• Volume 17, Issue 1

      July 1981,   pages  1-120

    • A study of Fermi level shifts in copper-gallium and copper-germanium alloys by x-ray absorption spectroscopy

      N R Lokhande Chintamani Mande

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      The K x-ray absorption discontinuities of copper, gallium and germanium in pure metals as well as in six copper-gallium and three copper-germanium alloys have been studied using a Cauchois-type bent crystal spectrograph. It is observed that in all the systems the K discontinuity of copper shifts towards the higher energy side, while the discontinuities of gallium and germanium shift to the lower energy side relative to their positions in the respective pure metals. The magnitudes of the shifts are found to increase with the decrease in the content of the absorbing atom in the alloys. The observed shifts are explained on the basis of the free-electron theory of metals and the rigid-band model for alloys.

    • Semi-quantitative theory of the structures of simple ionic crystals

      Ramesh Narayan

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      A simple theory is developed which shows that the regions of stability of the CsCl, NaCl and ZnS structures can be demarcated in a two-dimensional plot of the radius ratio versus the strength of the van der Waals interaction. There is good agreement with experiment. The effect of pressure on these structures is explained qualitatively. The increased occurrence of the ZnS structure and the decreased stability of the CsCl structure in the A2+ B2− crystals compared to the A+B crystals is also explained. Finally it is shown that the radius ratio and the polarizabilities of the ions are the important factors that determine the structures of AB2 crystals.

    • Elastic constants of galena down to liquid helium temperatures

      V C Padaki S T Lakshmikumar S V Subramanyam E S R Gopal

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      The elastic constants of single crystal galena have been determined from the measured ultrasonic velocities down to liquid helium temperature. A cryostat incorporating an arrangement to inject the liquid bonding material at low temperature is described. At 5 K, the values of elastic constants are C11=14.90, C12=3.51 and C44=2.92×1010 N/m2.

    • Behaviour of ultrasonic velocities in amorphous Se90Ge10 and Se85Ge15 alloys near their glass transition

      P E S Kartha V C Padaki S T Lakshmikumar E S R Gopal

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      Precise measurements of 10 MHz frequency longitudinal and shear wave velocities are reported in amorphous SeGe alloys near their glass transition temperature Tg. There is a sharp decrease of the velocities near Tg, but the reduction in velocities appears smaller than expected.

    • An optical topographic technique to study the dissolution kinetics at dislocation sites in sodium chloride crystals

      A K Shrivastava K G Bansigir

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      A technique has been developed to study the dissolution kinetics at dislocation sites in sodium chloride single crystals. A parallel beam of light is allowed to fall upon the silvered etched surface of the crystals and the reflected beam is received back on a photographic film. The pattern thus obtained on the photographic film gives information about (i) the structure of the etched surface (ii) the nature of the surface forming the pit and (iii) the slope of the pit at the source of dissolution.

    • Solvable models of temporally correlated random walk on a lattice

      V Balakrishnan

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      We seek the conditional probability functionP(m,t) for the position of a particle executing a random walk on a lattice, governed by the distributionW(n, t) specifying the probability ofn jumps or steps occurring in timet. Uncorrelated diffusion occurs whenW is a Poisson distribution. The solutions corresponding to two different families of distributionsW are found and discussed. The Poissonian is a limiting case in each of these families. This permits a quantitative investigation of the effects, on the diffusion process, of varying degrees of temporal correlation in the step sequences. In the first part, the step sequences are regarded as realizations of an ongoing renewal process with a probability densityψ(t) for the time interval between successive jumps.W is constructed in terms ofψ using the continuous-time random walk approach. The theory is then specialized to the case whenψ belongs to the class of special Erlangian density functions. In the second part,W is taken to belong to the family of negative binomial distributions, ranging from the geometric (most correlated) to the Poissonian (uncorrelated). Various aspects such as the continuum limit, the master equation forP, the asymptotic behaviour ofP, etc., are discussed.

    • Overall values of conventional discrepancy indices for related and unrelated models of a non-centrosymmetric crystal with a centrosymmetric group

      M N Ponnuswamy S Parthasarathy

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      Theoretical expressions for the overall values of the conventional discrepancy indicesR(F) andR(I) are derived for a non-centrosymmetric crystal with a centrosymmetric group by taking the centrosymmetric group and a part of the other atoms in the unit cell as the trial structure. These results are used to obtain tables of values of these indices in terms of the parameter σ1c2 and σ12 which define the fractional contribution to the local mean intensity from the centrosymmetric group and all the known atoms respectively.

    • Rotational excitation of H2 and N2 molecules by electron impact

      G P Gupta K C Mathur

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      The rotational excitation of the H2 and N2 molecules from (J→J+2,J=0) and (J→J+2,J=1) states by electron impact is studied, in the framework of the regional-plane-wave approximation. The empirical cut-off parameterρ, involved in these calculations, is varied to give the best fit to the experimental data of Srivastava and co-workers.

    • Sound velocity in water vapour

      Pankaj Pathak Om Singh

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      The effective potential is used to calculate the sound velocity of water vapour. The agreement between theoretical and experimental values confirms the validity of the effective potential.

    • Pre-equilibrium effects in (n, 2n) reactions at 14.2 MeV+

      N Lakshmana Das C V Srinivasa Rao B V Thirumala Rao J Rama Rao

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      With a view to study the pre-equilibrium effects in neutron-induced reactions, the activation cross-sections for (n, 2n) reactions at 14.2±0.2 MeV in the heavy mass region have been measured using the versatile mixed powder technique and high resolution Ge(Li) detection. The experimental cross-sections are found to be consistently smaller than the predictions based on the statistical theory and this is attributed to the effect of pre-equilibrium decay in these reactions. The cross-sections due to pre-equilibrium decay were estimated using exciton, hybrid and unified models. When this cross-section was included in comparing the experimental cross-sections with theory, we obtained better agreement, within the limitations of the present-day preequilibrium theories.

    • Recoil nuclear polarization in muon capture—Effect of target thickness and finite range of nuclear recoils

      V Devanathan P R Subramanian

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      The effect of target thickness and finite range of nuclear recoils is investigated in the study of average and longitudinal polarizations of recoiling nuclei resulting from the capture of muons by spin zero target nuclei.

    • An analysis of energetic cosmic ray interactions in graphite

      R Hasan M S Swami

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      Fifty two high energy cosmic ray interactions in graphite have been analysed. The analysis strongly suggests that some of the characteristics of high energy interactions are dependent on the total transverse momentum,Σpt, of allγ-rays in an interaction. Out of the 52 events analysed, 29 haveΣpt⩽2.5 GeV/c and the rest haveΣpt>2.5 GeV/c; the former are called smallpt events while the latter are designated as largept events. For these two types of events, the characteristics investigated are: (a) fractional energy distribution ofγ-rays, (b) the invariant mass ofγ-rays and (c) the energy distribution ofγ-rays in the emission system.

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