Average and local magnetic moments on Fe atoms in microcrystalline and amorphous Fe100−xPx alloys

We have measured the concentration dependence of the average magnetic moment per Fe atom _Fe(x) in microcrystalline and amorphous Fe-P alloys obtained over a wide concentration range using electrochemical deposition. The model of local magnetic moments has been used to described _Fe(x). On the basis of this model the effects of phosphorus on the value _Fe are explained in terms of the parameters of the local environment of the Fe atom.     

Europium-151 Mössbauer spectroscopic and XANES investigation of europium-exchanged Y-zeolite

Eu³⁺ in ca. 10 wt% europium-exchanged Y-zeolite is partially reduced by treatment in hydrogen at 600°C to Eu²⁺. The reduction of Eu³⁺ is more readily achieved in Y-zeolite than in europium(III) oxide. The discrepancy in the extent of reduction as revealed by¹⁵¹Eu Mössbauer spectroscopy and near edge X-ray absorption fine structure (XANES) is associated with any difference in the recoil free fractions of Eu²⁺ and Eu³⁺ which may exist at 298 K and the enhanced sensitivity of the XANES to changes in the europium oxidation state.     

Asymptotic expansions of integrals of two Bessel functions via the generalized hypergeometric and Meijer functions

Asymptotic expansions of certain finite and infinite integrals involving products of two Bessel functions of the first kind are obtained by using the generalized hypergeometric and Meijer functions. The Bessel functions involved are of arbitrary (generally different) orders, but of the same argument containing a parameter which tends to infinity. These types of integrals arise in various contexts, including wave scattering and crystallography, and are of general mathematical interest being related to the Riemann—Liouville and Hankel integrals. The results complete the asymptotic expansions derived previously by two different methods — a straightforward approach and the Mellin-transform technique. These asymptotic expansions supply practical algorithms for computing the integrals. The leading terms explicitly provide valuable analytical insight into the high-frequency behavior of the solutions to the wave-scattering problems.     

Monitoring of safety features,fissile material accumulation and concentration in nuclear reprocessing by active and passive neutron counting techniques

The application, the advantage and limits of neutron monitoring techniques, such as Hf-monitors, fissile material accumulation and concentration monitors are being discussed. The active neutron counting technique applied to emptied pulsed extraction columns containing Hf-sieve plates allows conclusive answers as to the position of the plates in the columns. Pu-accumulations on Hf-sieve plates in pulsed extraction columns can be estimated within a factor of two, whereby the detection limit is about equal to or less than 1 g Pu/plate. Fissile material concentration changes of 1.1 g/l can be detected in the case of²³⁵U in solution and of 0.4 Pu/l if a²³⁹Pu/²⁴⁰Pu ratio of 4 to 1 is assumed.     

Global optimization by random perturbation of the gradient method with a fixed parameter

The paper deals with the global minimization of a differentiable cost function mapping a ball of a finite dimensional Euclidean space into an interval of real numbers. It is established that a suitable random perturbation of the gradient method with a fixed parameter generates a bounded minimizing sequence and leads to a global minimum: the perturbation avoids convergence to local minima. The stated results suggest an algorithm for the numerical approximation of global minima: experiments are performed for the problem of fitting a sum of exponentials to discrete data and to a nonlinear system involving about 5000 variables. The effect of the random perturbation is examined by comparison with the purely deterministic gradient method.     

Semiconductor waveguide directional couplers for coherent receivers

Optical waveguide 3-dB couplers integrated on semicondutors have been studied, designed and fabricated, using both bulk and diluted multi-quantum-well InGaAsP/InP-based materials, grown by MOCVD. The device structure is based on the two-mode interference (TMI) principle and is fully compatible for integration with the optoelectronic and electronic components of a coherent receiver. Bulk material couplers provide an output balanced within 0.05 dB per nanometer and an excess loss of 1.4 dB, compared to a straight guide, while coupling loss to a tapered-lensed single mode fibre is 4 dB.Improved coupling efficiency to single-mode fibres is achieved by use of moderately diluted multi-quantum-well waveguides, which include InGaAsP wells and InP barriers: coupling loss to a tapered-lensed single-mode fibre as low as 0.5 dB and excess loss of 1.8 dB are featured. Couplers fabricated with this waveguide structure have a balance sensitivity of 0.03 to 0.04 dB per nanometer.A moderately diluted multi-quantum-well 3-dB coupler has been permanently pigtailed and butt-coupled to a dual balanced PIN photoreceiver. This hybrid assembly was tested in a coherent transmission system at 155 and 622 Mbits^-1 showing sensitivities, for 10^-9 BER, of about-38.0 dBm and-28.8 dBm, respectively.     

The influence of synthetic conditions on the stability of methotrexate-monoclonal antibody conjugates determined by reversed phase high performance liquid chromatography.

Methotrexate (MTX) has been convalently attached to an IgG-type monoclonal antibody (791T/36) directed to tumour-associated antigen gp72. Conjugates were synthesized by the active ester method using MTX N-succinimidyl ester at various pH values (7.5-10.5). Following purification by gel filtration, high performance liquid chromatography was used to assess the free drug or its derivatives in samples of MTX-791T/36 conjugates previously treated (or not) with hydroxylamine. Quantitative analysis, performed on a reversed phase column (pore size 300 A) with isocratic acetonitrile-sodium acetate buffer (pH 4.8) as mobile phase, indicated no detectable amount of free methotrexate in hydroxylamine-treated conjugates even six months after their preparation. Similar observations were made with conjugates, whose synthesis were performed at pH greater than or equal to 10. In contrast, the presence of increasing amounts of drug/metabolite could be demonstrated in samples produced at lower pH values. Based on these findings, the pH-dependent kinetics of MTX release has been determined and used to design conditions under which stable MTX-791T/36 conjugates could be prepared without hydroxylamine reaction.     

Flame Temperature Effect on the Structure of SiC Nanoparticles Grown by Laser Pyrolysis

Small SiC nanoparticles (10 nm diameter) have been grown in a flow reactor by CO₂ laser pyrolysis from a C₂H₂ and SiH₄ mixture. The laser radiation is strongly absorbed by SiH₄ vibration. The energy is transferred to the reactive medium and leads to the dissociation of molecules and the subsequent growth of the nanoparticles. The reaction happens with a flame. The purpose of the experiments reported in this paper is to limit the size of the growing particles to the nanometric scale for which specific properties are expected to appear. Therefore the effects of experimental parameters on the structure and chemical composition of nanoparticles have been investigated. For a given reactive mixture and gas velocity, the flame temperature is governed by the laser power. In this study, the temperature was varied from 875°C to 1100°C. The chemical analysis of the products indicate that their composition is a function of the temperature. For the same C/Si atomic ratio in the gaseous phase, the C/Si ratio in the powder increases from 0.7 at 875°C up to 1.02 at 1100°C, indicating a growth mechanism limited by C₂H₂ dissociation. As expected, X-ray diffraction has shown an improved crystallisation with increasing temperature. Transmission electron microscopy observations have revealed the formation of 10 nm grains for all values of laser power (or flame temperature). These grains appear amorphous at low temperature, whereas they contain an increasing number of nanocrystals (2 nm diameter) when the temperature increases. These results pave the way to a better control of the structure and chemical composition of laser synthesised SiC nanoparticles in the 10 nm range.