arXiv subjects

R. Wyss

Publications and source records attributed to R. Wyss.

At least 37 records · Page 2Linked to original sources

arXiv2009

Microscopic-Macroscopic Approach for Binding Energies with Wigner-Kirkwood Method

The semi-classical Wigner-Kirkwood $\hbar$ expansion method is used to calculate shell corrections for spherical and deformed nuclei. The expansion is carried out up to fourth order in $\hbar$. A systematic study of Wigner-Kirkwood averaged energies is presented as a function of the deformation degrees of freedom. The shell corrections, along with the pairing energies obtained by using the Lipkin-Nogami scheme, are used in the microscopic-macroscopic approach to calculate binding energies. The macroscopic part is obtained from a liquid drop formula with six adjustable parameters. Considering a set of 367 spherical nuclei, the liquid drop parameters are adjusted to reproduce the experimental binding energies, which yields a {\it rms} deviation of 630 keV. It is shown that the proposed approach is indeed promising for the prediction of nuclear masses.

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arXiv2007

Critical Survey of Isoscalar and Isovector Contributions to the Spin Orbit Potential in Relativistic Mean Field Theory

The spin-orbit (SO) interaction, emerging naturally from the Relativistic Mean Field (RMF) theory is examined critically in the light of the recently measured excitation energy differences between the terminating states built on two different configurations for nuclei belonging to the lower pf shell. The calculations are carried out using the cranked RMF framework. To further probe the iso-vector dependence of the spin-orbit potential, the energy spacing between the g_{7/2} and h_{11/2} states in the Sb-chain is compared to experiment. It is found that the calculation at the quantitative level deviates strongly from the experiment. In particular the balance of the iso-scalar and iso-vector strengths of the effective one body SO potential indicates that additional terms like tensor couplings may be needed to account for the experimental data.

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arXiv2006

Systematics of proton emission

A very simple formula is presented that relates the logarithm of the half-life, corrected by the centrifugal barrier, with the Coulomb parameter in proton decay processes. The corresponding experimental data lie on two straight lines which appear as a result of a sudden change in the nuclear shape marking two regions of deformation independently of the angular momentum of the outgoing proton. This feature provides a powerful tool to assign experimentally quantum numbers in proton emitters.

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arXiv2005

Mass number dependence of the Skyrme-force-induced nuclear symmetry energy

The global mass dependence of the nuclear symmetry energy and its two basic ingredients due to the mean-level spacing and effective strength of the isovector mean-potential is studied within the Skyrme-Hartree-Fock model. In particular, our study determines the ratio of the surface-to-volume contributions to the nuclear symmetry energy to be ~1.6 and reveals that contributions due to mean-level spacing and effective strength of the isovector mean-potential are almost equal after removing momentum-dependent effects by rescaling them with isoscalar and isovector effective masses, respectively.

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arXiv2005

A Complex Shell Model Representation Including Antibound States

A generalization of the Complex Shell Model formalism is presented which includes antibound states in the basis. These states, together with bound states, Gamow states, and the continuum background, represented by properly chosen scattering waves, form a representation where all states are treated on the same footing. Two-particle states are evaluated within this formalism and observable two-particle resonances are defined. The formalism is illustrated in the well known case of $^{11}$Li in its bound ground state and in $^{70}$Ca(gs), which is also bound. Both cases are found to have a halo structure. These halo structures are described within the generalized Complex Shell Model. We investigated the formation of two-particle resonances in these nuclei, but no evidence of such resonances was found.

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arXiv2004

Enhanced Stability of Superheavy Nuclei due to High-Spin Isomerism

Configuration-constrained calculations of potential-energy surfaces in even-even superheavy nuclei reveal systematically the existence at low excitation energies of multi-quasiparticle states with deformed axially symmetric shapes and large angular momenta. These results indicate the prevalence of long-lived, multi-quasiparticle isomers. In a quantal system, the ground state is usually more stable than the excited states. In contrast, in superheavy nuclei the multi-qausiparticle excitations decrease the probability for both fission and $α$ decay, implying enhanced stability. Hence, the systematic occurrence of multi-qausiparticle isomers may become crucial for future production and study of even heavier nuclei. The energies of multi-quasiparticle states and their $α$ decays are calculated and compared to available data.

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arXiv2004

Alpha Decay Hindrance Factors: A Probe of Mean Field Wave Functions

A simple model to calculate alpha-decay Hindrance Factors is presented. Using deformation values obtained from PES calculations as the only input, Hindrance Factors for the alpha-decay of Rn- and Po-isotopes are calculated. It is found that the intrinsic structure around the Fermi surface determined by the deformed mean field plays an important role in determining the hindrance of alpha-decay. The fair agreement between experimental and theoretical Hindrance Factors suggest that the wave function obtained from the energy minima of the PES calculations contains an important part of the correlations that play a role for the alpha-decay. The calculated HF that emerges from these calculations render a different interpretation than the commonly assumed n-particle n-hole picture.

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arXiv2002

Cranking in Isospace

The response of isovector and isoscalar pairing to generalized rotation in isospace is studied. Analytical expressions for non-selfconsistent solutions for different limiting cases of the model are derived. In particular, the connections between gauge relations among pairing gaps and the position of the isocranking axis are investigated in $N$=$Z$ nuclei. The two domains of collective and non-collective rotation in space are generalized to isospace. The amplitudes for pair-transfer of T=0 and T=1 pairs are also calculated. It is shown that the structure of the T=0 state in odd-odd nuclei prevents any enhancement of pairing transfer also in the presence of strong T=0 correlations. The energy differences of the T=0 and T=1 excitations in odd-odd nuclei are qualitatively reproduced by Total Routhian Surface calculations.

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arXiv2002

High Spin Proton Emitters in Odd-Odd Nuclei and Shape Changes

We present a formalism to describe proton emission from odd-odd nuclei based on a scattering like approach. Special emphasis is given to the case of transitions between states with different deformations. As an example we estimate the proton half-life of the odd-odd nucleus $^{58}$Cu. Our calculations show that the change of deformation in the decay process has a significant influence on the half-life. In addition, the angular momentum coupling of proton and neutron orbitals can result in an important $K$-hindrance of the decay. To fully account for the observed half-life in $^{58}$Cu, one has to consider a shape mixing in the final state.

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arXiv2002

Comments on the nuclear symmetry energy

According to standard textbooks, the nuclear symmetry energy originates from the {\it kinetic} energy and the {\it interaction} itself. We argue that this view requires certain modifications. We ascribe the physical origin of the {\it kinetic} term to the discreteness of fermionic levels of, in principle arbitrary binary fermionic systems, and relate its mean value directly to the average level density. Physically it connects this part also to the isoscalar part of the interaction which, at least in self-bound systems like atomic nuclei, decides upon the spatial dimensions of the system. For the general case of binary fermionic systems possible external confining potentials as well as specific boundary conditions will contribute to this part. The reliability of this concept is tested using self-consistent Skyrme Hartree-Fock calculations.

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arXiv2002

The T=0 neutron-proton pairing correlations in the superdeformed rotational bands around 60Zn

The superdeformed bands in 58Cu, 59Cu, 60Zn, and 61Zn are analyzed within the frameworks of the Skyrme-Hartree-Fock as well as Strutinsky-Woods-Saxon total routhian surface methods with and without the T=1 pairing correlations. It is shown that a consistent description within these standard approaches cannot be achieved. A T=0 neutron-proton pairing configuration mixing of signature-separated bands in 60Zn is suggested as a possible solution to the problem.

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arXiv2000

Rotations in isospace: a doorway to the understanding of neutron-proton superfluidity in N=Z nuclei

The $T$=2 excitations in even-even $N$=$Z$ nuclei are calculated within the isospin cranked mean-field approach. The response of pairing correlations to rotation in isospace is investigated. It is shown that whereas the isovector pairing rather modestly modifies the single-particle moment of inertia in isospace, the isoscalar pairing strongly reduces its value. This reduction of the moments of inertia in isospace with respect to its rigid body value is a strong indicator of collective isoscalar pairing correlations. Beautiful analogies between the role of isovector pairing for the case of spatial rotations and the role of isoscalar pairing for the case of iso-rotations are underlined.

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arXiv2000

Isovector and isoscalar superfluid phases in rotating nuclei

The subtle interplay between the two nuclear superfluids, isovector T=1 and isoscalar T=0 phases, are investigated in an exactly soluble model. It is shown that T=1 and T=0 pair-modes decouple in the exact calculations with the T=1 pair-energy being independent of the T=0 pair-strength and vice-versa. In the rotating-field, the isoscalar correlations remain constant in contrast to the well known quenching of isovector pairing. An increase of the isoscalar (J=1, T=0) pair-field results in a delay of the bandcrossing frequency. This behaviour is shown to be present only near the N=Z line and its experimental confirmation would imply a strong signature for isoscalar pairing collectivity. The solutions of the exact model are also discussed in the Hartree-Fock-Bogoliubov approximation.

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arXiv2000

A number projected model with generalized pairing interaction

A mean-field model with a generalized pairing interaction that accounts for neutron-proton pairing is presented. Both the BCS as well as number-projected solutions of the model are presented. For the latter case the Lipkin-Nogami projection technique was extended to encompass the case of non-separable proton-neutron systems. The influence of the projection on various pairing phases is discussed. In particular, it is shown that number-projection allows for mixing of different pairing phases but, simultanously, acts destructively on the proton-neutron correlations. The basic implications of proton-neutron pairing correlations on nuclear masses are discussed. It is shown that these correlations may provide a natural microscopic explanation of the Wigner energy lacking in mean-field models. A possible phase transition from isovector to isoscalar pairing condensate at high angular momenta is also discussed. In particular predictions for the dynamical moments of inertia for the superdeformed band in $^{88}$Ru are given.

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arXiv1999

Quadrupole Pairing Interaction and Signature Inversion

The signature inversion in the πh11/2 \otimes νh11/2 rotational bands of odd-odd Cs and La isotopes and the πh11/2 \otimes νi13/2 bands of odd-odd Tb, Ho and Tm nuclei is investigated using pairing and deformation self consistent mean field calculations. The model can rather satisfactorily account for the anomalous signature splitting, provided that spin assignments in som of the bands are revised. Our calculations show that signature inversioncan appear already at axially symmetric shapes. It is found that this is due to the contribution of the λμ=22 component of the quadrupole pairing interaction to the mean field potential.

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arXiv1999

T=0 Neutron-Proton Correlations at High Angular Momenta

The properties of T=0 neutron-proton correlations are discussed within the frame work of different model calculations. Single-j shell calculations reveal that the T=0 correlations remain up to the highest frequencies. They are more complex and cannot be restricted to L=0 pairs only. Whereas it may be difficult to find clear evidence for T=0 pairing at low spins, T=0 correlations are found to induce a new excitation scheme at high angular momenta.

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arXiv1999

Shape and blocking effects on odd-even mass differences and rotational motion of nuclei

Nuclear shapes and odd-nucleon blockings strongly influence the odd-even differences of nuclear masses. When such effects are taken into account, the determination of the pairing strength is modified resulting in larger pair gaps. The modified pairing strength leads to an improved self-consistent description of moments of inertia and backbending frequencies, with no additional parameters.

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arXiv1998

Onset of T=0 Pairing and Deformations in High Spin States of the N=Z Nucleus 48Cr

The yrast line of the N=Z nucleus 48Cr is studied up to high spins by means of the cranked Hartree-Fock-Bogoliubov method including the T=0 and T=1 isospin pairing channels. A Skyrme force is used in the mean-field channel together with a zero-range density-dependent interaction in the pairing channels. The extensions of the method needed to incorporate the neutron-proton pairing are summarized. The T=0 pairing correlations are found to play a decisive role for deformation properties and excitation energies above 16hbar which is the maximum spin that can be obtained in the f7/2 subshell.

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