• Volume 60, Issue 1

      January 2003,   pages  1-166

    • Classical stochastic approach to cosmology revisited

      Moncy V John C Sivakumar K Babu Joseph

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      The classical stochastic model of cosmology recently developed by us is reconsidered. In that approach the parameterw defined by the equation of statep = wp was taken to be fluctuating with mean zero and we compared the theoretical probability distribution function (PDF) for the Hubble parameter with observational data corresponding to a universe with matter and vacuum energy. Even though qualitative agreement between the two was obtained, an attempt is herein made to introduce a more realistic assumption for the mean ofw and use it for the calculations. In the present theory the mean values of bothp andw are taken to be nonzero. The theoretical and observational PDFs are compared for different epochs and values of the Hubble parameter. The corresponding values of the diffusion constantD obtained are approximately constant. We use the scatter in the observed redshift-magnitude data of Type Ia supernova to place limits on the stochastic variation in expansion rate and consequently, on the stochastic variation of the equation of state.

    • Invariance properties of the Dirac equation with external electro-magnetic field

      N D Sen Gupta

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      In this paper, we attempt to obtain the nature of the external field such that the Dirac equation with external electro-magnetic field is invariant. The Poincaré group, which is the maximal symmetry group for field free case, is constrained by the presence of the external field. Introducing infinitesimal transformation ofx and ψ, we apply Lie’s extended group method to obtain the class of external field which admit of the invariance of the equation. It is important to note that the constraints for the existence of invariance are explicity on the electric and magnetic field, though only potentials explicity appears in the equation.

    • Cosmological constant in the Bianchi type-I-modified Brans-Dicke cosmology

      A K Azad J N Islam

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      In 1961, Brans and Dicke [1] provided an interesting alternative to general relativity based on Mach’s principle. To understand the reasons leading to their field equations, we first consider homogeneous and isotropic cosmological models in the Brans-Dicke theory. Accordingly we start with the Robertson-Walker line element and the energy tensor of a perfect fluid. The scalar field φ is now a function of the cosmic time only.

      Then we consider spatially homogeneous and anisotropic Bianchi type-I-cosmological solutions of modified Brans-Dicke theory containing barotropic fluid. These have been obtained by imposing a condition on the cosmological parameter Λ(φ). Again we try to focus the meaning of this cosmological term and to relate it to the time coordinate which gives us a collapse singularity or the initial singularity. On the other hand, our solution is a generalization of the solution found by Singh and Singh [2]. As far as we are aware, such solution has not been given earlier.

    • Riccion from higher-dimensional space-time with D-dimensional sphere as a compact manifold and one-loop renormalization

      S K Srivastava

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      Lagrangian density of riccions is obtained with the quartic self-interacting potential using higher-derivative gravitational action in (4 +D)-dimensional space-time withSD as a compact manifold. It is found that the resulting four-dimensional theory for riccions is one-loop multiplicatively renormalizable. Renormalization group equations are solved and its solutions yield many interesting results such as (i) dependence of extra dimensions on the enegy mass scale showing that these dimensions increase with the increasing mass scale up toD = 6, (ii) phase transition at 3.05 × 1016 GeV and (iii) dependence of gravitational and other coupling constants on energy scale. Results also suggest that space-time above 3.05 × 1016 GeV should be fractal. Moreover, dimension of the compact manifold decreases with the decreasing energy mass scale such thatD = 1 at the scale of the phase transition. Results imply invisiblity of S1 at this scale (which is 3.05 × 1016 GeV).

    • Spectroscopic investigation of the Dergaon meteorite with reference to 10 μm and 20 μm bands

      A Gohain Barua B R Boruah S Bhattacharyya G D Baruah

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      Analysis of a part of the meteorite which fell at Dergaon (India) on March 2, 16.40 local time (2001) is presented with the help of FTIR, absorption and atomic spectra. The FTIR spectrum exhibits prominent absorption bands in the region 800–1100 cm−1, originating from the valence vibration of SiO4, a basic component of the silicate lattice.

    • Hermitian quark matrices

      Narendra Singh

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      Assuming a relation between the quark mass matrices of the two sectors a unique solution can be obtained for the CKM flavor mixing matrix. A numerical example is worked out which is in excellent agreement with experimental data.

    • O(12) limit and complete classification of symmetry schemes in proton-neutron interacting boson model

      V K B Kota

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      It is shown that the proton-neutron interacting boson model (pnIBM) admits new symmetry limits withO(12) algebra which breakF spin but preserves theFz quantum numberMF. The generators ofO(12) are derived and the quantum numberU ofO(12) for a given boson numberN is determined by identifying the corresponding quasi-spin algebra. TheO(12) algebra generates two symmetry schemes and for both of them, complete classification of the basis states and typical spectra are given. With theO(12) algebra identified, complete classification of pnIBM symmetry limits with goodMF is established.

    • The phenomenon of nucleon emission at high angular momentum states of fused compound systems

      T R Rajasekaran S Selvaraj S Santhosh Kumar

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      Nucleon emission from high spin fused compound systems is analyzed in the framework of the statistical theory of hot rotating (STHR) nuclei. This is an elaborate version of our earlier work and we present our results for156Er,166Er,168 Yb and188Hg. We predict an increase in neutron emission for166Er due to the abrupt decrease in neutron separation energy aroundI55ℏ. Since the drop in the separation energy is closely associated with the structural changes in the rotating nuclei, relative increase in neutron emission probability around certain values of angular momentum may be construed as evidence for the shape transition. A similar effect is predicted for168Yb aroundI55ℏ. We also extend the microscopic cranked Nilsson method (CNM) to hot nuclear systems and compare the results with that of the STHR method. The two methods yield different results for triaxially deformed nuclei although for biaxial deformations the results are identical. This is illustrated for186Hg.

    • Estimation of rotational temperature of121Sb16O molecule

      M B Sureshkumar N R Shah Ashish Jaituni

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      The (0,1), (0,0), (1,0) and (2,0) bands ofBX system of antimony monoxide have been excited in rf discharge tube source and photographed in the 9th order of a two metre plane grating spectrograph (PGS-2). Intensity records of the rotational lines have been obtained. The intensity measurements ofQ andP branch lines of the above four bands andJ numberings are used to estimate the effective rotational temperature of the source emitting the spectrum of121Sb16O molecule. It is concluded that121Sb{su16}O may be present inK type stars.

    • Operational characteristics and power scaling of a transverse flow transversely excited CW CO2 laser

      Jai Khare R Sreedhar C P Paul T Reghu A K Nath

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      Transverse flow transversely excited (TFTE) CO2 lasers are easily scalable to multikilowatt level. The laser power can be scaled up by increasing the volumetric gas flow and discharge volume. It was observed in a TFTE CW CO2 laser having single row of pins as an anode and tubular cathode that the laser power was not increasing when the discharge volume and the gas volumetric flow were increased by increasing the electrode separation keeping the gas flow velocity constant. The discharge voltage too remained almost constant with the change of electrode separation at the same gas flow velocity. This necessitated revision of the scaling laws for designing this type of high power CO2 laser. Experimental results of laser performance for different electrode separations are discussed and the modifications in the scaling laws are presented.

    • Characteristics of the fast electron emission produced during the cleavage of crystals

      B P Chandra N L Patel S S Rahangdale R P Patel V K Patle

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      The present paper reports the fast electron emission produced during the cleavage of alkali halide crystals and models the dynamics of the process. The mechano-emission arises as a result of the ionization of surface traps at the expense of the energy which is released in the annihilation of the defects which are formed during cleavage. The slow electrons which appear upon the ionization of surface traps are subsequently accelerated in the field of negatively charged segment of the freshly cleaved surface. Considering the basic mechanism of fast electron emission, expressions are derived which are able to explain satisfactorily the temporal, thermal, charge, surface, coloration, water adsorption and other characteristics of the fast electron emission produced during the cleavage of crystals. The decay time of the charges on the newly created surfaces, and the velocity of cracks can be determined from the measurements of fast electron emission produced during the cleavage of crystals. It is shown that two types of diffusing centres are responsible for the charge relaxation and thereby for the emission of fast electrons produced during the cleavage of alkali halide crystals.

    • Strain field due to transition metal impurities in Ni and Pd

      Hitesh Sharma S Prakash

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      The strain field due to body centered substitutional transition metal impurities in Ni and Pd metals are investigated. The calculations are carried out in the discrete lattice model of the metal using Kanzaki lattice static method. The effective ion-ion interaction potential due to Wills and Harrison is used to evaluate dynamical matrix and the impurity-induced forces. The results for atomic displacements due to 3d, 4d and 5d impurities (Fe, Co, Cu, Nb, Mo, Pd, Pt and Au) in Ni and (Fe, Co, Cu, Ni, Nb, Mo, Pt and Au) impurities in Pd are given up to 25 NN’s of impurity and these are compared with the available experimental data. The maximum displacements of 4.6% and 3.8% of 1NN distance are found for NiNb and PdNb alloys respectively, while the minimum displacements of 0.63% and 0.23% of 1NN distance are found for NiFe and PdFe alloys respectively. Except for Cu, the atomic displacements are found to be proportional to the core radii and d state radius. The relaxation energies for 3d impurities are found less than those for 4d and 5d impurities in Ni and Pd metals. Therefore, 3d impurities may easily be solvable in these metals.

    • An empirical fit to estimated neutron emission cross sections from proton induced reactions

      Moumita Maiti Maitreyee Nandy S N Roy P K Sarkar

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      Neutron emission cross section for various elements from9Be to209Bi have been calculated using the hybrid model code ALICE-91 for proton induced reactions in the energy range 25 MeV to 105 Me V. An empirical expression relating neutron emission cross section to target mass number and incident proton energy has been obtained. The simple expression reduces the computation time significantly. The trend in the variation of neutron emission cross sections with respect to the target mass number and incident proton energy has been discussed within the framework of the model used.

    • Expressions for third-order aberration theory for holographic images

      S K Tripathy S Ananda Rao

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      Expressions for third-order aberration in the reconstructed wave front of point objects are established by Meier. But Smith, Neil Mohon, Sweatt independently reported that their results differ from that of Meier. We found that coefficients for spherical aberration, astigmatism, tally with Meier’s while coefficients for distortion and coma differ.

    • Strain field due to self-interstitial impurity in Ni

      Hitesh Sharma S Prakash

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      The embedded-atom method have been applied to study the strain field produced by the self-interstitial impurity at the octahedral site in Ni. The calculation have been carried out consistently on the basis of discrete lattice theory, using Kanzaki method. The atomic force constants are evaluated using Wills and Harrison interatomic potential. The dynamical matrix and external force are evaluated considering the interaction up to first nearest neighbors. The atomic displacements are tabulated up to 20NN’s. These displacements are of oscillatory nature and of decreasing magnitude with NN’s distance. The physical properties such as self-interstitial formation energy and volume change calculated using atomic displacements are in accordance with the earlier studies.

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