Searcharxiv⌕ Search

arXiv subjects

W. Scheid

Publications and source records attributed to W. Scheid.

At least 19 recordsLinked to original sources
arXiv2016

Application of the triaxial quadrupole-octupole rotor to the ground and negative-parity levels of actinide nuclei

In this work we examine the possibility to describe yrast positive- and negative-parity excitations of deformed even-even nuclei through a collective rotation model in which the nuclear surface is characterized by triaxial quadrupole and octupole deformations. The nuclear moments of inertia are expressed as sums of quadrupole and octupole parts. By assuming an adiabatic separation of rotation and vibration degrees of freedom we suppose that the structure of the positive- and negative- parity bands may be determined by the triaxial-rigid-rotor motion of the nucleus. By diagonalizing the Hamiltonian in a symmetrized rotor basis with embedded parity we obtain a model description for the yrast positive- and negative-parity bands in several actinide nuclei. We show that the energy displacement between the opposite-parity sequences can be explained as the result of the quadrupole-octupole triaxiality.

nucl-th↗
arXiv2014

Why does neutron transfer play different roles in sub-barrier fusion reactions $^{32}$S+$^{94,96}$Zr and $^{40}$Ca+$^{94,96}$Zr?

The sub-barrier capture (fusion) reactions $^{32}$S+$^{90,94,96}$Zr, $^{36}$S+$^{90,96}$Zr, $^{40}$Ca+$^{90,94,96}$Zr, and $^{48}$Ca+$^{90,96}$Zr with positive and negative $Q$-values for neutron transfer are studied within the quantum diffusion approach and the universal fusion function representation. For these systems, the s-wave capture probabilities are extracted from the experimental excitation functions and are also analyzed. Different effects of the positive $Q_{xn}$-value neutron transfer in the fusion enhancement are revealed in the relatively close reactions $^{32}$S+$^{94,96}$Zr and $^{40}$Ca+$^{94,96}$Zr.

nucl-th↗
arXiv2013

Non-yrast spectra of odd-A nuclei in a model of coherent quadrupole-octupole motion

The model of coherent quadrupole and octupole motion (CQOM) is applied to describe non-yrast split parity-doublet spectra in odd-mass nuclei. The yrast levels are described as low-energy rotation-vibration modes coupled to the ground single-particle (s.p.) state, while the non-yrast parity-doublet structures are obtained as higher-energy rotation-vibration modes coupled to excited s.p. states. It is shown that the extended model scheme describes both the yrast and non-yrast quasi parity-doublet spectra and the related B(E1) and B(E2) transition rates in different regions of heavy odd-A nuclei. The involvement of the reflection-asymmetric deformed shell model to describe the single-particle motion and the Coriolis interaction on a deeper level is discussed.

nucl-th↗
arXiv2013

Fusion at near-barrier energies within quantum diffusion approach

The nuclear deformation and neutron-transfer process have been identified as playing a major role in the magnitude of the sub-barrier fusion (capture) cross sections. There are a several experimental evidences which confirm the importance of nuclear deformation on the fusion. The influence of nuclear deformation is straightforward. If the target nucleus is prolate in the ground state, the Coulomb field on its tips is lower than on its sides, that then increases the capture or fusion probability at energies below the barrier corresponding to the spherical nuclei. The role of neutron transfer reactions is less clear. The importance of neutron transfer with positive Q-values on nuclear fusion (capture) originates from the fact that neutrons are insensitive to the Coulomb barrier and therefore they can start being transferred at larger separations before the projectile is captured by target-nucleus. Therefore, it is generally thought that the sub-barrier fusion cross section will increase because of the neutron transfer. The fusion (capture) dynamics induced by loosely bound radioactive ion beams is currently being extensively studied. However, the long-standing question whether fusion (capture) is enhanced or suppressed with these beams has not yet been answered unambiguously. The study of the fusion reactions involving nuclei at the drip-lines has led to contradictory results.

nucl-th↗
arXiv2013

Sub-barrier capture reactions with $^{16,18}$O and $^{40,48}$Ca beams

Various sub-barrier capture reactions with beams $^{16,18}$O and $^{40,48}$Ca are treated within the quantum diffusion approach. The role of neutron transfer in these capture reactions is discussed. The quasielastic and capture barrier distributions are analyzed and compared with the recent experimental data.

nucl-th↗
arXiv2012

Search of systematic behavior of breakup probability in reactions with weakly bound projectiles at energies around Coulomb barrier

Comparing the capture cross sections calculated without the breakup effect and experimental complete fusion cross sections, the breakup was analyzed in reactions with weakly bound projectiles $^{6,7,9}$Li, $^{9,11}$Be, and $^{6,8}$He. A trend of a systematic behavior for the complete fusion suppression as a function of the target charge and bombarding energy is not achieved. The quasielastic backscattering is suggested to be an useful tool to study the behavior of the breakup probability in reactions with weakly bound projectiles.

nucl-th↗
arXiv2012

Threshold energy for sub-barrier fusion hindrance phenomenon

The relationship between the threshold energy for a deep sub-barrier fusion hindrance phenomenon and the energy at which the regime of interaction changes (the turning-off of the nuclear forces and friction) in the sub-barrier capture process, is studied within the quantum diffusion approach. The quasielastic barrier distribution is shown to be a useful tool to clarify whether the slope of capture cross section changes at sub-barrier energies.

nucl-th↗
arXiv2012

Quasifission at extreme sub-barrier energies

With the quantum diffusion approach the behavior of the capture cross-section is investigated in the reactions $^{92,94}$Mo + $^{92,94}$Mo, $^{100}$Ru + $^{100}$Ru, $^{104}$Pd + $^{104}$Pd, and $^{78}$Kr + $^{112}$Sn at deep sub-barrier energies which are lower than the ground state energies of the compound nuclei. Because the capture cross section is the sum of the complete fusion and quasifission cross sections, and the complete fusion cross section is zero at these sub-barrier energies, one can study experimentally the unique quasifission process in these reactions after the capture.

nucl-th↗
arXiv2012

Non-yrast nuclear spectra in a model of coherent quadrupole-octupole motion

A model assuming coherent quadrupole-octupole vibrations and rotations is applied to describe non-yrast energy sequences with alternating parity in several even-even nuclei from different regions, namely $^{152,154}$Sm, $^{154,156,158}$Gd, $^{236}$U and $^{100}$Mo. Within the model scheme the yrast alternating-parity band is composed by the members of the ground-state band and the lowest negative-parity levels with odd angular momenta. The non-yrast alternating-parity sequences unite levels of $\beta$-bands with higher negative-parity levels. The model description reproduces the structure of the considered alternating-parity spectra together with the observed B(E1), B(E2) and B(E3) transition probabilities within and between the different level-sequences. B(E1) and B(E3) reduced probabilities for transitions connecting states with opposite parity in the non-yrast alternating-parity bands are predicted. The implemented study outlines the limits of the considered band-coupling scheme and provides estimations about the collective energy potential which governs the quadrupole-octupole properties of the considered nuclei.

nucl-th↗
arXiv2011

Structure effects in the region of superheavy elements via the $\alpha$-decay chain of $^{293}$118

The $\alpha$-decay chain of $^{293}$118, first proposed in the Berkeley cold fusion experiment $^{208}$Pb($^{86}$Kr,1n) and now retracted, is calculated by using the preformed cluster model (PCM) of one of us (RKG). Also, the possible branchings of $\alpha$-particles to heavier cluster decays of all the parents in this chain are calculated for the first time. The calculated Q-values, penetrabilities and preformation factors for $\alpha$-decays suggest that the $^{285}$114 nucleus with Z=114, N=171 is a magic nucleus, either due to the magicity of Z=114, or of N=172 or of both. The N=172 is proposed to be a magic number in certain relativistic mean-field calculations, but with Z=120. The calculated cluster decays point to new interesting possibilities of $^{14}$C decay of the $^{281}$112 parent, giving rise to a (reasonably) deformed Z=106, N=161, $^{267}$106 daughter (N=162 being now established as the deformed magic shell) or to a doubly magic $^{48}$Ca cluster emitted from any of the parent nucleus in the $\alpha$-decay chain. Apparently, these are exciting new directions for future experiments.

nucl-th↗
arXiv2011

The formation and decay of superheavy nuclei produced in $^{48}Ca$-induced reactions

The formation of superheavy nuclei in $^{48}Ca+^{232}Th$, $^{238}U$, $^{242,244}Pu$ and $^{248}Cm$ reactions and their subsequent decay are studied within the quantum mechanical fragmentation theory (QMFT) and the QMFT based preformed cluster-decay model (PCM) of Gupta and collaborators. According to QMFT, all these $^{48}Ca$-induced reactions are cold fusion reactions with relative excitation energies larger than for the $Pb$-induced cold fusion reactions and smaller than for the lighter beam i.e. $Mg$, $Si$ or $S$-induced hot fusion reactions. The same reactions were first suggested by Gupta et al. in 1977 on the basis of QMFT, and this study re-establishes the same result. In fact, for such heavy isotopes of Z=110 to 116, $^{50}Ca$ is shown to be a better beam for cold fusion, but $^{50}Ca$ is a radioactive nucleus. The $\alpha$-decay half-lives of these nuclei after 3n and/ or 4n evaporations, i.e. of the evaporation residues of these compound systems, calculated on PCM compare reasonably well with experiments published by Dubna group and another recent calculation. As expected for such rare decays, PCM calculations show that the $\alpha$-preformation factors are small, $\sim 10^{-8}$ to $10^{-10}$. The possible competition of $\alpha$-decays with heavy cluster emissions from these superheavy nuclei is also probed from the point of view of searching for new nuclear structure information and possible future experiments with such exotic nuclei. The decay half-lives for some clusters are in fact shown to be lower than the limits of experiments for nuclei with enough available atoms.

nucl-th↗
arXiv2011

Correlation between muonic levels and nuclear structure in muonic atoms

A method that deals with the nucleons and the muon unitedly is employed to investigate the muonic lead, with which the correlation between the muon and nucleus can be studied distinctly. A "kink" appears in the muonic isotope shift at a neutron magic number where the nuclear shell structure plays a key role. This behavior may have very important implications for the experimentally probing the shell structure of the nuclei far away from the $\beta$-stable line. We investigate the variations of the nuclear structure due to the interaction with the muon in the muonic atom and find that the nuclear structure remains basically unaltered. Therefore, the muon is a clean and reliable probe for studying the nuclear structure. In addition, a correction that the muon-induced slight change in the proton density distribution in turn shifts the muonic levels is investigated. This correction to muonic level is as important as the Lamb shift and high order vacuum polarization correction, but is larger than anomalous magnetic moment and electron shielding correction.

nucl-th↗
arXiv2011

Effects of nuclear deformation and neutron transfer in capture process, and origin of fusion hindrance at deep sub-barrier energies

The roles of nuclear deformation and neutron transfer in sub-barrier capture process are studied within the quantum diffusion approach. The change of the deformations of colliding nuclei with neutron exchange can crucially influence the sub-barrier fusion. The comparison of the calculated capture cross section and the measured fusion cross section in various reactions at extreme sub- barrier energies gives us information about the fusion and quasifission.

nucl-th↗
arXiv2011

Collinear cluster tripartition as sequential binary fission in the $^{235}$U(n$_{\rm th}$,f) reaction

The mechanism leading to the formation of the observed products of the collinear cluster tripartition is carried out within the framework of the model based on the dinuclear system concept. The yield of fission products is calculated using the statistical model based on the driving potentials for the fissionable system. The minima of potential energy of the decaying system correspond to the charge numbers of the products which are produced with large probabilities in the sequential fission (partial case of the collinear cluster tripartition) of the compound nucleus. The realization of this mechanism supposes the asymmetric fission channel as the first stage of sequential mechanism. It is shown that only the use of the driving potential calculated by the binding energies with the shell correction allows us to explain the yield of the true ternary fission products. The theoretical model is applied to research collinear cluster tripartition in the reaction $^{235}$U(n$_{\rm th}$,f). Calculations showed that in the first stage of this fission reaction, the isotopes $^{82}$Ge and $^{154}$Nd are formed with relatively large probabilities and in the second stage of sequential fission of the isotope Nd mainly Ni and Ge are formed. This is in agreement with the yield of the isotope $^{68}$Ni which is observed as the product of the collinear cluster tripartition in the experiment.

nucl-th↗
arXiv2011

Sub-barrier capture with quantum diffusion approach

With the quantum diffusion approach the behavior of capture cross sections and mean-square angular momenta of captured systems are revealed in the reactions with deformed and spherical nuclei at sub-barrier energies. With decreasing bombarding energy under the barrier the external turning point of the nucleus-nucleus potential leaves the region of short-range nuclear interaction and action of friction. Because of this change of the regime of interaction, an unexpected enhancement of the capture cross section is found at bombarding energies far below the Coulomb barrier. This effect is shown its worth in the dependence of mean-square angular momentum on the bombarding energy. From the comparison of calculated capture cross sections and experimental capture or fusion cross sections the importance of quasifission near the entrance channel is demonstrated for the actinidebased reactions and reactions with medium-heavy nuclei at extreme sub-barrier energies.

nucl-th↗
arXiv2011

Sub-barrier capture with quantum diffusion approach: actinide-based reactions

With the quantum diffusion approach the behavior of capture cross sections and mean-square angular momenta of captured systems are revealed in the reactions with deformed nuclei at subbarrier energies. The calculated results are in a good agreement with existing experimental data. With decreasing bombarding energy under the barrier the external turning point of the nucleusnucleus potential leaves the region of short-range nuclear interaction and action of friction. Because of this change of the regime of interaction, an unexpected enhancement of the capture cross section is expected at bombarding energies far below the Coulomb barrier. This effect is shown its worth in the dependence of mean-square angular momentum of captured system on the bombarding energy. From the comparison of calculated and experimental capture cross sections, the importance of quasifission near the entrance channel is shown for the actinide-based reactions leading to superheavy nuclei.

nucl-th↗