• Volume 19, Issue 3

September 1982,   pages  203-302

• Temperature variation of the Debye-Waller factors of Ba++ and F ions in BaF2 powder by x-ray diffraction

The temperature variation of the Debye-Waller factors of Ba++ and F ions in BaF2 powder has been studied using x-ray powder diffraction over the temperature range 77°–298°K. A continuous flow cryostat has been specially fabricated for this purpose for the YPC 50 NM powder diffractometer available in the department. The Debye-Waller factors of Ba++ and F between room temperature and 879°K have been measured using single crystal neutron diffraction by Cooperet al. Theoretical lattice dynamics shell model calculations using a 7-parameter model in a quasiharmonic approximation have been done over a temperature range 77° to 879°K. The theoretical values have been compared with the present x-ray measurements and the single crystal neutron diffraction values and the results are discussed.

• Mechanical properties of semicrystalline polymer-polypropylene

The elastic properties of polypropylene have been investigated in terms of a phenomenological model, which is a modified form of the Takayanagi’s two-phase model. In the present model both the change of crystallinity and orientation of the crystalline chains have been taken into account. The orientation effect at the drawn state has been considered in a simplified manner. It has been shown that a partially aligned crystalline phase may be considered as a superposition of a perfectly aligned crystalline region in an elastically isotropic randomly oriented crystalline phase. The predicted values of elastic modulus agree with experimental values.

• Volume variation of nuclear Grüneisen parameter for equation of state studies

The method of first principle pseudopotential is used to simulate the volume variation of nuclear Grüneisen parameter. The elements Al and Be, for which Neal’s experimental data exist are investigated. The nuclear Grüneisen obtained from the details of phonon frequencies are in better agreement with the experimental data as compared to those obtained from approximate Slater and Dugdale-MacDonald methods, often used in shock wave studies.

• On the theory of electron tunnelling

A generalized expression for the differntial tunnelling current density based on the problem of electron energy distribution introduced during the process has been presented. This is directly applicable to junction devices for the evaluation of their tunnelling I–V characteristics.

• Neutron-antineutron oscillations and SO (10) grand unification

Within the framework of the survival hypothesis for Higgs scalars we comprehensively examine the following question: could there be neutron-antineutron oscillations in SO(10) grand unified theories which would be detectable in the forthcoming experiments? In the process of answering this, we critically discuss and supplement the existing knowledge of the relevant patterns of SO(10) symmetry breakdown in relation to the said oscillations. However, our conclusions are negative with the oscillation period being 1025 years or higher.

• The (α–d) cluster model of6Li and muon capture

The (α–d) cluster model with parameters determined from electron scattering and pion photoproduction processes is used to calculate the muon capture rate in6Li. The result is found to be better than the results calculated in other models and is in agreement with the experimental data.

• Two-body effects on baryon magnetic moments and radiative decays of mesons

We demonstrate that the two-body interaction effects involving spectator quark can account for the discrepancy between theory and experiment for the baryon magnetic moments and meson radiative decays.

The high energy elastic scattering of pp,$$p\bar p$$, pπ+, pπ, pk+ andpk processes is studied at forward directions. The expressions for total scattering cross-sections and the ratios of real-to-imaginary parts of the forward amplitude are derived inP+f model. For the Regge part of the scattering amplitude, the standard form is taken. For the pomeron part, the Harari-Freund conjecture is assumed. The background is assumed to get dominant contributions from the multiparticle exchanges in that channel. These contributions are obtained by parametrising the branch cuts by conformal mapping and polynomial expansion methods. The agreement with the experiment is good. The fits suggest strong exchange degeneracy forpp andkp which in turn is consistent with the Harari-Freund duality.

• Nonlinear distribution functions for the Vlasov-Poisson system

The nonlinear distribution function introduced by Allis has been used to investigate the stability of the solution of Vlasov-Poisson’s equations. The ‘average’ Lagrangian is calculated on the basis of this distribution function, and the ‘average’ variational principle of Witham is applied to discuss modulational stability. It is found that the distribution function of Allis exactly gives rise to the Lighthill’s stability condition of non-linear waves.

• Ion source scaling laws

Simple theory and basic plasma physics experiments are used to deduce scaling laws for ion source discharges.

• Many-body study of van der Walls interaction involving lithium and rare-gas atoms and its contribution to hyperfine shifts

Using linked cluster many-body perturbation theory, the frequency-dependent dipole polarizabilitiesa(ω) has been calculated for the lithium atom. The value ofa(ω) at the static limit (169.04a03) matches well with other available theoretical values and experimental results. These values have been used to calculate the van der Waals constants for interactions of lithium, helium and neon atoms. The values of the van der Waals constants for dipole-dipole interaction in atomic units are −22.9, −44.8, −1465.8, 184950.0, 2011.8, 3896.5, 30.3, 59.0 and 115.1 for Li-He, Li-Ne, Li-Li, Li-Li-Li, Li-Li-He, Li-Li-Ne, Li-He-He, Li-He-Ne and Li-Ne-Ne interactions respectively. Obtaining the suitable response functions for lithium and helium atoms, the long range contribution to Δa(r)/a0 in the study of fractional frequency shift in hyperfine pressure and temperature shift measurements is obtained as −541 atomic units.

• # Pramana – Journal of Physics

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