The role of free carbon in the transport and magnetic properties of boron carbide

Boron carbide is a ceramic which has a wide field of application because of its mechanical and nuclear properties. This material is difficult to characterise due to the presence of different levels of disorder and inhomogeneities which are found in the usual available samples. The transport and magnetic properties of several samples of boron carbide have been measured from liquid helium to room temperature as a function of temperature and composition. We have attempted to attribute the different features of these properties to the different levels of disorder. The role of free carbon, in form of thin layers of graphite within the disordered semi-conducting matrix, was investigated in particular details, because it was either ignored or neglected by others. Free carbon is found to dominate the D.C. transport when its concentration is larger than 5%; while the principal features of the electron spin resonance (E.S.R.) line show a dominance of free carbon when the concentration is larger than 3.5%. Below these concentrations conductivities as well as spin relaxation rates do not depend very much on free carbon; neither these have been found to be correlated in a simple way to the stoichiometry: the disorder in the boron sublattice is probably responsible for the electronic properties in this range of concentrations.     

A quasi random search technique for functional fitting of detector responses

The problem of nonlinear optimization is encountered while fitting the observed response of certain radiation detectors. In this paper the development of an optimizing technique based on quasi random search is described. The performance of this technique is compared with other existing methods of gradient search and linear Taylor differential correction technique in the case of fitting the photopeak responses of Nal(Tl) detectors. Our results are favourable especially when the observed data are associated with poor statistics.     

Use of threshold activation technique in estimating energy distributions of accelerator produced neutrons

A study of effective implementation of threshold activation technique for neutron spectral analysis in the environment of an alpha-cyclotron target is presented. The activation data are analysed using LOUHI-82 code. Optimal choices of the regularization parameters of the code are studied and discussed. Energy distribution of neutrons emitted from thick targets of Be, C and Ta irradiated by 40–45 MeV alphas is discussed.     

Measurement of ultrasonic absorption in a gel by light diffraction and resonator methods

A new sample cell construction for a light diffraction method permits absolute measurements of the ultrasonic absorption coefficient for a strongly absorbing transparent gel. By means of this cell the absorption coefficient at 6.2 MHz has been measured as a function of the temperature in the gelling temperature range for a 25% (w/w) aqueous solution of the ABA polyoxyethylene-polyoxypropylene block copolymer PLURONIC^R F 127. The resonator method has been used under the same experimental conditions. The strongly absorbing gel causes overlapping of the resonance signals. A calibration based on resonance experiments compared with the absolute measurements yields a solution to that problem and thus makes the resonator method applicable to a strongly absorbing gel.     

Finite quantum field theory and neutrinodynamics

In quantum neutrinodynamics (photon-neutrino weak coupling) all the renormalization constants vanish and therefore the field equations cannot be expressed in terms of unrenormalized field quantities. This helps us to formulate quantum neutrinodynamics as a convergent quantum field theory. It is also pointed out that from the viewpoint of the unified model of weak and electromagnetic interaction as developed on the basis of the photon-neutrino weak coupling by Bandyopadhyay, quantum electrodynamics also manifests itself as a convergent field theory.     

Distribution of Shower Particles as Obtained in the Analysis of a 10 + 13p Shower Event

We present here the analysis of an accelerator produced 10 + 13p shower event observed in nuclear emulsion. The DULLER -WALKER method is applied to obtain the distribution of secondary shower particles, and the particles having the closest log₁₀ tan θ_L values are taken to form a cluster. This cluster formation is also confirmed by the target diagram method. If this cluster is conceived to be a high spin resonance and the secondary pions to be the decay products of this resonance, then the azimuthal integral distribution curve fits fairly well with our experimental data.     

Non-standard asymptotics in an inhomogeneous gamma process

Nonhomogeneous Poisson process (NHPP) is a commonly used stochastic model that is utilized to describe the pattern of repeated occurrence of certain events or conditions. Aninhomogeneous gamma process evolves as a generalization to NHPP, where the observed failure epochs correspond to every successive κ-th event of the underlying Poisson process, κ being an unknown parameter to be estimated from the data. This article focuses on a special class of inhomogeneous gamma process, calledmodulated power law process (MPLP) that assumes the Weibull form of the intensity function. The traditional power law process is a popular stochastic formulation of certain empirical relationships between the time to failure and the cumulative number of failures, often observed in industrial experiments. The MPLP retains this underlying physical basis and provides a more flexible modeling environment potentially leading to a better fit to the failure data at hand. In this paper, we investigate inference issues related to MPLP. The maximum likelihood estimators (MLE’s) of the model parameters are not in closed form and enjoy the curious property that they are asymptotically normal with a singular variance-covariance matrix. Consequently, the derivation of the large-sample results requires non-standard modifications of the usual arguments. We also propose a set of simple closed-form estimators that are asymptotically equivalent to the MLE’s. Extensive simulation results are carried out to supplement the theoretical findings. Finally, we implement our inference results to a failure dataset arising from a repairable system.     

Analysis of a model scheme incorporating reactant inhibition: Instability of homogeneous solution and formation of spatio–temporal structures

A model scheme incorporating reactant inhibition in the rate process has been analyzed with a view to study the instability of homogeneous solution due to diffusion. Conditions for the occurrence of Turing as well as phase instability are derived and show the existence of multiplicity in the parameter space. The Ginzburg-Landau equation for the system is developed and solved numerically in various regions of the parameter space. The simple model system shows the existence of very rich behavior including normal and inverted bifurcations in the super and subcritical regimes. The various results are analyzed and discussed. © 1993 John Wiley & Sons, Inc.