• Volume 9, Issue 4

      October 1977,   pages  321-439

    • Total toK-shell photoelectric cross section ratios in Zr, Ag, Ta, and Th

      K Sivasankara Rao V Radha Krishna Murty K Parthasaradhi J Rama Rao V Lakshminarayana

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      Total photon atomic cross sections in elements Zr, Ag, Ta and Th were determined around theirK-edges in the energy region 6 to 400 keV with a good geometry set-up using proportional counter or Ge(Li) detector system. From these values the photoelectric cross sections were obtained by subtracting the theoretical values of coherent and incoherent scattering cross sections. The resulting photoelectric cross sections were fitted to curves above and below theK-edge for each element and extrapolated on either side and the total toK-shell photoelectric cross section ratios were determined with an error not exceeding 2%. The ratios were compared with the theoretical values obtained by Grodstein and others, as also from the empirical relation of Hubbell. The present values show good agreement with those of Scofield. A definite trend of decrease of the ratio as the energy increases is observed in the case of Th unlike in the previous experimental studies.

    • Use ofDJ, DJK constants for the unique fixing of intramolecular forces inXY3 pyramidal molecules

      T R Ananthakrishnan C M Paul C P Girijavallabhan

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      A mathematical formalism is developed to evaluate all the possible sets of intramolecular force fields fitting fundamental vibrational frequencies and centrifugal distortion constants in the case ofXY3 pyramidal type molecules. The method is applied to PF3 molecule as an example. It is found that there exist as many as four sets of force field fitting all the above experimental data. A few general criteria are suggested to eliminate the suprious sets, thus making further experimental data unnecessary to fix the true physical force field.

    • Fourier transform of the collagen triple-helical structure and its significance

      M Bansal

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      The Fourier transforms of the collagen molecular structure have been calculated taking into consideration various side chain atoms, as well as the presence of bound water molecules. There is no significant change in the calculated intensity distribution on including the side chain atoms of non-imino-acid residues. Taking into account the presence of about two bound water molecules per tripeptide unit, the agreement with the observed x-ray pattern is slightly improved. Fourier transforms have also been calculated for the detailed molecular geometries proposed from other laboratories. It is found that there are no major differences between them, as compared to our structure, either in the positions of peak intensity or in the intensity distribution. Hence it is not possible to judge the relative merits of the various molecular geometries for the collagen triple helix from a comparison of the calculated transforms with the meagre data available from its x-ray fibre pattern. It is also concluded that the collagen molecular structure should be regarded as a somewhat flexible chain structure, capable of adapting itself to the requirements of the different side groups which occur in each local region.

    • CNDO calculations of some thioamides—III

      L Srinivas Rao B K Gupta

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      CNDO calculations are made for thioacetamide, N-methyl-thioacetamide (both in cis and trans forms) and N, N dimethylthioacetamide. The charges, bond orders and dipole moments of the molecules are discussed. The contributions of changes in pi-energy and monoatomic energy to the barier to internal rotation about C-N bond in thioacetamide are calculated. The assignments of first and second ionization potentials and photoelectron spectra of thioamides are discussed.

    • On the perception and measurement of the accelerated observer of the clock problem

      Alberto Chamorro

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      The situation of the abruptly accelerated observer usually considered when dealing with the clock problem is analysed at the moment of the acceleration. It is shown that, contrary to some views held in the past, special relativity theory consistently predicts that this observer will not see any jump in the readings of a remote clock. It is also made clear how current statements on discontinuities measured by the observer do not have the same operational validity as measurements obtained by unaccelerated observers. If the accelerated observer uses a single operational prescription he does not have to get discontinuities in his measurements of the readings or positions of the distant stationary clock at the acceleration instant. This is related to the essential fact that the process of measurement of the space-time coordinates of an event takes to the observer a time interval that contains the instant at which the event happens. And if an observer is to obtain operational results consistent with the Lorentz transformations he must be inertial during that whole time interval. Finally, another operational asymmetry between an inertial observer and an accelerated one is pointed out.

    • Solutions of scalar and electromagnetic wave equations in the metric of gravitational and electromagnetic waves

      T Padmanabhan

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      The wave equation for a scalar field ϕ and vector potentialA* are solved in the background metric of a gravitational wave. The corresponding solutions when the metric is generated by a plane electromagnetic wave, is obtained from these solutions. The solution for the scalar wave is discussed in detail. It is found that because of the interaction, two new waves are generated in the lower order approximations. One of them has the same phase dependence as the original wave while the other shows a transient character. There is no interaction when the waves are along the same direction.

    • Aspects of symmetry in the problem of phase transitions in improper ferroelectrics

      I S Zheludev

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      The term ‘improper’ ferroelectrics has recently come to be used for ferroelectrics in which the spontaneous polarizationPs is not a transition parameter. From the standpoint of symmetry therefore, the new (polar) symmetry group is no longer the highest common sub-group of the crystal symmetry and the spontaneous polarization symmetry (polar vector symmetry group ∞mm). The concept of an improper ferroelectric has therefore been revised to include transitions from polar to polar groups. It is shown that in the general cases where other parameters are permissible, the problem has to be solved using the symmetry of directions. The present analysis on the bases of irreducible representation of polar crystals establishes all the possible cases where spontaneous polarization is accompanied by spontaneous magnetisation, optical activity and more complex phenomena. The symmetry changes and the polarization configurations in Gd(MoO4)3, boracites, (NH4)2BeF4, alkali trihydrogen selenite family have been discussed in the light of the extended concept of an ‘improper’ ferroelectric.

    • Kinetics of oxygen chemisorption and its effects on electrical conductivity of thin evaporated CdS films

      V G Bhide S Jatar A C Rastogi

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      The mechanism and kinetics of chemisorption of oxygen on thin evaporated films of CdS have been investigated through changes in electrical conductivity caused by chemisorbed oxygen. Electrical conductivity of CdS films, when exposed to oxygen atmosphere falls exponentially with time, with a characteristic time determined by several factors such as temperature, electric field applied to the sample, illumination, etc. Electric field applied in appropriate direction is shown to enhance oxygen chemisorption. Complete theoretical treatment for these changes has been provided and the agreement between theory and experiment is shown to be excellent.

    • Reflectance spectra and thermoluminescence of NaBr coloured in an electrodeless discharge

      S V Moharil B T Deshmukh

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      A comparative study of the optical and thermoluminescent properties ofγ irradiated NaBr and NaBr coloured in an electrodeless discharge is reported. In discharge coloured NaBrF centre absorption maxima shifted with the colouration time. This is tentatively attributed to the formation ofE centres. Correspondingly, an additional peak was observed in thermoluminescence glow curve. It is suggested that the results are not characteristic of the method of colouration but rather of the imperfections inherent to the powders, and in a perfect (undeformed) single crystal such a phenomenon should not be observed.

    • Total (p, n) reaction cross section study on51V over the incident energy range 1·56 to 5·53 MeV

      M K Mehta S Kailas K K Sekharan

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      The total (p, n) reaction cross section for51V has been measured as a function of proton energy in the energy range 1·56 to 5·53 MeV with thick and thin targets. The fluctuations in the fine resolution excitation functions were analysed, to extract 〈Γ〉, the coherence width. The thick target excitation function suitably averaged over appropriate energy intervals has been compared with the optical model, Hauser-Feshbach and Hauser-Feshbach-Moldauer calculations. The strong isobaric analog resonance atEp ∼ 2·340 has been shape analysed to extract the proton width Γp, the spreading withW and the spectroscopic factor.

    • Shape of the 935 keV beta transition of143Pr

      K Venkata Ramaniah K Venkata Reddy

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      The beta spectrum shape of the 7/2+ → 7/2 beta transition in the decay of143Pr is measured employing a thoroughly tested intermediate image beta ray spectrometer of the Siegbahn-Slatis type and using the source produced by irradiation of highly enriched stable142Ce sample. The spectrum exhibits a definite energy dependence and is well fitted with shape factorC(W) = 1 − (0·057 ± 0·006)W whereW is in m0c2 units for an end point energy of 935 ± 2 keV agreeing well with the results of Hamiltonet al and Spejweskiet al and disagreeing with the results of Persson and Reynolds. The single particle configurations for the low lying levels of143Pr could not give quantitative results.

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