• C L Thaper

      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.

    • 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.

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