Applied Nuclear Physics

A re-scan of the well-known Mach band illusion has led to the proposal of a Bi-Laplacian of Gaussian operation in early vision. Based on this postulate, the human visual system at low level has been modeled from two approaches that give rise to two new tools. On one hand, it leads to the construction of a new image sharpening kernel, and on the other, to the explanation of more complex brightness-contrast illusions and the possible development of a new algorithm for robust visual capturing and display systems.

Abstract: Based upon physiological observation on time dependent orientation selectivity in the cells of macaque's primary visual cortex together with the psychophysical studies on the tuning of orientation detectors in human vision we suggest that time dependence in ...

We are proposing a biologically inspired multi-scale derivative filter in which the higher order derivatives are expressed as a linear combination of a smoothing function at various scales. One of the functions in the summation has been approximated to a Dirac-delta function to finally yield the new filter. This modification has some support from the point of view of authentic edge detection as well as from neurophysiological and psychophysical experiments at the retinal level. Besides, it improves the quality of the filter in a number of ways. The proposed filter can be optimized at any desired scale. Hence it is very effective in extracting the features from a noisy picture. The filter is rotationally symmetric. Zero-crossing map of any picture filtered with the proposed model gives a half-toning effect to the retrieved image and hence preserves the intensity information in the image even in the edge map.

The phenomenon of coherence resonance (CR) is investigated in an unijunction transistor relaxation oscillator (UJT-RO) and quantified by estimating the normal variance (NV). Depending upon the measuring points two types of NV curves have been obtained. We have observed that the degradations in coherency at higher noise amplitudes in our system is probably the result of direct interference of coherent oscillations and the stochastic perturbation. Degradation of coherency may be minimal if this direct interference of noise and coherent oscillations is eliminated.

Abstract. Evidences favouring a dynamic receptive field model of reti-nal ganglion cells and the cells of Lateral Geniculate Nucleus (LGN) have been presented based on the perception of some brightness-contrast il-lusions. Of the different kinds of such stimuli, four, namely ...

In this paper, a new low-pass FIR filter design technique for achieving variable stopband attenuation without altering the passband and stopband edges, is proposed. The filter function is a linear combination of multi-scale Gaussian derivatives. In the frequency domain, the spectral modes corresponding to the continuous Gaussian derivatives have monotonic tails. In discrete domain, this amounts to variable stopband attenuation which depends upon the truncation length of the continuous kernel, the 'scale' and the order of Gaussian derivatives. The proposed algorithm consists of derivation of some useful mathematical relations between the basic design parameters and the parameters of the Gaussian derivatives and an optimization technique that finally produces an equiripple passband lowpass FIR filter with a higher falloff rate at the beginning of the transition band. Such a filter with variable stopband attenuation may be effective at the time of reconstruction of the desired passband signal in a S-D modulator output. r

Study was conducted of surface structural modification of layered sheets of mica that had been subjected to titanium (Ti) sputter coating, use being made of a variable but mono energetic slow positron beam. Comparison has been drawn with the structure of pure muscovite mica sheet. Doppler broadening (DB) of the energy of annihilation radiation has been used as a parameter to assay changes in the surface characteristics within a depth profile of 500 Å . In support of these investigations, X-ray reflectivity data was used to assay titanium film thickness and the electron density profile of the film on mica. Finally, changes involving the surface micro-morphology of the system have been corroborated by scanning electron microscopy. r

The observed decrease in intensity of the long-lived component in positronium decay at a critical surfactant concentration in non-polar solvents, concommitant with the formation of reverse micelles, is described through a diffusion-recombination model. It is emphasized that phenomenological studies, such as the present one, are important for the development of an understanding of positronium formation in liquids for which as yet no satisfactory theory exists, and strengthens the use of positron annihilation as a tool, for the study of the aggregational behaviour of surfactant molecules, the characteristics of which are different for polar and non-polar solvents. q 1998 Elsevier Science B.V. 0009-2614r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved.

Positronium (Ps) reactions with the diamagnetic acceptor molecule, namely, nitrobenzene, have been critically studied by us at various temperatures in water and water/methanol mixture (1:1 v/v), with the special emphasis on the Arrhenius plot of the overall reaction rate constant. While the Ps-Ac reaction rate constant in water takes over an anomalous course at higher temperatures (departing from normal Arrhenius paradigm), the reaction in water/methanol mixture, however, shows an entirely different systematics. The temperature dependences of surface tensions and viscosities of these solvents when taken in conjunction with the other physical parameters, enable us to explain and compare the plausible mechanism in the respective solvents.

. The observed anti-Arrhenius temperature dependence of the rate for positronium-acceptor Ps-Ac complex formation above, and a normal Arrhenius behaviour below, a solvent-dependent critical temperature T , is argued to be ascribable to a 0 Kramers' turnover, relating to viscosity effects unleashed by temperature-dependent surface tension, in the context of the bubble model of the positronium atom in liquids, provided one uses a more realistic and robust version thereof developed by us, which is also in closer accord with lifetime and gamma-gamma angular correlation data in pure solvents. q 2000 Published by Elsevier Science B.V.

A systematic investigation on the microstructural assay of aromatic polyamide membrane material has been presented. The study constitutes important aspects like critical micro-void size determination and the relative concentration of the same in the different samples synthesized using positron annihilation lifetime spectroscopy (PALS). Furthermore, assay of the partial crystallinity with the wide angle powder diffraction method and electron diffraction pattern along with the transmission electron micrograph (TEM) yields the micro-morphological description of the membrane material under investigation. The effect of ionizing radiation ( 60 Co ␥-rays, ∼110 kGy) on the membrane material is found to have a significant increase in the relative population of the voids as examined through PALS. An FT IR spectroscopic analysis before and after irradiation of the sample is also presented. These a priori informations are expected to be useful in determining the efficacy of the desalination performance of this polyamide membrane material.

FeC&-doped polypyrrole samples have been synthesized at different temperatures, at 308 K and below. The induced defects in the doped structure were analyzed by positron lifetime and Doppler broadening of the annihilation radiation. The magnitude of the conductivity measured in the samples was found to increase monotonically with the induced defect COIKentlXtiOIl. 0 1997 Ehevier Science Lid

Microstructural studies of silica gel powder were carried out using positron annihilation lifetime spectroscopy (PALS) in conjunction with transmission electron microscopy (TEM). It is argued that the two distinct longlived components found (labeled by τ 3 and τ 4 ) may be ascribed to ortho positronium annihilation in microcavities within the grains and intergranular mesoscopic pores, respectively. In the latter type of void, a significant fraction decays via the three-photon mode. A simple physical picture of positronium annihilation in the larger pores is put forward, while the situation vis-à-vis the smaller cavities is shown to be well described by a modification of the currently prevailing model for the pick-off process. The simple parametrization finally arrived at provides a sharpening of the use of the positron as a useful probe for microstructural study of porous substances. It is emphasized that two different positron annihilation mechanisms prevail in the microcavities and mesopores.

the long polyoxyethylene chain. If such a micellar solution This is a report on the spectroscopic characteristics of UO 2/ 2 in be utilized for isolation and compartmentalization of a metal the excited state in Triton X-100 micellar medium. It also indicates ion species, one must know about the micelle-metal ion some important results of viscosity and surface tension measureinteraction and the structural behavior with respect to the ments of the system which have direct relevance to the spectroconcentration range of the surfactant. scopic investigation in the excited state. The quenching of the This means that in order to frame an application-oriented UO 2/ 2 fluorescence due to Triton X-100, upon micellization in the fluorometric determination of metal ion complexation, one aqueous medium, reveals two kinds of microenvironments of the must characterize the particular system. Of late, there have fluorophore from the Stern-Volmer plot. This has been verified been numerous fluorometric (9, 10) analytical applications by flash photolytic measurements. A blue shift of the quenched emission spectrum is ascribed to the collisional encounter of to determination of low-level uranium in water from interfer-UO 2/ 2 with the head groups of Triton X-100. ᭧ 1996 Academic ing elements and to discriminating it from organic matter Press, Inc. fluorescence by an electronic gating system. These attempts Key Words: micelles; Triton X-100; UO 2/ 2 ; emission spectros-reveal the importance of uranyl ion chelation in various syscopy; viscosity; surface tension. tems projecting to their analytical applications in radioactive waste disposals. There have been a few other reports on hydrolysis of uranium(VI), complexation (11) of uranium(VI) with aqueous humic acids (12, 13), and its absorp-377

with the mixed solvents of n-heptane-butanol, the phase The positron annihilation technique (PAT) has been applied to behavior and stable microemulsion formation possibility study the molecular association phenomenon of Triton X-100 (TXhave been worked out very recently (9), which steered us 100) and formation of reversed micelles in the mixed solvent of to attempt on the systematics of the aggregation behavior in butanol and n-heptane. The results indicate the sensitivity of posithe same nonaqueous phase. tronium (Ps) parameters to the phase transition region due to the Positron annihilation technique is a sensitive spectroself-aggregation phenomenon of TX-100 within the system. The scopic tool to investigate the microphase transitional behavintensity of the long-lived ortho-Ps component, I 3 , and its lifetime, ior and corresponding structural changes associated in such t 3 , show a remarkable change at a critical concentration of the media (10-12). It is the bound state of (e / e 0), the positrosurfactant at Ç1.5 mM coined as the operational CMC of TX-100 in both (1:1) and (1:2) butanol-n-heptane (BuHp) systems. The nium (Ps) atom, its reaction and formation, which plays narrow component/para-Ps intensity as computed from Doppler the key role in elucidating the structural changes associated broadening of annihilation radiation indicates discernable changes within the medium (11, 13). Parameters such as the orthoat the same concentration region (Ç1.5 mM) of TX-100 in the Ps lifetime component (vide its pick-off lifetime) and its system. Further, microphase changes due to the association of intensity and the Doppler broadening of annihilation radiawater molecules within the nonaqueous phase has been studied by tion (DBAR) are greatly dependent on the electronic envi-Ps parameters, which reveals a clear demarcation of the polar and ronment of the medium; therefore, our intention has been to nonpolar zones.

The bubble model, which is conventionally used to ®t the observed characteristics of the picko component of ortho-Positronium decay in liquids, is subjected in this study to a critical assessment. It is demonstrated that in its present form (namely that of a bubble with a sharp boundary) the model is untenable when confronted jointly with experimental data on the lifetime and angular correlation of the decay gammas. A modi®ed version of the model, free from such shortcomings, is presented. Further, the dramatic turnover in the temperature dependence of the rate for the formation of positronium±acceptor complex in the case of nitro-benzene in dierent solvents is interpreted as a Kramers' turnover in the context of the improved Bubble Model.