1,558 research outputs found
Suppression of complete fusion due to breakup in the reactions B + Bi
Above-barrier cross sections of -active heavy reaction products, as
well as fission, were measured for the reactions of B with
Bi. Detailed analysis showed that the heavy products include components
from incomplete fusion as well as complete fusion (CF), but fission originates
almost exclusively from CF. Compared with fusion calculations without breakup,
the CF cross sections are suppressed by 15% for B and 7% for B. A
consistent and systematic variation of the suppression of CF for reactions of
the weakly bound nuclei Li, Be, B on targets of
Pb and Bi is found as a function of the breakup threshold
energy
Relating breakup and incomplete fusion of weakly-bound nuclei through a classical trajectory model with stochastic breakup
A classical dynamical model that treats break-up stochastically is presented
for low energy reactions of weakly-bound nuclei. The three-dimensional model
allows a consistent calculation of breakup, incomplete and complete fusion
cross sections. The model is assessed by comparing the breakup observables with
CDCC quantum mechanical predictions, which are found to be in reasonable
agreement. Through the model, it is demonstrated that the breakup probability
of the projectile as a function of its distance from the target is of primary
importance for understanding complete and incomplete fusion at energies near
the Coulomb barrier.Comment: Accepted in Physical Review Letter
Influence of entrance-channel magicity and isospin on quasi-fission
The role of spherical quantum shells in the competition between fusion and
quasi-fission is studied for reactions forming heavy elements. Measurements of
fission fragment mass distributions for different reactions leading to similar
compound nuclei have been made near the fusion barrier. In general, more
quasi-fission is observed for reactions with non-magic nuclei. However, the
Ca+Pb reaction is an exception, showing strong evidence for
quasi-fission, though both nuclei are doubly magic. Time-dependent Hartree-Fock
calculations predict fast equilibration of in the two fragments early in
the collision. This transfer of nucleons breaks the shell effect, causing this
reaction to behave more like a non-magic one in the competition between fusion
and quasi-fission. Future measurements of fission in reactions with exotic
beams should be able to test this idea with larger asymmetries.Comment: accepted for publication in Physics Letters
Novel insights into transfer processes in the reaction 16O+208Pb at sub-barrier energies
The collision of the doubly-magic nuclei O+Pb is a benchmark
in nuclear reaction studies. Our new measurements of back-scattered
projectile-like fragments at sub-barrier energies show show that transfer of 2
protons () is much more probable than -particle transfer.
transfer probabilities are strongly enhanced compared to expectations for the
sequential transfer of two uncorrelated protons; at energies around the fusion
barrier absolute probabilities for two proton transfer are similar to those for
one proton transfer. This strong enhancement indicates strong pairing
correlations in O, and suggests evidence for the occurrence of a nuclear
supercurrent of two-proton Cooper pairs in this reaction, already at energies
well below the fusion barrier.Comment: 5 pages, 3 figure
Importance of Non-Linear Couplings in Fusion Barrier Distributions and Mean Angular Momenta
The effects of higher order coupling of surface vibrations to the relative
motion on heavy-ion fusion reactions at near-barrier energies are investigated.
The coupled channels equations are solved to all orders, and also in the linear
and the quadratic coupling approximations. It is shown that the shape of fusion
barrier distributions and the energy dependence of the average angular momentum
of the compound nucleus can significantly change when the higher order
couplings are included. The role of octupole vibrational excitation of ^{16}O
in the ^{16}O + ^{144}Sm fusion reaction is also discussed using the all order
coupled-channels equations.Comment: 8 pages, 6 figures, To be published in the Proceedings of the FUSION
97 Conference, South Durras, Australia, March 1997 (J. Phys. G
Atomic spectral-product representations of molecular electronic structure: metric matrices and atomic-product composition of molecular eigenfunctions
Recent progress is reported in development of ab initio computational methods for the electronic structures of molecules employing the many-electron eigenstates of constituent atoms in spectral-product forms. The approach provides a universal atomic-product description of the electronic structure of matter as an alternative to more commonly employed valence-bond- or molecular-orbital-based representations. The Hamiltonian matrix in this representation is seen to comprise a sum over atomic energies and a pairwise sum over Coulombic interaction terms that depend only on the separations of the individual atomic pairs. Overall electron antisymmetry can be enforced by unitary transformation when appropriate, rather than as a possibly encumbering or unnecessary global constraint. The matrix representative of the antisymmetrizer in the spectral-product basis, which is equivalent to the metric matrix of the corresponding explicitly antisymmetric basis, provides the required transformation to antisymmetric or linearly independent states after Hamiltonian evaluation. Particular attention is focused in the present report on properties of the metric matrix and on the atomic-product compositions of molecular eigenstates as described in the spectral-product representations. Illustrative calculations are reported for simple but prototypically important diatomic (H_2, CH) and triatomic (H_3, CH_2) molecules employing algorithms and computer codes devised recently for this purpose. This particular implementation of the approach combines Slater-orbital-based one- and two-electron integral evaluations, valence-bond constructions of standard tableau functions and matrices, and transformations to atomic eigenstate-product representations. The calculated metric matrices and corresponding potential energy surfaces obtained in this way elucidate a number of aspects of the spectral-product development, including the nature of closure in the representation, the general redundancy or linear dependence of its explicitly antisymmetrized form, the convergence of the apparently disparate atomic-product and explicitly antisymmetrized atomic-product forms to a common invariant subspace, and the nature of a chemical bonding descriptor provided by the atomic-product compositions of molecular eigenstates. Concluding remarks indicate additional studies in progress and the prognosis for performing atomic spectral-product calculations more generally and efficiently
An evaporation-based model of thermal neutron induced ternary fission of plutonium
Ternary fission probabilities for thermal neutron induced fission of
plutonium are analyzed within the framework of an evaporation-based model where
the complexity of time-varying potentials, associated with the neck collapse,
are included in a simplistic fashion. If the nuclear temperature at scission
and the fission-neck-collapse time are assumed to be ~1.2 MeV and ~10^-22 s,
respectively, then calculated relative probabilities of ternary-fission
light-charged-particle emission follow the trends seen in the experimental
data. The ability of this model to reproduce ternary fission probabilities
spanning seven orders of magnitude for a wide range of light-particle charges
and masses implies that ternary fission is caused by the coupling of an
evaporation-like process with the rapid re-arrangement of the nuclear fluid
following scission.Comment: 25 pages, 12 figures, accepted for publication in IJMP
A practical guide for the study of human and murine sebaceous glands in situ
The skin of most mammals is characterised by the presence of sebaceous glands (SGs), whose predominant constituent cell population is sebocytes, that is, lipid-producing epithelial cells, which develop from the hair follicle. Besides holocrine sebum production (which contributes 90% of skin surface lipids), multiple additional SG functions have emerged. These range from antimicrobial peptide production and immunomodulation, via lipid and hormone synthesis/metabolism, to the provision of an epithelial progenitor cell reservoir. Therefore, in addition to its involvement in common skin diseases (e.g. acne vulgaris), the unfolding diversity of SG functions, both in skin health and disease, has raised interest in this integral component of the pilosebaceous unit. This practical guide provides an introduction to SG biology and to relevant SG histochemical and immunohistochemical techniques, with emphasis placed on in situ evaluation methods that can be easily employed. We propose a range of simple, established markers, which are particularly instructive when addressing specific SG research questions in the two most commonly investigated species in SG research, humans and mice. To facilitate the development of reproducible analysis techniques for the in situ evaluation of SGs, this methods review concludes by suggesting quantitative (immuno-)histomorphometric methods for standardised SG evaluation
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