Investigation of the 208Pb(18O, f) fission reaction: Mass-energy distributions of fission fragments and their correlation with the gamma-ray multiplicity

The mass-energy distributions of fragments originating from the fission of the compound nucleus ²²⁶Th and their correlations with the multiplicity of gamma rays emitted from these fragments are measured and analyzed in ¹⁸O + ²⁰⁸Pb interaction induced by projectile oxygen ions of energy in the range E _lab = 78–198.5 MeV. Manifestations of an asymmetric fission mode, which is damped exponentially with increasing E _lab, are demonstrated. Theoretical calculations of fission valleys reveal that only two independent valleys, symmetric and asymmetric, exist in the vicinity of the scission point. The dependence of the multiplicity of gamma rays emitted from both fission fragments on their mass, M_γ(M), has a complicated structure and is highly sensitive to shell effects in both primary and final fragments. A two-component analysis of the dependence M_γ(M) shows that the asymmetric mode survives in fission only at low partial-wave orbital angular momenta of compound nuclei. It is found that, for all E _lab, the gamma-ray multiplicity M_γ as a function of the total kinetic energy (TKE) of fragments, M_γ(TKE), decreases linearly with increasing TKE. An analysis of the energy balance in the fission process at the laboratory energy of E _lab = 78 MeV revealed the region of cold fission of fragments whose total kinetic energy is TKE ～Q _max.     

Structure and chemical composition of LiB3O5 surfaces

The electronic and structural properties of LiB₃O₅ (LBO) surfaces have been studied by X‐ray photoemission spectroscopy (XPS) and reflectance high‐energy electron diffraction (RHEED). The as‐grown (110) crystal face and mechanically polished (001) surfaces have been investigated comparatively. Electronic structure of LBO has been determined on as‐grown (110) crystal face previously cleaned by chemical etching with RHEED control. The correlation of valence band structure and measured binding energies with earlier reported results has been discussed. Core‐level spectroscopy reveals strong enriching of mechanically polished LBO surface with carbon, when nanodiamond powder is used as an abrasive. So high carbon level as C:B = 0.7 has been observed at the surface while the ratio Li:B:O remains according to LBO chemical composition. The association of LBO Kikuchi‐lines with strong background has been shown by RHEED analysis of the surface. Thus, the polished LBO surface constitutes a high structure quality LBO with the inclusions of some amorphous carbon compound. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Ising magnets with mobile defects

Motivated by recent experiments on cuprates with low-dimensional magnetic interactions, a new class of two-dimensional Ising models with short-range interactions and mobile defects is introduced and studied. The non-magnetic defects form lines, which, as temperature increases, first meander and then become unstable. Using Monte Carlo simulations and analytical low- and high-temperature considerations, the instability of the defect stripes is monitored for various microscopic and thermodynamic quantities in detail for a minimal model, assuming some of the couplings to be indefinitely strong. The robustness of the findings against weakening the interactions is discussed as well. Received 22 August 2002 / Received in final form 4 October 2002 Published online 19 November 2002     

Formation of TiO<Subscript>2</Subscript> and KTiOPO<Subscript>4</Subscript> nanoclusters on the (001) surface of KTiOPO<Subscript>4</Subscript> crystal upon annealing

Morphology and crystallographic characteristics of (001) KTiOPO₄ air-annealed surface were investigated. The autoepitaxy of nanosized KTiOPO₄ islands was revealed in samples annealed at 550°C for 2–20 h. When annealing at 650°C takes ～20 h, the TiO₂ particles are observed to form on the substrate surface. This indicates the onset of the thermal decomposition of KTP at this temperature.     

Interaction of functionally-substituted 4-alkyl-2,6-di-<Emphasis Type="Italic">tert</Emphasis>-butylphenols with hydrohalic acids

Reactions of 4-alkyl-2,6-di-tert-butylphenols containing OH, SH, COOH, and COOMe groups in their para substituents with hydrogen chloride and hydrohalic acids were studied. One-step transformations of 2,6-di-tert-butyl-4-(ω-hydroxyalkyl)phenols to the corresponding 4-(ω-halogenoalkyl)phenols, as well as of 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionic acid and its esters to phloretic acid were proposed. 4-(3-Mercaptopropyl)phenol upon heating with conc. HBr undergoes condensation to 3-(4-hydroxyphenyl)propyl 4-(3-mercaptopropyl)phenyl sulfide as the main product. Dedicated to the memory of Academician N. N. Vorozhtsov on the 100th anniversary of his birth. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1078–1083, June, 2007.     

Sorption of Cadmium from Aqueous Solutions at Different Temperatures by Mexican HEU-type Zeolite Rich Tuff

Many factors may affect the heavy metals sorption on natural zeolites among them the temperature, for this reason in this paper the cadmium retention behavior on Mexican zeolitic rich tuff as a function of temperature is considered. The kinetic and the isotherms were determined at 303, 318, and 333 K, the remaining cadmium in the solution samples was analyzed by atomic absorption spectrometry. The pseudo-second order rate constant, k, as well as the apparent diffusion coefficients were calculated from the cadmium uptake by the zeolitic rock as a function of the contact time and temperature, the highest amounts were found for the experiments done at 333 K. The maximum cadmium adsorption capacity by the zeolitic material was 12.2 mg Cd²⁺/g at 318 K corresponding to 20% of the effective ion exchange capacity of the Chihuahua zeolitic rock. In order to explain the cadmium sorption behavior different kinetics and isotherm models were considered.     

Thermodynamic modeling of actinide complexation with oxalate at high ionic strength

A geochemical model describing the solubility of actinides in underground water at the Waste Isolation Pilot Plant (WIPP) Project is under development. The database for this model consists of standard chemical potentials and Pitzer model parameters for hundreds of species that may be present in the WIPP disposal room. Organic ligands used in separation and decontamination processes may be present in the nuclear wastes placed in the WIPP site and could have a significant impact on mobile actinide concentrations. In this work the ₁ and ₂ stability constants of NpO₂ ⁺, UO₂ ²⁺, Am³⁺ and Th⁴⁺ with the oxalate anion have been measured in 0.3–5.0M NaCl media at 25 °C by a solvent extraction technique. For the 1:1 complexation, the values of the stability constants increased in the order: NpO₂ ⁺<Am³⁺<UO₂ ²⁺<Th⁴⁺, in accordance with the actinide charge density and reflecting the strongly ionic bonding of the complexes. The Pitzer ionic interaction model was used to model the data. Because the data were collected mainly in the high ionic strength region, values of ⁽¹⁾ were estimated from these plus literature values. The Pitzer model gives a good representation of the data using three interaction parameters ⁽⁰⁾, ⁽¹⁾, and c.     

Core level spectroscopy and RHEED analysis of KGd0.95 Nd0.05(WO4)2 surface

A study of the surface structure and electronic properties of (010) KGd_0.95Nd_0.05(WO₄)₂ (Nd:KGW) using RHEED analysis and XPS is presented. It is shown that Nd doping has a negligible effect on the core levels of the basic elements. A bombardment of the Nd:KGW crystal with 3-keV Ar ions results in surface amorphization accompanied by the generation of tungsten ions in lower valence states.     

Zeta potential of anoxygenic phototrophic bacteria and Ca adsorption at the cell surface: possible implications for cell protection from CaCO3 precipitation in alkaline solutions

Electrophoretic mobility measurements and surface adsorption of Ca on living, inactivated, and heat-killed haloalkaliphilic Rhodovulum steppense, A-20s, and halophilic Rhodovulum sp., S-17-65 anoxygenic phototrophic bacteria (APB) cell surfaces were performed to determine the degree to which these bacteria metabolically control their surface potential equilibria. Zeta potential of both species was measured as a function of pH and ionic strength, calcium and bicarbonate concentrations. For both live APB in 0.1M NaCl, the zeta potential is close to zero at pH from 2.5 to 3 and decreases to -30 to -40 mV at pH of 5-8. In alkaline solutions, there is an unusual increase of zeta potential with a maximum value of -10 to -20 mV at a pH of 9-10.5. This increase of zeta potential in alkaline solutions is reduced by the presence of NaHCO(3) (up to 10 mM) and only slightly affected by the addition of equivalent amount of Ca. At the same time, for inactivated (exposure to NaN(3), a metabolic inhibitor) and heat-killed bacteria cells, the zeta potential was found to be stable (-30 to -60 mV, depending upon the ionic strength) between pH 5 and 11 without any increase in alkaline solutions. Adsorption of Ca ions on A-20s cells surface was more significant than that on S-17-65 cells and started at more acidic pHs, consistent with zeta potential measurements in the presence of 0.001-0.01 mol/L CaCl(2). Overall, these results indicate that APB can metabolically control their surface potential to electrostatically attract nutrients at alkaline pH, while rejecting/avoiding Ca ions to prevent CaCO(3) precipitation in the vicinity of cell surface and thus, cell incrustation.