• Volume 10, Issue 2

      February 1978,   pages  115-226

    • A search for heavy leptons in cosmic radiation underground

      P N Bhat P V Ramana Murty

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      An experiment to search massive long-lived, weakly interacting particles (leptons) in cosmic radiation has been conducted at Kolar Gold Fields at a depth of 7.6 hg cm−2 (1 hg cm−2=100 g cm−2) below surface. The apparatus was senstive to sub-relativistic (velocity<0.75 c) charged leptons of mass greater than that of a proton and life times greater than a microsecond. The method consists of selecting charged particles using a scintillator counter telescope and vetoing relativistic particles (velocity >0.75 c) by using a water Čerenkov detector. The range of the particle is observed in arrays of neon flash tubes interspersed with iron absorbers. During 3000 hours of observation 28 events were recorded satisfying the trigger and event selection criteria. Bulk of these events were interpreted as due to recoil protons (low energy) from the inelastic scattering of high energy muons in the overhead absorber. The remaining events were interpreted as either atmospheric stopping protons or stopping muons that failed to generate a Čerenkov signal. The observed events are thus consistent with the background and no heavy leptons were seen. From our observations an upper limit of 2.12×10−7 (with 90% confidence level) is set on the ratio of the flux of heavy leptons to that of all muons at this depth.

    • A new interpretation of an event attributed to a magnetic monopole

      N Durgaprasad M V S Rao

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      An alternate and a new interpretation is given for the event attributed to a magnetic monopole by Price and coworkers found in an emulsion plastic sandwich stack flown from Sioux City, Iowa, USA on 18 September 1973. The electron pick-up and stripping cross-sections of nuclei ofZ∼70–80 andv∼0.6–0.7c in Lexan polycarbonate are calculated using the formulae given by Nikolaev. It is shown that the corresponding mean free paths are of the order of thickness (∼250µ) of Lexan plastic sheets used by them. In such a case asnapshot of these processes is believed to have been observed in plastic sheets. Monte-Carlo simulations of the event have been made for three values of charges at the top of the main Lexan stack, namelyZ=83, 78 and 70 respectively. The event is thus interpreted as a cosmic ray nucleus ofZ=70–83 andv=0.6–0.7c losing and capturing electrons (mainly the latter) as it passes through the stack. The probability of the occurrence of such an event is estimated by several methods.

    • Homogeneous instabilities in nematic liquid crystals

      U D Kini

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      The effect of a magnetic field on the instability threshold in shear flow and plane Poiseuille flow of nematics has been worked out for the case of unperturbed director orientation normal to the plane of shear. The critical shear rate for the onset of instability has been studied as a function of destabilising field. The dependence of the time constantν on shear and field for time dependent perturbations in shear flow is found to be explained by two solutions which differ from one another to the extent that wave vectors which are real for one solution are imaginary for the other; these solutions exist for different ranges of shear and field but have one common point where they yield the sameν value making possible a continuous description ofν. In Poiseuille flow even for a destabilising field applied along the primary velocity the occurrence of an instability associated with net secondary flow (flow transverse to the primary velocity) is found to be more favourable than one having no secondary flow. This trend is also observed for low destabilising fields applied along the velocity gradient but for higher fields the occurrence of an instability without net secondary flow becomes more favourable. Simultaneous application of a stabilising electric field does not change the qualitative nature of the results.

    • Electron paramagnetic resonance of Mn2+ and Gd3+ in Pr2Zn3(NO3)12.24 H2O single crystals

      V K Jain T M Srinivasan

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      The electron paramagnetic resonance of Mn2+ and Gd3+ doped in Pr2Zn3(NO3)12.24H2O single crystals has been studied at X-band. Mn2+ substitutes for two Zn2+ sites, while Gd3+ substitutes for single type of Pr3+ sites. The spin-Hamiltonian analysis of the EPR spectra is presented at 298 K as well as 77 K.

    • Domain structure in ferroelectric PbNb2O6

      S G Ingle B M Bangre

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      Single crystals of ferroelectric PbNb2O6 were grown employing a modification of the technique of Goodman. The results obtained on the domain structure were analysed and compared with those reported on BaTiO3 and KNbO3. The domain structure observed here corresponds to the twinning on (110) plane of the unit cell reported by Francombe and Lewis or the subcell reported by Labbe and others. The (001) planes were observed, as the crystal habit is such as to produce (001) planes, and the cleavage plane is also (001). Also the analysis of the observations can be done easily under these conditions. The domains observed are 90° domains with polar axis in (001) plane. Wedge shaped domains and spikes are present as in BaTiO3 and KNbO3. The twinning can occur also on$$\left( {\bar 110} \right)$$ plane producing a domain line at 90° with that due to twinning on (110). This gives patterns of perpendicular lines similar to those in KNbO3 and BaTiO3. Crystal structure considerations show that the domain structures with polarization in and out of the observed (001) plane are not possible, and also were not observed. In this sense, it is a two dimensional ferroelectric. The studies showed a peculiar grain structure in the crystals, and it can be explained on the basis of the growth habit of the crystal. The polarizing microscope is particularly useful in analysing the domain structure along with the grain structure.

    • Thermoelectric power and ac conductivity of A-type Nd2O3

      Virendra Pratap B K Verma

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      Measurement of thermoelectric power Θ of pressed pellets of A-type Nd2O3 from 550 to 1180K and electrical conductivity (σ) at dc, 50 Hz, 1.542 kHz and 3 kHz at different temperatures is reported. It is concluded that electrical conduction at high temperature (T>600K) in this solid is due to positive large polarons in O2− : 2p (valence) band and negative intermediate polarons in Nd3+ : 5d (conduction band). The energy band gap of the solid has been found to be 2.44 eV. At low temperatures, conduction by hopping of charge carriers from one impurity centre to another has been predicted.

    • Surface exciton modes for plane and spherical semiconductor-metal interfaces

      S Srinivasan Sudhanshu S Jha

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      An approximate method is developed for investigating the nature of interface exciton modes in a composite spatially dispersive medium. The method is general enough to be applicable to any composite system, in which each component is described by an arbitrary bulk dielectric functionε(q, ω). It is based on the extension of the usual electrostatic-image method of solving the Poisson’s equation, in the presence of an external point charge in the system. We have applied our general method to a composite system of a finite metal slab surrounded by a semiconductor on one side and the vacuum on the other side. Similarly, we have also considered the case of a metallic sphere of radiusR, surrounded by a semiconductor, with a spherical interface between them. With assumed spatially dispersive model dielectric functions for the bulk metal and the bulk semiconductor, the nature of the electron-electron interaction and the interface exciton modes in the metallic region are obtained in both the cases. For the relevant size of the metal large compared to the atomic dimensions over which the bulk dielectric functions are non-local due to the spatial dispersion, it is shown that one can obtain the interface exciton modes by first defining new effective dielectric functions for each of the media making the particular interface, and then using the usual expression which determines the modes in the non-dispersive case.

    • Use of atomic fues potential within the floating spherical gaussian orbital method: Study of some two-valence-electron diatomics and triatomic ions

      N K Ray S P Mehandru

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      Atomic Fues potential type model potential is used within the floating spherical gaussian orbital formalism to study a series of two-valence-electron diatomics and triatomic ions. Calculated equilibrium geometries and force constants are in excellent agreement with available experimental data and results of all-electronab initio studies. Electron populations are given for fourteen diatomic species and comparison has been made with the results of other theoretical studies. The predicted results are generally satisfactory.

    • Infrared intensity analysis of some MXY2 type molecules

      Pratibha Naik N Rajeswara Rao

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      Infrared intensity analysis of MXY2 type molecules has been attempted. The dipole momentμ of C=O is obtained to be different for different moleculesviz. for CH2O, 1.9052; for COCl2, 1.1517; for COF2, 0.6340; and for COBr2, 0.7687. Similarly in the case of CSF2 and CSCl2μC=S it was found to be 0.2473 and 0.2983 respectively. This shows that the effect of the electro-negative halogen atoms is very important.

    • Particle-hole calculations in4He and16O with centre of mass subtracted kinetic energy

      J Dey A Ansari

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      It is found that in light nuclei it is very important to use a relative kinetic energy operator. This kinetic energy is mass number dependent and contributes in all odd parity (J*,T) channels. In particular it is seen to produce the spurious state exactly at zero energy in the (1, 0) channel in TDA both in4He and16O. The effect of the relative KE is to decrease the central force attraction and increase the relative importance of the tensor force. The latter is important in RPA. Sussex matrix elements without the hard core are used.

    • Detection of direction of motion and zero order fringe identification in holographic displacement measurement

      C S Vikram

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      Two methods have been suggested to detect the direction (forward or backward) of uniform velocity motion along with identification of no motion positions with other intensity maxima positions in the reconstruction. The first method requires two continuous exposures while the other is a combination of one static and one continuous recording.

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