The proton-neutron residual interaction associated with the appearance of dynamical symmetry is compared to effective proton-neutron forces.

The competition between collective and single particle behaviour in the odd-odd isotopes 128,130La

Excited states in the odd-odd nuclei 128,130La have been investigated to high spin using gamma -ray spectroscopy. Two distinct bands are seen, one based on a pi h11/2(X) nu h11/2 structure and the other on a pi h11/2(X) nu g7/2 configuration. Backbends are observed in both configurations for both nuclei and the nature of the particles responsible can be conclusively proved. Experimental variation in the energy-level signature splitting and B(M1)/B(E2) ratios are discussed in terms of the polarising effects of the odd proton and odd neutron on the soft collective core. The results are discussed in terms of the cranked shell model, odd-odd particle-rotor calculations and total Routhian surface calculations. They are also compared to neighbouring odd and odd-odd systems.

Calculation of β−-decay half-lives with the proton-neutron quasiparticle RPA

Abstract The β decay half-lives of neutron-rich isotopes with Z = 24–28 are calculated in the QRPA with a Gamow-Teller residual interaction. For odd-mass and odd-odd systems QRPA phonon correlations are introduced to quasiparticle transitions in first-order perturbation. The calculated half-lives agree very well with the experimental values. For later application of this model to nuclei far from stability, we have examined the dependence of the calculated half-lives on the model parameters.

Spin(6) boson-fermion dynamical symmetry for odd-odd nuclei

Abstract Spin (6) boson-fermion dynamical symmetry is extended to odd-odd nuclei. An algorithm is constructed for a subsequent reduction appropriate for the boson-fermion group chain associated with odd protons and neutrons in j = 3 2 configurations and with SO(6) boson symmetry of the core. The corresponding energy formula for the odd-odd system is constructed and discussed. The possibility of generalization to extended supersymmetry encompassing even-even, odd-even and odd-odd nuclei is pointed out.

Anomalous ground-state configurations in some odd-odd deformed nuclei

The ground-state configuration in certain odd-odd nuclei is found to differ from that expected from the superposition of the ground states in the neighbouring odd-A nuclei. Such anomalies are pointed out for a few nuclei in both the rare-earth and actinide regions for which either the proton or the neutron configurations for the ground state in the odd-odd system correspond to that of the excited state in the respective odd-A system. The origin of this anomaly is attributed to an unusual contribution from the residual n-p interaction term to the band-head energy and the results of specific calculations in a parameter-independent formulation which satisfactorily explain the observed re-ordering of the two-quasiparticle states in the spectra of odd-odd nuclei.