Shell Structure Effects Research Articles

Ultrastructural characteristics of ostrich eggshell: outer shell membrane and the calcified layers.

The ultrastructure of the eggshell of the domestic hen has been well researched and structural studies of other avian species, such as the ostrich, often base their interpretation of egg shell structure on that of the chicken. In the ostrich, lowered hatchability and hatching trauma may be due to shell ultrastructural abnormalities. In the present study the ultrastructure of the calcified portion, and the outer shell membrane (OSM), of domesticated ostrich eggshells was investigated using standard electron microscopic techniques. Transmission and scanning electron microscopy studies demonstrated intimate contact between cup-shaped structures present on the OSM and the mammillary layer of the calcified portion of the shell. The initial calcium carbonate growth of the calcified shell was of a dendritic nature with nucleation sites on the surface of the cup's contents. The dendritic growth gave way to a more randomly-orientated, smaller crystallite growth structure, which changed in form as it neared the vertical crystal layer (VCL). The VCL is described as being both amorphous and 'crumbly' depending on the plane of fracture. These observations suggest that firstly, initial calcification is contained within the cups and is then directed outwards to form the shell and that secondly, the VCL may contain an evolutionary, calcified cuticular layer. These observations serve as a baseline for studies investigating the effect of shell structure and strength on hatchling trauma and the influence of maternal diet.

Effect of shell structure on energy dissipation in heavy-ion collisions

The effect of shell structure on the distribution of the excitation energy between fragments of the deep inelastic collisions is analysed in the microscopic approach. It is shown that the density of the single-particle levels of the proton and neutron subsystems near the Fermi surface determines the ratio between the excitation energies of fragments at the initial stage of the collision. It is shown also that the shell structure strongly influences the correlations between the width of the charge distributions and the total kinetic energy losses. Calculations are performed for the 40,48Ca+248Cm reactions. The results obtained suggest a possible interpretation for the observed concentration of the excitation energy in the light fragment in deep inelastic collisions for a wide range of the total kinetic energy losses.

Investigation of the fission fragment properties of the reaction 238U(n,f) at incident neutron energies up to 5.8 MeV

The fission fragment properties of the reaction 238U(n,f) have been studied, at different incident neutron energies ranging from E n=1.2 to 5.8 MeV. The pre-neutron emission mass, kinetic energy and fission fragment angular distributions have been investigated with a double Frisch-gridded ionization chamber. The influence of the subthreshold vibrational resonances and of the proton pairing effect on the fission fragment properties is clearly visible. The total kinetic energy averaged over all fission fragment masses ( TKE ) shows an increasing trend up to E n=3.5 MeV with a sudden drop at roughly E n=3.8 MeV which has been attributed to the onset of pair breaking at the barrier. Above E n=3.8 MeV, the TKE is again continuously increasing. The changes in the mass yield and TKE ( A) distributions have been studied as a function of the compound nuclear excitation energy and their contribution to the observed variations in the TKE have been determined. The two-dimensional mass-TKE distributions have been described in terms of fission modes and compared with theoretical calculations performed recently in the frame of the multi-modal random neck-rupture model. Although theoretically six asymmetric fission modes are predicted which all surpass individual outer barriers, an interpretation in terms of only two asymmetric modes has physical meaning. This points to an influence of shell structure effects to the observed distributions. In any case, the super-long symmetric mode has to be included, in order to explain the dip in TKE ( A) distribution close to symmetry.

Stimuli-Response of Microsphere Having Poly(N-isopropylacrylamide) Shell

Several kinds of submicron-sized microspheres having environment-sensitive shells were prepared and the effect of shell structure on the properties of microspheres was studied. The shell was composed of crosslinked or hairy poly(N-isopropyl acrylamide) (PNIPAM) whose lower critical solution temperature was about 32°C but depended on the architecture of the shell. The resposivity of the shell to the change in environmental condition was sluggish when the shell was crosslinked. The decrease in the crosslink density of the shell was slightly effective in sharpening the response. But even non-crosslinked, hairy PNIPAM shell did not exhibit discontinuous transition at the critical condition. The response of hairy PNIPAM shell to environmental change was drastically sharpened by incorporating a slight amount of acrylic acid (AAc) in the hair.

Shell structure and exchange-related spin effects for quantum dots in a magnetic field

Ground-state calculations based on current spin density-functional theory for circular parabolic quantum dots are presented which reveal interesting spin effects in dependence on an external magnetic field: Hund's first rule (well-known from atomic physics for zero magnetic field) is related to the level occupancy near the Fermi energy and determines the spin configuration of the ground state in finite magnetic field. Consequently, the addition spectrum of the quantum dot exhibits a specific non-monotonous dependence on magnetic field, which is visible in recent quantum transport experiments.

Spin-density waves in superdeformed quantum dots

Electronic shell structure and spin effects in deformed quantum dots are investigated using spin-density functional theory. We recently suggested (Koskinen et al., Phys. Rev. Lett. 79 (1997) 1389) that for circular dots, depending on the density of the two-dimensional electron gas and the electron number, a spin-density wave-like state can occur as a possible ground state. Here these studies are extended to deformed and superdeformed dots, which approach the limit of a finite quantum wire.

Relativistic and Electron-Correlation Effects in the Ground States of Lanthanocenes and Actinocenes

Lanthanide and actinide sandwich complexes exhibit fundamental differences in their electronic structure which may be explained by taking into account the effects of shell structure, relativity and electron correlations (depicted schematically on the right). The results of recent theoretical as well as experimental investigations are used to establish a general picture of the electronic structure for the neutral complexes [M(C8H8)2] as well as their anions.

Role of shell structure in the 2νββ nuclear matrix elements

Significance of the nuclear shell structure in the ββ nuclear matrix elements is pointed out. The 2νββ processes are mainly mediated by the low-lying 1 + states. The shell structure also gives rise to concentration or fragmentation of the 2νββ components over intermediate states, depending on nuclide. These roles of the shell structure are numerically confirmed by realistic shell model calculations. Some shell structure effects are suggested for 0νββ matrix elements; dominance of low-lying intermediate states and nucleus-dependence of their spin-parities.

Ab Initio Study of the Lanthanide and Actinide Contraction

Results for the spectroscopic constants of the monohydrides, monoxides, and monofluorides of the lanthanide elements lanthanum and lutetium as well as the actinide elements actinium and lawrencium from all-electron and valence-only ab initio electronic structure calculations are presented. The valence-only investigations were carried out using energy-consistent pseudopotentials for the heavy atoms. Electron correlation was accounted for by means of the coupled-cluster method. Multireference averaged coupled-pair functional calculations were carried out to describe effects due to the mixing of the energetic close-lying ns2-m (n−1)dm (0 ≤ m ≤ 2) configurations of the lanthanide (n = 6) and actinide (n = 7) cations. The counterpoise correction was applied to estimate the size of the basis set superposition errors. Nonrelativistic as well as quasirelativistic calculations were performed. The latter included also corrections for spin−orbit effects for the actinide compounds derived from limited configuration interaction calculations. The reliability of the pseudopotential approach is demonstrated by a comparison to the results of corresponding nonrelativistic and scalar-relativistic Douglas−Kroll−Hess all-electron self-consistent-field calculations performed with large basis sets. The influence of shell-structure effects, i.e. the filling of the 4f and 5f shell, and relativistic effects on the molecular properties is discussed. The values for the lanthanide and actinide contractions are found to depend strongly on the ligand and to vary from 6 to 11 pm for lanthanides and from 11 to 17 pm for actinides. Relativistic effects play a significant role; for example, their neglect even leads to a slight lanthanide/actinide expansion in the case of the monoxides.

Decay Modes of 180–194Hg Nuclei — A New Possibility of Deformed Daughter Radioactivity

Cluster decays of both the normal deformed and superdeformed mercury nuclei ranging from 180 Hg to 194 Hg are studied. It is shown that they are all α-emitters, the superdeformed parents being poorer α-emitters. Also, though almost all the superdeformed 190–194 Hg nuclei are stable against all the heavier clusters, the normal deformed 180–188 Hg nuclei present measurable instabilities against 8 Be and 12 C clusters. The 8 Be and 12 C decays of 180 Hg with T1/2 ~ 1023–1028 s offer as best cases for measurements with the present-day experimental facilities. More significant decays are [Formula: see text] and [Formula: see text], and perhaps also [Formula: see text], which all exhibit shell structure effects and hence predict deformed shell closures at Z=76 or 74 and N=100–104 referring to daughter nuclei. The very stable nature of superdeformed 190–194 Hg parents stresses the presence of (spherical) shell closures at N=110–114 and the very unstable nature of normal deformed 188 Hg parent suggests no (spherical/deformed) shell closure at N=108.

Onset of rotational damping in superdeformed nuclei

We discuss damping of the collective rotational motion in A ∼ 150 superdeformed nuclei by means of a shell-model combining the cranked Nilsson mean field and the surface and volume delta two-body residual forces. It is shown that, because of the shell structure associated with the superdeformed mean field, onset energy of the rotational damping becomes E x ∼ 1.5–3 MeV above the yrast line, with significant variation for different neutron and proton numbers. The mechanism of the shell structure effect is investigated through detailed analysis of level densities in superdeformed nuclei. The variation in onset of damping is associated with variation in the single-particle structure at the Fermi surface.

Fixed-J level densities beyond spin cut-off approximation

It is well known that the expression for fixed-J level densityIl(E, J)=−I(E)\(\frac{\partial }{{\partial M}}\)I(M|E)|M=J+1/2 reduces to familiar Bethe’s formula provided the conditionalM-distributionI(M|E) is approximated by a Gaussian form, so called spin cut-off approximation (M is thez-component of total angular momentumJ andE is the excitation energy). After a detailed analysis, we find that the Bethe’s formula which overestimatesIl(E, J) at highJ values, in particular near the yrast line, can be significantly improved by including a few higher-order moment terms in a suitable expansion forI(M|E) with the lowest-order term to be a Gaussian, e.g., Edgeworth expansions. We also find that, except at very low excitation energies, reasonable values (close to exact) of the moments ofI(M|E) can easily be obtained when multiple Laplace-back transform of the partition function for grand canonical ensemble is evaluated within the saddle point approximation. Furthermore, we study the effects of shell structure as well as residual interaction on the excitation energy dependence of these moments.

Effects of shell structure and N/Z ratio of a projectile on the excitation energy distribution between interacting nuclei in deep-inelastic collisions.

Deep inelastic collisions of stable and radioactive projectiles with heavy targets are considered. The effects of shell structure and the N/Z ratio of a projectile on the excitation energy distribution between interacting nuclei, and on the centroid position and variance of the charge (mass) distribution of binary reaction products are explored. The role of nucleon exchange and particle-hole excitation mechanisms in the transformation of relative motion kinetic energy into the internal excitation energy of nuclei is studied. It is shown that a change in the mass number of the projectile nucleus causes a sufficient change both in the sharing of the excitation energy between fragments and in the charge (mass) distribution of reaction products. The nucleon exchange process between interacting nuclei is the main mechanism of dissipation.

Single-particle and collective properties of drip-line nuclei.

We study the effect of the unique shell structure as well as the very low particle threshold on collective modes in drip-line nuclei, first performing the Hartree-Fock (HF) calculation with Skyrme interactions and, then, using the random phase approximation solved in the coordinate space with the Green's function method. We examine also one-particle resonant states in the HF potential. The properties of both isoscalar and isovector monopole giant resonance (GR) are found to change drastically in nuclei around the neutron drip line. The characteristic feature of the isovector dipole modes as well as the isoscalar quadrupole modes in drip-line nuclei is also studied. \textcopyright{} 1996 The American Physical Society.

Excitation functions for the production of some radioisotopes of Cs, Xe and I in proton bombardment of natXe up to 100 MeV

Integral excitation functions for the production of 16 radioisotopes of Cs, Xe and I in the bombardment of natXe with protons were measured up to 100 MeV. The results were compared with geometry-dependent hybrid-model calculations performed by means of the computer code ALICE/85/300, employing both the standard methods offered by the code to take nucleon pairing and shell-structure effects into account. Good overall agreement was obtained in both cases.

Consistent exciton model calculations with shell structure, pairing and isospin effects

Shell structure effects are included in a consistent way for all stages of preequilibrium particle emission in the exciton model. Also included are pairing, isospin, and finite well depth effects. Calculations have been performed for a variety of (N,N) reactions around N=50 and Z=50 at 18 and 25 MeV incident energies. Comparisons with experimental angle-integrated energy spectra demonstrate the utility of the current methodology for including shell effects when pairing and isospin are also included. They also show that shell structure effects wash out in about D/2d mass units. The sensitivity to and interconnectedness of model input were studied, and adjusted parameter values were applied uniformly to all systems. In particular, in order to reproduce the relative yields in the inelastic and exchange channels, the effective residual interaction between like nucleons must be larger than between a neutron and a proton. This is contrary to earlier assumptions.

Shell-corrected particle-hole state densities for pre-equilibrium reaction calculations

Calculated particle-hole state densities in the shell-shifted equi-spacing model (s2-ESM) potentially provide a fast and simple way to consistently include shell structure effects in pre-equilibrium reaction calculations. Results from the existing state density formalism were compared with results from the exact counting of states, and the methodology for evaluating the effective single particle state densities in the s2-ESM state density formula was completely revised. Calculated state densities were then compared with literature results obtained from more realistic sets of single particle states, indicating the model's utility. Finally, model-independent estimates of the sizes of the shell gaps were obtained from a set of nuclear mass tables so that the s2-ESM can be used in reaction calculations without the introduction of any new free parameters.

Lanthanide and Actinide Contractions: Relativistic and Shell Structure Effects

= 103 shows that these elements reveal some interesting and often unexpected chemistry.’ Lifetimes of superheavy nuclei beyond Z = 109 may be much higher than previously ~redicted,~-~ opening the way for atom- at-a-time chemistry.’ While the chemistry of element 104 has been studied in some detail,’ss the chemistry of the elements with nuclear charge greater than 105 is virtually unknown and only theoretical estimates of physical and chemical properties have been established from relativistic

Neutron-halo nuclei in cold synthesis and cluster decay of heavy nuclei: Z=104 nucleus as an example.

Nuclei at the neutron-drip line are studied. The light neutron-halo nuclei are found to play an important role for both cold fusion reactions and exotic cluster decay studies of heavy nuclei at the neutron-drip line. For cold fusion reactions, beams of neutron-halo nuclei are shown to occur as natural extensions of the conventional lighter beams but with the corresponding target nuclei as the heavy neutron-rich radioactive nuclei. Thus, in synthesizing the various isotopes of a neutron-rich cool compound nucleus, both the target and projectile nuclei have to be richer in neutrons, with their proton numbers remaining the same. On the other hand, neutron-halo (cluster) decays are favored for a relatively less neutron-rich parent nucleus. Possible consequences of this work for the shell structure effects in neutron-rich heavy nuclei are also pointed out. This follows from the fact that the so far observed phenomena of both cold fusion and cluster radioactivity are associated with closed or nearly closed shell nuclei. Calculations are made for {sup 274,288}104, using the quantum mechanical fragmentation theory for cold fusion reaction studies and a performed cluster model for cluster decay studies.

Chemisorption of molecular oxygen on small copper clusters and the Cu(100) surface

Calculations within the local density approximation are presented for the chemisorption of O 2 on Cu 7Cu 10 and a cluster model for the Cu(100) surface. The charge transfer from the copper atoms to the antibonding π g ∗ orbital of the oxygen molecule is investigated in terms of a partial density of state analysis. The copper 4p orbital is found to be of importance for the formation of the bond between metal and adsorbate. The calculations indicate that shell structure effects are of importance for the chemisorption on the clusters.