Shell Stabilization Research Articles

FISSION OF SUPER-HEAVY NUCLEI EXPLORED WITH SKYRME FORCES

We present a large scale survey of life-times for spontaneous fission in the regime of super-heavy elements (SHE), i.e. nuclei with Z =104-122. This is done on the basis of the Skyrme-Hartree-Fock model. The axially symmetric fission path is computed using a quadrupole constraint. Self-consistent cranking is used for the collective masses and associated quantum corrections. The actual tunneling probability is estimated by the WKB approximation. Three typical Skyrme forces are used to explore the sensitivity of the results. Benchmarks in the regime Z =104-108 show an acceptable agreement. The general systematics reflects nicely the islands of shell stabilization and the crossover from α-decay to fission for the decay chains from the region of Z / N =118/176.

Structural Features of the Scaffold Interaction Domain at the N Terminus of the Major Capsid Protein (VP5) of Herpes Simplex Virus Type 1

Protein-protein interactions drive the assembly of the herpes simplex virus type 1 capsid. A key interaction occurs between the C terminus of the scaffold protein and the N terminus of the major capsid protein (VP5). Results from alanine-scanning mutagenesis of hydrophobic residues in the N terminus of VP5 revealed seven residues (I27, L35, F39, L58, L65, L67, and L71) that reside in two predicted alpha helices (helix 1(22-42) and helix 2(58-72)) that are important for this bimolecular interaction. The goal of the present study was to further characterize the VP5 scaffold interaction domain (SID). Amino acids at the seven positions were replaced with L, M, V or P (I27); I, M, V, or P (L35, L58, L65, L67, and L71); and H, W, Y, or L (F39). Replacement with a hydrophobic side chain did not affect the interaction with scaffold protein in yeast cells or the ability of a virus specifying the mutation from replicating in cells. The mutation to the proline side chain abolished the interaction in all cases and was lethal for virus replication. Mutant viruses with proline substitutions in helix 1(22-42) at positions 27 and 35 assembled large open capsid shells that did not attain closure. Proline substitutions in helix 2(58-72) at either position 59, 65, or 67 abolished the accumulation of VP5 protein, and, at 58 and 71, although VP5 did accumulate, capsid shells were not assembled. Thus, the second SID, SID2, is highly structured, and this alpha helix (helix 2(58-72)) is likely involved in capsomere-capsomere interactions during shell accretion. Conserved glycine G59 in helix 2(58-72) was also mutated. G59 may act as a flexible "hinge" in helix 2(58-72) because decreasing the movement of this side chain by replacement with valine impaired capsid assembly. Thus, the N terminus of VP5 and the alpha helices embedded in this domain, as in the capsid shell proteins of some double-stranded DNA phages, are a key regulator of shell accretion and stabilization.

Advances in the numerical modeling of field-reversed configurations

The field-reversed configuration (FRC) is a compact torus with little or no toroidal magnetic field. A theoretical understanding of the observed FRC equilibrium and stability properties presents significant challenges due to the high plasma beta, plasma flows, large ion gyroradius, and the stochasticity of the particle orbits. Advanced numerical simulations are generally required to describe and understand the detailed behavior of FRC plasmas. Results of such simulations are presented in this paper. It is shown that 3D nonlinear hybrid simulations using the HYM code [E. V. Belova et al., Phys. Plasmas 7, 4996 (2000)] reproduce all major experimentally observed stability properties of elongated (theta-pinch-formed) FRCs. Namely, the scaling of the growth rate of the n=1 tilt mode with the S*∕E parameter (S* is the FRC kinetic parameter, E is elongation, and n is toroidal mode number), the nonlinear saturation of the tilt mode, ion toroidal spin-up, and the growth of the n=2 rotational mode have been demonstrated and studied in detail. The HYM code has also been used to study stability properties of FRCs formed by the counterhelicity spheromak merging method. A new stability regime has been found for FRCs with elongation E∼1, which requires a close-fitting conducting shell and energetic beam ion stabilization.

Preparation of Shell Cross-Linked Nano-Objects from Hybrid-Peptide Block Copolymers

Supramolecular structures formed by self-assembly of diblock copolymers in solution are stable over restricted environmental conditions: concentration, temperature, pH, or ion strength among others. To enlarge their domain of application, it appears necessary to develop stabilization strategies. We report here different strategies to stabilize the shell of micelles formed by self-assembly of amphiphilic polydiene-b-polypeptide diblock copolymers. For this purpose, covalent bonds can be formed between either amine or carboxylic acid groups distributed along the soluble peptide block and a cross-linking agent that contains respectively aldehyde or amine functions. Shell stabilization affords systems with unique properties that combine three main advantages: shape persistence, control of the porosity, and stimuli-responsive behavior. The covalent capture of such macromolecular objects has been studied by light scattering, AFM, and conductimetry measurements.

Shell stabilization and the survival of heavy compound nuclei

Knowledge of the influence of nuclear shell structure on the survival probability of heavy compound nuclei against fission is important for a quantitative understanding of the production rates of spherical super-heavy elements (SHE). Fission probabilities of N = 126 isotones beyond astatine can be used as test cases for the production of spherical super-heavy elements, as those isotones possess a strong shell correction energy and are highly fissile. Here, we report on two new experimental approaches which probe the effect of the closed neutron shell at N = 126 on the competition between fission and particle evaporation using projectile fragmentation and electromagnetic-induced fission of radioactive beams. We conclude that these nuclei lose at least a great part of their stability against fission at low excitation energies and angular momenta - mostly due to the influence of collective contributions in the level density. Implications on the production of spherical SHE will be discussed.

Tokamak NOVA-UNICAMP recent results

Tokamak NOVA-UNICAMP is a small tokamak with iron core and conducting shell stabilization built to study plasma-wall interaction and optical diagnostic development. Characteristic plasma behavior was the appearance of 2-3 kHz spike oscillation during entire discharge with very high X-ray activity which prevent our optical diagnostic development. By modifying capacitance and charging voltage on OH and vertical field banks, in order to obtain similar discharge current shape, most of 2-3 kHz spikes has been taken out, although some activities still remains to be analyzed. Nevertheless, the high X-ray activity which jeopardize our optical measurements is completely eliminated and time varying parameter as ion temperature is now possible to be followed during whole discharge.

Shell stabilization of super- and hyperheavy nuclei without magic gaps

Quantum stabilization of superheavy elements is quantified in terms of the shell-correction energy. We compute the shell correction using self-consistent nuclear models: the non-relativistic Skyrme-Hartree-Fock approach and the relativistic mean-field model, for a number of parametrizations. All the forces applied predict a broad valley of shell stabilization around Z=120 and N=172-184. We also predict two broad regions of shell stabilization in hyperheavy elements with N approx 258 and N approx 308. Due to the large single-particle level density, shell corrections in the superheavy elements differ markedly from those in lighter nuclei. With increasing proton and neutron numbers, the regions of nuclei stabilized by shell effects become poorly localized in particle number, and the familiar pattern of shells separated by magic gaps is basically gone.

Shell corrections of superheavy nuclei in self-consistent calculations

Shell corrections to the nuclear binding energy as a measure of shell effects in superheavy nuclei are studied within the self-consistent Skyrme-Hartree-Fock and Relativistic Mean-Field theories. Due to the presence of low-lying proton continuum resulting in a free particle gas, special attention is paid to the treatment of single-particle level density. To cure the pathological behavior of shell correction around the particle threshold, the method based on the Green's function approach has been adopted. It is demonstrated that for the vast majority of Skyrme interactions commonly employed in nuclear structure calculations, the strongest shell stabilization appears for Z=124, and 126, and for N=184. On the other hand, in the relativistic approaches the strongest spherical shell effect appears systematically for Z=120 and N=172. This difference has probably its roots in the spin-orbit potential. We have also shown that, in contrast to shell corrections which are fairly independent on the force, macroscopic energies extracted from self-consistent calculations strongly depend on the actual force parametrisation used. That is, the A and Z dependence of mass surface when extrapolating to unknown superheavy nuclei is prone to significant theoretical uncertainties.

Projectile-fragment yields as a probe for the collective enhancement in the nuclear level density

Production cross sections of several hundred nuclei produced in the fragmentation of uranium and lead projectiles are analysed with the abrasion-ablation model taking into account fission in the deexcitation chain. The survival probability against fission of excited compound nuclei is a sensitive probe for shell and collective effects in the nuclear level density. The expected stabilization against fission for neutron-deficient actinides near the magic number N = 126 due to large ground-state shell effects is not found in the experimental data. With the inclusion of collective enhancement in the level density the experimental data can be described well. The damping of the collective enhancement with excitation energy has to be treated as essentially independent of nuclear deformation, a new finding which contradicts the current theoretical picture. A schematic description of the vibrational enhancement in spherical nuclei is deduced from the data. The lack of shell stabilization against fission observed for the 126-neutron shell is expected to have consequences for the production cross sections of spherical superheavy elements around N = 184.

Possible synthesis of new and super-heavy elements via cluster decay

Cluster decays of some heavy and super-heavy, formed in heavy-ion 'cold fusion' reactions, are studied within a preformed cluster decay model. The cold super-asymmetric fission of these nuclei is also investigated. Some of the calculated exotic cluster decays from the ground to ground or excited to excited states are found to have half-liver T12/<1025 s, which are measurable with presently available experimental techniques. If such decays of the compound systems are observed as the long lived, super asymmetric, spontaneous fission activity, or simply as the cold fission process, it could give rise to some other new, heavy and super-heavy nuclei. Shell stabilization effects due to both the spherical and deformed shell closures are found to be responsible for selecting both the favourable cold fusion reactions as well as the most probable cluster decays.

Lipid classes and fatty acid profiles in cryptobiotic cysts of Streptocephalus dichotomus and Streptocephalus proboscideus (crustacea: anostraca)

Polyunsaturated fatty acids (PUFA) are required for growth in lobsters (Casteel & Covey, 1976), prawns (Guary et al., 1976; Kanazawa et al., 1979), marine fish (Owen et al., 1972) and the brine shrimp Artemia (Schauer et al., 1980; Abatzopoulos et al., 1989). Comparable studies on fresh water anostracan species are not available. Studies are available on the fairy shrimp S. dichotomus, dealing with egg shell stabilization, and with the involvement of lipids in accelerating hydration of cysts (Munuswamy & Subramoniam, 1984; Munuswamy, 1987). The present investigation provides an analysis of the fatty acid composition of cysts of the fairy shrimps Streptocephalus dichotomus Baird and Streptocephalus proboscideus (Frauenfeld) with an aim at understanding their functional value.

Andinodesma radicostata n. gen. n. sp., a grammysiid taxon from the Lower Devonian Catavi-Formation (Bolivia) and its autecological and phylogenetic implications

Andinodesma radicostata from the lowermost Devonian Cata vi Formation of Bolivia is a grammysiid genus, which is radially ribbed on the central parts of its valves and which exhibits a rather elongate anterior end. The umbones are placed near the middle of the valves and suggest an evolutionary state by Lower Devonian times, that rather parallels trends inGrammysia de Verneuil than in the morphologically closer relatedGrammysioidea Williams & Breger. Thus, the idea is supported, that not all taxa, beingGrammysia-shaped except for the conspicuous “cinctures” may easily be assigned toGrammysioidea. Living in an unstable shifting substratumA. radicostata evolved features that display a compromise between adaptation to free and relatively rapid burrowing and physical shell stabilization. The pattern of ornamentation suggests thatA. radicostata changed its mode of penetrating the sediment during one burrowing act. Anteriorly situated concentric ridges aided in more or less vertical down-probing during an initial phase of penetration whereas the radially ribbed central part of the valves indicates that rocking and forward movement predominated in later stages.

Part I Conceptual design of the R-tokamak

For the study of the alpha particles behavior in DT plasma the ‘Reacting Plasma Project’ (R-project in short) was initiated in 1981. The design study of the ‘R-tokamak’ was performed from 1981 to 1985, and evolved through three versions. The first version is based upon the usual tokamak and is similar to TFTR. The remote maintenance and dismantling however is considered very difficult. For the reduction of the residual radiation level the design of DT tokamak with aluminum alloy was conducted. It was found that hand-on maintenance after 1000 DT shots was feasible. Another effect of the aluminum alloy is the increase of the shell effect and stabilization of the variously shaped tokamak. The 3rd version tokamak with the extremely shaped cross-section (the crescent tokamak) is proposed for tokamak improvement (higher beta, better confinement and low radioactivity).

Part I. Conceptual design of the R-tokamak

For the study of the alpha particles behavior in DT plasma the ‘Reacting Plasma Project’ (R-project in short) was initiated in 1981. The design study of the ‘R-tokamak’ was performed from 1981 to 1985, and evolved through three versions. The first version is based upon the usual tokamak and is similar to TFTR. The remote maintenance and dismantling however is considered very difficult. For the reduction of the residual radiation level the design of DT tokamak with aluminum alloy was conducted. It was found that hand-on maintenance after 1000 DT shots was feasible. Another effect of the aluminum alloy is the increase of the shell effect and stabilization of the variously shaped tokamak. The 3rd version tokamak with the extremely shaped cross-section (the crescent tokamak) is proposed for tokamak improvement (higher beta, better confinement and low radioactivity).

A shell stabilized island of α-active heavy deformed nuclei: the discovery of elements 107 to 109

Results of experiments to synthesize the heaviest elements are reported. The high stability against fission not only of the odd and odd-odd nuclei but also of even isotopes of even elements is surprising, α-decay data allowed an increasing stabilization by shell effects to be established for nuclei up to 266109, the heaviest known element. Theoretically, the high stability is explained by an island of nuclei with large quadrupole and hexadecapole deformations around Z = 109 and N = 162. Future experiments are planned to establish further the shell stabilization of superheavy nuclei in the new island, and limitations set to their production by nuclear reaction mechanisms are discussed.

Shell Stabilization of Tilting Mode Instability in a Moving Ring Reactor

The shell stabilization of the tilting mode in a moving ring reactor due to the arrangement of a conductor close to the plasma is studied by numerically calculating the stabilizing torque by the eddy current induced on the conductor surface. The tilting mode instability can be successfully suppressed in the slender ring plasma with the aspect ratio of four by either an internal rod conductor or an external annular conductor. The arrangement of both rodand annular-type conductors is required for stabilizing the tilting mode in a ring plasma with the aspect ratio of three. The effect of the mutual interaction among the eddy current is shown to be so small as to be safely neglected in calculating the eddy current induced by the tilted plasma, and the simplified treatment of the eddy current is suggested for the computation of the stabilizing torque due to the shell effect.

Experimental Study on Limit of the Safety Factor in JFT-2 Tokamak

In medium q a ( q a ≃3) discharges, enhanced radiation loss power due to increased plasma-wall interaction plays an important role in the growth of low frequency oscillations. The m =2 tearing mode seems to be the most plausible cause of the disruption. The reduction of the impurity is essential in realizing the stable discharges of medium q a . Attainable q a is reduced with decreasing radiation loss power. In low q a ( q a <2) discharges, no enhancement of the radiation loss power was observed until the disruption. The plasma disrupts at \(q_{a}{\lesssim}2\). The m =2 kink mode is the probable cause of the disruption. Attainable q a is reduced with decreasing the ratio of shell radius to plasma radius. The possibility of the shell stabilization against the m =2 kink mode is proposed,

Lifetime predictions for super-heavy elements

Lifetime extrapolations are presented for elements of Z=104 to 115 based on the masses of Myers and Swiatecki. The letter mass values were calculated with the assumption of shell closures at Z=114, N=184. The extrapolation formulae of Viola and Seaborg were used for α and spontaneous fission lifetimes; the Moszkowski nomographs were used for EC and β lifetimes. Calculations were performed both with the shell stabilization predicted by the Myers-Swiatecki mass formula and with one half the predicted stabilization energy for each isotope. Since many isotopes of Z≦115 are predicted to have lifetimes of minutes or longer, it is concluded that proposed synthesis experiments stand a very good chance of success.