• B A Dasannacharya

      Articles written in Pramana – Journal of Physics

    • Dynamics of liquid ammonia from cold neutron scattering

      C L Thaper B A Dasannacharya P S Goyal

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      The scattering of 4.1 Å neutrons by liquid ammonia has been measured at 218 K in the angular range of 30–90° using the Trombay rotating crystal spectrometer. The experimental data, after correcting for multiple scattering, have been compared with model calculations, and it is shown that it is possible to get detailed information about the rotational correlation function on the basis of neutron experiments alone. The model assumes Langevin diffusion for translational motions. Rotational motions are described by means of an orientational correlation function which has a gaseous behaviour for times up toτ0 and then changes over to a diffusive character with a rotational diffusion constantDr. Within the framework of the model the correlation function can be described withDr=0.28×1013sec−1 andτ0=0.57×10−18sec. Corrections for multiple scattering and their dependence on model parameters are discussed.

    • Lattice dynamics of beryllium

      B A Dasannacharya P K Iyengar R V Nandedkar K R Rao A P Roy C L Thaper

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      Phonon frequencies in beryllium along the principal symmetry directions have been determined by means of the slow neutron inelastic scattering technique. The data are analysed in terms of a six-neighbour force constant model and the force constants are evaluated. It is concluded that strong tensor forces are present in beryllium and the importance of this finding to basic theories of lattice dynamics is pointed out.

    • Normalisation and assessment of neutron diffraction data from liquids

      K R Rao B A Dasannacharya

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      A couple of normalisation criteria have been proposed to assess liquid structure data over the past decade. In this paper, a critical examination of these and other plausible criteria is made. Neutron diffraction data from liquid krypton measured by Clayton and Heaton is used to study these criteria. It is shown that if the structure factorS (Q)=1+γ (Q) satisfies the Krogh-Moe relation, namely$$\rho _{0 = \frac{1}{{2\pi ^2 }}} {\text{ }}\mathop \smallint \limits_{\text{0}}^\infty {\text{ }}Q^{\text{2}} {\text{ }}\gamma {\text{ (}}Q{\text{) }}dQ$$ whereQ is the wavevector transfer andρ0 the mean atomic number density,S (Q) is properly normalised. Subjecting such data to the other criteria would reveal, however, the quality of data in so far as systematic errors and/or termination errors are concerned. If the data is measured with reasonable accuracy to sufficiently large values of the wavevector transfer all these criteria would assess the quality of the data to almost the same extent. This is established in this paper by improving and extending the liquid krypton data by procedures similar to that of Kaplow, Strong and Averbach and subjecting this revised data to the various criteria. It is concluded, thereby, that (i) one can study the neutron diffraction data from liquids using any of the criteria mentioned in the paper to improve the quality of the data and (ii) for normalisation purposes the simple relation {ie328-2} would suffice.

    • Dynamics of liquid silane

      C L Thaper B A Dasannacharya

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      Neutron scattering of cold neutrons from liquid silane at 137° K and 98°K is explained on the basis of a simple model. The rotational diffusion constant,Dr, and the delay time,τ0, after which rotational diffusion may be said to occur are derived on the basis of this model. At 137° K we getDr=0.22×1013 sec−1 andτ0=0.68×10−13 sec. At 98°KDr (=0.06×1013 sec−1) is down by a factor of more than three butτ0=(0.54×10−13 sec) shows only a small change. By comparison with data on liquid CH4 it is concluded that the law of corresponding states is not applicable for describing rotational dynamics of CH4 and SiH4. Rotational motions in SiH4 are more hindered than in CH4 at the same reduced temperature.

    • High resolution measurement of bragg cut-off in beryllium

      C L Thaper P S Goyal B A Dasannacharya

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      Bragg cut-off for$$(10\bar 10)$$ plane of polycrystalline beryllium of various lengths of 300 and 116 K has been measured with an energy resolution of 5 μeV. The natural width of the cut-off is 12.5±1.5 μeV, independent of temperature and length of beryllium and also of physical characteristics and certain metallurgical treatments of the powder. Such blocks of beryllium would be suitable for designing a ΔT-window spectrometer with resolution ⩾20 μeV. Bragg cut-offs corresponding to (0002) and$$(10\bar 11)$$ planes of beryllium have been separated for the first time. These can also be used for producing additional energy windows in a ΔT-window spectrometer, thus increasing its efficiency.

    • ΔT-window neutron spectrometer

      P S Goyal C L Thaper B A Dasannacharya

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      A high resolution neutron spectrometer making use of a ΔT-window filter for the analyser and time-of-flight technique for analysing incident neutron energy has been designed. The spectrometer will provide a continuously variable energy resolution ΔE from 40–50µeV at ∼ 5230µeV. The range of energy transfer allowed is −1450µeV to +2950µeV and the range of wavevector transferQ allowed is 0·82–3·06 Å−1. Depending on the resolution used, the counting rates are expected to vary from 28–60 × 103 counts/hr if one assumes 10% isotropic elastic scattering from the sample.

    • Magnetic field dependence of critical current density in YBa2Cu3O7

      P K Mishra G Ravi Kumar P Chaddah B A Dasannacharya M K Malik

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      We have measured the transport critical current densityJcof sintered YBa2Cu3O7, in various applied fields up to 185 Oe at 77 K. We find a sharp decay ofJcwith magnetic field. We show that this sharp decay is consistent with the low field hysteresis results of Groveret al. We argue that the observed field dependence is not caused by intragranular weak links.

    • Field and temperature dependence of intergranular hysteresis in YBa2Cu3O7

      Shailendra Kumar G Ravikumar P K Mishra P Chaddah B A Dasannacharya Ram Prasad N C Soni

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      We have studied the variation of low field magnetization hysteresis in YBa2Cu3O7 as a function of the maximum magnetic field applied during a hysteresis cycle (1 G<Hmax<7.3 G) and also as a function of temperature (77 K<T<95 K). The remnant magnetization is studied as a function ofHmax andT and the measured dependences are explained using the extended critical state model. The potential of this technique as a contactless method of probing the temperature dependence ofJc is discussed.

    • Form-factors for different aggregation models of micelles

      P S Goyal K Srinivasa Rao B A Dasannacharya V K Kelkar

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      Spherical micelles in ionic micellar solutions, often aggregate to form spherical, cylindrical or chain-like aggregates on addition of salt to the solution. It is known that the technique of small angle neutron scattering (SANS) can be used to distinguish spherical and cylindrical aggregates. To examine if SANS can be used to distinguish the latter two aggregation processes, we have calculated the angular distribution of scattered neutrons from 0.002 M CTAB solutions. These calculations show that aggregation of CTAB micelles results in large changes in SANS spectra. The shapes of SANS spectra are different for the above three types of aggregates, suggesting that technique of SANS can indeed be used to distinguish the three aggregation processes. The size of the aggregate can also be obtained from such studies.

    • Small angle neutron scattering from micellar solutions of triton X-100

      K Srinivasa Rao P S Goyal B A Dasannacharya V K Kelkar C Manohar S xxxV G Menon

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      Micellar solutions of non-ionic surfactant triton X-100 (8% by weight) show phase separation at cloud pointTcp ∼ 335 K. This paper reports results of small angle neutron scattering (SANS) experiments from this solution as a function of temperature between 298 and 332 K. The range of wave-vector transferQ, covered in these experiments is from 0.02 to 0.15 Å−1. It is seen that as one approachesTcp, the neutron scattering cross section diverges in the region of lowQ (<0.06 Å−1) while it is independent of temperature in region of largeQ(>0.06 Å−1). We believe that the divergence of scattering at lowQ with an increase in temperature is because of changes in the structure factorS(Q) of the solution. The measured distributions have been analyzed using four different models for inter-micellar potential. The models used to calculate the structure factorS(Q) are (1) mean spherical approximation (MSA) with Yukawa tail for attractive potential, (2) MSA with an attractive square well potential, (3) random phase approximation (RPA) with an attractive square-well potential and (4) Sticky hard sphere model (attractive square-well potential with Percus-Yevick approximation). The strengths of the attractive potential required to fit the SANS data are (−6.6 to − 14.4)/kt for model (1), (− 6.6 to − 15.0)/kt for model (2), (− 3.8 to − 7.3)/kB T for model (3) and (−2 to −2.7)/kt for model (4). On the basis of reasonableness of the derived strength of the potential near the phase separation temperature and its relative temperature dependence, it is concluded that present data favour the Sticky hard sphere model.

    • Raman study of phase transition in ferroelectric Ba0.95Ca0.05TiO3

      Rekha Rao A P Roy B A Dasannacharya S Balakumar R Ilangovan P Ramasamy

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      Raman scattering investigation of phase transition in the ferroelectric Ba0.95Ca0.05TiO3 is reported. The results suggest onset of significant dynamic disorder at 105°C. This corroborates findings of recent structural study regarding large positional disorder associated with Ti and O1 atoms well below the tetragonal to cubic transition temperature (∼150°C).

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