• Volume 40, Issue 4

      April 1993,   pages  1-327

    • Rotational excitations of NH4+ in (NH4)xK1−xI

      S L Narasimhan R Mukhopadhyay

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      We propose a gaussian model for the distribution of rotational tunneling lines which constitute the experimentally observed inelastic neutron scattering spectra in the range 0 to 0.6 meV from the mixed system (NH4)xK1−xI. The intensity profiles generated by this model are in remarkable agreement with the experimentally observed ones.

    • On a simple derivation of master equations for diffusion processes driven by white noise and dichotomic Markov noise

      V Balakrishnan

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      A very simple way is presented of deriving the partial differential equations (the master equations) satisfied by the probability density for certain kinds of diffusion processes in one dimension, in which the driving term is a Gaussian white noise, or a dichotomic noise, or a combination of the two. The method involves the use of certain ‘formulas of differentiation’ to derive the equations obeyed by the characteristic functions of the processes concerned, and thence the corresponding master equations. The examples presented cover a substantial number of diffusion processes that occur in physical modelling, including some master equations derived recently in the literature for generalizations of persistent diffusion.

    • Floquet states of a periodically kicked particle

      A K Sikri M L Narchal

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      We present explicit expressions for the Floquet states of a periodically kicked particle in coordinate and momentum representations. These states have been used to evaluate the energy of the particle after an arbitrary number of kicks.

    • Gravitational field of a tachyon in a de Sitter universe

      K S Virbhadra

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      An exact solution of Einstein’s equations is interpreted as describing the gravitational field of a tachyon in a de Sitter universe. Switching off the cosmological constant yields the gravitational field of a tachyon in flat spacetime background.

    • Astrophysically significant solutions of the Einstein and Einstein-Maxwell equations from the Laplace’s seed

      K C Das S Chaudhuri

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      The stationary solutions given by Amenedo and Manko generated from known solutions of Laplace’s equation as seed have been generalised to include the electromagnetic field. Further, the exterior solution of an axially symmetric rotating body with higher multipole moments and a solution corresponding to a Kerr object embedded in a gravitational field are given. We also give a method for constructing stationary vacuum solutions from static magnetovac solutions and vice versa and discuss a specific application of this method.

    • Strange solitons in a chiral quark-meson model

      J Segar M S Sri Ram

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      We consider aSU(3) quark soliton model based on chiral invariant quark-meson coupling. We find soliton solutions with nonzero strangeness andB=1 in the model with nontrivial kaonic fields, for values of the coupling constant greater than the phenomenologically acceptable number. Hence they do not correspond to known strange baryons.

    • Prompt neutron emission spectra and multiplicities in the thermal neutron induced fission of235U

      M S Samant R P Anand R K Choudhury S S Kapoor K Kumar D M Nadkarni A Saxena

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      The emission spectra of prompt fission neutrons from mass and kinetic energy selected fission fragments have been measured in235U(nth,f). Neutron energies were determined from the measurement of the neutron time of flight using a NE213 scintillation detector. The fragment energies were measured by a pair of surface barrier detectors in one set of measurements and by a back-to-back gridded ionization chamber in the second set of measurements. The data were analysed event by event to deduce neutron energy in the rest frame of the emitting fragment for the determination of neutron emission spectra and multiplicities as a function of the fragment mass and total kinetic energy. The results are compared with statistical model calculations using shell and excitation energy dependent level density formulations to deduce the level density parameters of the neutron rich fragment nuclei over a large range of fragment masses.

    • Collective dipole oscillations in atomic nuclei and small metal particles

      R S Bhalerao Mustansir Barma

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      The systematics of photon absorption cross sections in nuclei and small metal particles are examined as a function of the number of constituent fermionsA. It is pointed out that the shell-structure-linked oscillations in the full width at half maximum (FWHM) of the photoneutron cross section in nuclei, earlier recognized forA>63, in fact persist down to the lightest nuclei. Averaging over the oscillations or focusing on the lower envelope of the oscillating curve (magic nuclei), the FWHM is seen to generally decrease with increasingA, consistent withA−1/3, a dependence which was earlier known to hold in metal particle systems. If the FWHMs are scaled by the respective Fermi energies and the inverse radii by the Fermi wave vectors, the two data sets become comparable in magnitude. A schematic theoretical description of the systematics is presented.

    • Electron impact ionization of Zn+ and Ga+

      Gopaljee S K Shrivastava B N Roy

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      We have investigated the contribution of excitation-autoionization to the electron impact ionization of Zn+ and Ga+ using the binary encounter approximation. Hartree-Fock velocity distributions for the bound electrons have been used throughout the calculations of direct and indirect ionization cross-sections. The calculated cross-sections are in good agreement with recent experiments. We have also compared our results with other theoretical calculations.

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