• Volume 1, Issue 4

      October 1973,   pages  165-207

    • The effect of rotating fields in magnetic resonance

      N D Sen Gupta

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      The phenomenon of magnetic resonance is studied by considering the transverse oscillatory field as superposition of two oppositely rotating fields. One of the rotating fields is taken as strong and the other relatively weak.

    • Application of the three body force shell model to the lattice dynamics of calcium oxide

      S K Agarwal M P Verma

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      The lattice dynamics of GaO has been studied on the basis of the three body force shell model, which takes into account the effect of many body interactions in the lattice potential. The dispersion curves obtained by plottingω vsq agree fairly well with the experiments. It is concluded that the value of the molecular electronic polarizability of the solid must be must small than that determined experimentally which suggests that the interaction system in the solid may have a substantial covalent character.

    • ESR study of exchange coupled pairs in copper diethyldithiocarbamate

      R Kumari Cowsik R Srinivasan

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      ESR investigations on exchange coupled pairs of Cu ions in single crystals of Cu(dtc)2, isomorphously diluted with the corresponding diamagnetic zinc salt, are reported. The spin Hamiltonian parameters for the coupled species (S=1) are:g=2.1025,g+=2.031,A=75.1×10−4 cm−1,B=14.8×10−4,D=276.0×10−4 cm−1 andE=46.7×10−4 cm−1. While theg andA tensors show tetragonal symmetry, the zeor-field splitting tensor is rhombic and has principal axes different from those of theg andA tensors. Intensity measurements made down to 4.2 K indicate that the exchange is ferromagnetic with |FFF| ∼ 10 cm−1. Direct dipole-dipole interaction appears to be the major contribution to the zero-field splitting. A calculation on the distributed point dipole model shows that dipolar interaction is considerably modified by the high covalency of the Cu-S bond and accounts for the rhombic nature of the tensor. The possible exchange mechanisms in Cu(dtc)2—direct exchange and superexchange through the bridging sulphurs—are discussed.

    • A semi-empirical formula for the neutron total cross section and the interpretation of the empirical formula forA ⩾ 40

      I Ahmad N Bano A N Saharia

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      Weaknesses in Angeli and Csikai’s interpretation of their empirical formula for the neutron total cross section (σT) are pointed out. Using the Fourier-Bessel re-presentation of the scattering amplitude a semi-empirical formula forσT is obtained which has greater applicability and also explains the success of the empirical formula for mass numberA ⩾ 40 in terms of the established trends in neutron optical potential parameters.

    • Comments on the capture cross-section of electrons in silver bromide grains

      A P Sharma G P Sharma

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      The values in the range of 10−15 cm2 to 10−14 cm2 have been reported by various authors for the capture cross-section of electrons at the traps in silver bromide grains. In this paper we have given a suitable value of this cross-section which is used for computing the track characteristics in nuclear emulsions (e.g. effective ionization, probability of development, grain density and mean gap length) and the ratio of the rate constant of recombination and trapping. Theoretical results agree well with the experimental observations and the available data of other workers and give a convincing support to our choice of this cross-section parameter.

    • A simple heater attachment to precession camera for high temperature diffraction studies up to 1000°C

      A M Shaikh M A Viswamitra

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      A simple heating attachment suitable for taking x-ray precession photographs at high temperatures is described. It consists of a platinum wire-wound heater, split at the middle for sample insertion and x-ray irradiation. The heater is mounted on a quartzsupport and is independant of both the goniometer head and collimator. This makes crystal mounting and alignment at high temperature as easy as at room temperature and precession photographs up to precession angle$$\bar \mu = 30^ \circ $$=30° can be taken without any interference from the heater assembly, up to 1000°C.

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