313 research outputs found
Towards exotic nuclei via binary reaction mechanism
Assuming a binary reaction mechanism, the yield of isotopes near the heaviest
neutron-deficit nucleus Sn is studied with a microscopic
transport model. The large influence of nuclear shell structure and isotope
composition of the colliding nuclei on the production of exotic nuclei is
demonstrated. It is shown that the reaction Fe+Cd seems to be
most favourable for producing primary exotic Sn isotopes which may survive if
the excitation energy in the entrance reaction channel is less than about 100
MeV. In the case of large differences in the charge (mass) numbers between
entrance and exit channels the light fragment yield is essentially fed from the
decay of excited primary heavier fragments. The existence of optimal energies
for the production of some oxygen isotopes in the binary mechanism is
demonstrated for the S+Au reaction.Comment: 17 pages, RevTex, 8 Postscript figures, submitted to Phys. Rev.
Production of exotic isotopes in complete fusion reactions with radioactive beams
The isotopic dependence of the complete fusion (capture) cross section is
analyzed in the reactions
Xe+Ca with stable and
radioactive beams. It is shown for the first time that the very neutron-rich
nuclei W can be reached with relatively large cross sections by
complete fusion reactions with radioactive ion beams at incident energies near
the Coulomb barrier. A comparison between the complete fusion and fragmentation
reactions for the production of neutron-rich W and neutron-deficient Rn
isotopes is performed.Comment: 13 pages, 6 figures, accepted in PR
Disagreement between capture probabilities extracted from capture and quasi-elastic backscattering excitation functions
Experimental quasi-elastic backscattering and capture (fusion) excitation
functions are usually used to extract the s-wave capture probabilities for the
heavy-ion reactions. We investigated the
O+Sn,Sm,Pb systems at energies near and below
the corresponding Coulomb barriers and concluded that the probabilities
extracted from quasi-elastic data are much larger than the ones extracted from
fusion excitation functions at sub and deep-sub barrier energies. This seems to
be a reasonable explanation for the known disagreement observed in literature
for the nuclear potential diffuseness derived from both methods.Comment: 9 pages, 3 figure
Unified control/structure design and modeling research
To demonstrate the applicability of the control theory for distributed systems to large flexible space structures, research was focused on a model of a space antenna which consists of a rigid hub, flexible ribs, and a mesh reflecting surface. The space antenna model used is discussed along with the finite element approximation of the distributed model. The basic control problem is to design an optimal or near-optimal compensator to suppress the linear vibrations and rigid-body displacements of the structure. The application of an infinite dimensional Linear Quadratic Gaussian (LQG) control theory to flexible structure is discussed. Two basic approaches for robustness enhancement were investigated: loop transfer recovery and sensitivity optimization. A third approach synthesized from elements of these two basic approaches is currently under development. The control driven finite element approximation of flexible structures is discussed. Three sets of finite element basic vectors for computing functional control gains are compared. The possibility of constructing a finite element scheme to approximate the infinite dimensional Hamiltonian system directly, instead of indirectly is discussed
The effect of cutting temperature on hole quality when drilling CFRP/metal stack
The carbon fiber reinforced plastic (CFRP) are widely used in stacks with metals. That allows obtaining components with high strength and reduces weight. Holes’ drilling is a basic operation of CFRP/metal and metal/CFRP stacks machining. The most common problems of CFRP/metal stacks drilling are CFRP delamination, fiber pull – out, thermal degradation and low quality of hole surface. In this study the effect of cut ting temperature on the hole quality was provided. It was experimentally established that drilling of CFRP/metal stack was accompanied with a significant change of cutting temperature in the cutting zone during the transition of drill from CFRP to metal plate.Волокнисті полімерні композиційні матеріали (вуглепластики) широко використовуються в пакетах з металами. Це дозволяє отримати компоненти з високою міцністю, разом з тим забезпечивши зниження ваги. Свердління отворів є основною механічною операцією при обробці пакетів вуглепластик / метал і метал / вуглепластика. Найбільш поширені проблеми механічної обробки пакетів вуглепластик / метал є розшарування вуглепластика, витягування волокон, термічна деструкція і низька якість поверхні отвори. У статті представлено дослідження впливу дію температури різання на якість отворів. Експериментально було встановлено, що свердління пакета вуглепластик / сталь супроводжувався істотною зміною температури різання в зоні різання при переході свердла з вуглепластика до металевої пластини
Peculiarities of sub-barrier fusion with quantum diffusion approach
With the quantum diffusion approach the unexpected behavior of fusion cross
section, angular momentum, and astrophysical S-factor at sub-barrier energies
has been revealed. Out of the region of short-range nuclear interaction and
action of friction at turning point the decrease rate of the cross section
under the barrier becomes smaller. The calculated results for the reactions
with spherical nuclei are in a good agreement with the existing experimental
data.Comment: 11 pages, 5 figure
On nucleon exchange mechanism in heavy-ion collisions at near-barrier energies
Nucleon drift and diffusion mechanisms in central collisions of asymmetric
heavy-ions at near-barrier energies are investigated in the framework of a
stochastic mean-field approach. Expressions for diffusion and drift
coefficients for nucleon transfer deduced from the stochastic mean-field
approach in the semiclassical approximation have similar forms familiar from
the phenomenological nucleon exchange model. The variance of fragment mass
distribution agrees with the empirical formula . The comparison with the time-dependent Hartree-Fock calculations
shows that, below barrier energies, the drift coefficient in the semiclassical
approximation underestimates the mean number of nucleon transfer obtained in
the quantal framework. Motion of the window in the dinuclear system has a
significant effect on the nucleon transfer in asymmetric collisions.Comment: 10 pages, 10 figures, submitted for publicatio
Cluster Interpretation of Properties of Alternating Parity Bands in Heavy Nuclei
The properties of the states of the alternating parity bands in actinides,
Ba, Ce and Nd isotopes are analyzed within a cluster model. The model is based
on the assumption that cluster type shapes are produced by the collective
motion of the nuclear system in the mass asymmetry coordinate. The calculated
spin dependences of the parity splitting and of the electric multipole
transition moments are in agreement with the experimental data.Comment: 29 pages, 10 figure
RHYTHM—a server to predict the orientation of transmembrane helices in channels and membrane-coils
RHYTHM is a web server that predicts buried versus exposed residues of helical membrane proteins. Starting from a given protein sequence, secondary and tertiary structure information is calculated by RHYTHM within only a few seconds. The prediction applies structural information from a growing data base of precalculated packing files and evolutionary information from sequence patterns conserved in a representative dataset of membrane proteins (‘Pfam-domains’). The program uses two types of position specific matrices to account for the different geometries of packing in channels and transporters (‘channels’) or other membrane proteins (‘membrane-coils’). The output provides information on the secondary structure and topology of the protein and specifically on the contact type of each residue and its conservation. This information can be downloaded as a graphical file for illustration, a text file for analysis and statistics and a PyMOL file for modeling purposes. The server can be freely accessed at: URL: http://proteinformatics.de/rhyth
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