• S Singh

      Articles written in Pramana – Journal of Physics

    • Second virial coefficient for a mixture of nonspherical molecules of arbitrary symmetry

      S Singh K P Shukla

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      The mixture and interaction second virial coefficients of a binary gas mixture of nonspherical molecules of arbitrary symmetry have been calculated for a set of unlike force parameters which is obtained from the force parameters for like interactions by using empirical combination rules. In the calculation molecular anisotropy of very general type has been accounted. The relative contribution of each branch of interactions has been evaluated as a function of temperature. The theoretical values of interaction second virial coefficient have been compared with the experimental data of N2+A, N2+He, N2+H2, N2+O2, N2+C2H4, N2+C2H6, CO2+A, CO2+H2, CO2+N2, CO2+O2, CO2+CO, CO2+C2H4, CO2+C2H6, O2+A and H2+CO. The agreement between theory and experiment is satisfactory for all the systems. Numerical estimations of the mixture second virial coefficients as a function of temperature and composition are given for the systems CO2+A, CO2+H2, CO2+N2, CO2+O2 and CO2+CO.

    • Interaction second virial co-efficient of polar-quadrupolar gas mixtures

      S Singh

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      The interaction second virial coefficient of a binary polar-quadrupolar gas mixtures of non-spherical molecules of arbitrary symmetry has been calculated for a set of unlike force parameters which is obtained from the force parameters for like interactions by using empirical combination rules. In the calculation the influence of anisotropic interactions has been considered. The relative contribution of each branch of interactions has been evaluated as a function of temperature. The theoretical results have been compared with the experimental data of CH3F + N2, CH3F + CO2 and CH3Cl + CS2. The agreement between theory and experiment is satisfactory.

    • On the virial coefficients of the equation of state of a system of non-axial molecules

      S Singh

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      A thermodynamic perturbation theory in which all angle-dependent interactions are considered as a perturbation of the central potential is applied to calculate the second and third virial coefficients of a fluid composed of non-axial molecules. The influence of a large number of anisotropic pair- and three-body non-ādditive interactions has been considered. Experimental values have been used for the dipole moment, quadrupole tensor and for anisotropic polarizability. The parameters for the central Lennard-Jones (12-6) potential have been determined from the viscosity data. The relative contribution of each branch of pair and triplet interactions has been evaluated as a function of temperature for molecules possessing both the axial- and non-axial symmetries. It has been shown that the non-axial approximation is an improvement over the axial one. Theoretical results have been compared with the experimental data of CH3OH.

    • Clustering phenomena in radioactive and stable nuclei and in heavy-ion collisions

      S S Malik S Singh R K Puri S Kumar Raj K Gupta

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      A theory for clustering formation in nuclei and in heavy-ion collisions has been worked out in terms of the quantum-mechanical fragmentation process. Treating the mass fragmentation and relative separation coordinates as weakly coupled, the spontaneous cluster-decay of radioactive nuclei has been considered as a two-step process of clustering formation and tunnelling of the confining nuclear interaction barrier. This model has also been applied to “stable” nuclei, lighter than lead. The effects of adding more and more neutrons to collidingN =Z,A = 4n nuclei are studied for theα-clustering transfer phenomenon.

    • Bulk viscosity of neutron stars

      V K Gupta A Wadhwa S Singh J D Anand

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      Viscosity of neutron stars has been a continuing area of research for many years now. Recently interest in this field has revived because of the possibility of URCA processes in neutron stars. In this paper we report calculation of the bulk viscosity of neutron stars from these processes. For this purpose we have used theβ-decay rates which were calculated without making the usual approximations of neglecting the neutrino momentum and using the nuclear mean field theory for the description of interacting nuclear matter. Also we have not restricted our calculation to the linear regime which corresponds to the assumption that fluctuations in the chemical potential away fromβ-equilibrium remain small: Δμ/kT ≪ 1. We find that for large amplitude fluctuations, where the linear approximation is not valid, bulk viscosity increases by many orders of magnitude. Also, as against strange matter stars, where the viscosity first increases with increasing temperature and then starts decreasing beyond 0.1 MeV, we find that the viscosity increases uniformly with temperature at least up to 2MeV. We discuss the implications of these results for the stability of neutron stars.

    • Bulk viscosity of strange quark matter in density dependent quark mass model

      J D Anand N Chandrika Devi V K Gupta S Singh

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      We have studied the bulk viscosity of strange quark matter in the density dependent quark mass model (DDQM) and compared results with calculations done earlier in the MIT bag model where u, d masses were neglected and first order interactions were taken into account. We find that at low temperatures and high relative perturbations, the bulk viscosity is higher by 2 to 3 orders of magnitude while at low perturbations the enhancement is by 1–2 order of magnitude as compared to earlier results. Also the damping time is 2–3 orders of magnitude lower implying that the star reaches stability much earlier than in MIT bag model calculations.

    • Radial oscillations of neutron stars in strong magnetic fields

      VK Gupta Vinita Tuli S Singh JD Anand Ashok Goyal

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      The eigen frequencies of radial pulsations of neutron stars are calculated in a strong magnetic field. At low densities we use the magnetic BPS equation of state (EOS) similar to that obtained by Lai and Shapiro while at high densities the EOS obtained from the relativistic nuclear mean field theory is taken and extended to include strong magnetic field. It is found that magnetized neutron stars support higher maximum mass whereas the effect of magnetic field on radial stability for observed neutron star masses is minimal.

    • Cooling of fermionic $^{83}\rm{Kr}$ and bosonic $^{84}\rm{Kr}$ isotopes in a magneto-optical trap

      S SINGH V B TIWARI S R MISHRA

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      Simultaneous laser cooling of two isotopes of krypton, $^{83}\rm{Kr}$ and $^{84}\rm{Kr}$, is reported here in a two-isotope magneto-optical trap (TIMOT). The number of cold metastable $^{83}\rm{Kr}$ atoms in this TIMOT is dependent on the power of the repumping laser beams used, which is maximised for our set-up by varying the powers of repumping lasers. These studies may be useful to investigate cold collisions between fermionic $^{83}\rm{Kr}$ and bosonic $^{84}\rm{Kr}$ atoms in the metastable state.

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