Showing papers in "Pramana in 1977"

Liquid crystals of disc-like molecules

Abstract: Thermotropic mesomorphism has been observed in pure compounds consisting of simple disc-like molecules, viz., benzene-hexa-n-alkanoates. Thermodynamic, optical and x-ray studies indicate that the mesophase is a highly ordered lamellar type of liquid crystal. Based on the x-ray data, a structure is proposed in which the discs are stacked one on top of the other in columns that constitute a hexagonal arrangement, but the spacing between the discs in each column is irregular. Thus the structure has translational periodicity in two dimensions and liquid-like disorder in the third.

The Kerr metric in cosmological background

Abstract: A metric satisfying Einstein’s equations is given which in the vicinity of the source reduces to the well-known Kerr metric and which at large distances reduces to the Robertson-Walker metric of a homogeneous cosmological model. The radius of the event horizon of the Kerr black hole in the cosmological background is found out.

Charged particle trajectories in a magnetic field on a curved space-time

Abstract: In this paper we have studied the motion of charged particles in a dipole magnetic field on the Schwarzscbild background geometry. A detailed analysis has been made in the equatorial plane through the study of the effective potential curves. In the case of positive canonical angular momentum the effective potential has two maxima and two minima giving rise to a well-defined potential well rear the event horizon. This feature of the effective potential categorises the particle orbits into four classes, depending on their energies. (i) Particles, coming from infinity with energy less than the absolute maximum ofV eff, would scatter away after being turned away by the magnetic field. (ii) Whereas those with energies higher than this would go into the central star seeing no barrier. (iii) Particles initially located within the potential well are naturally trapped, and they execute Larmor motion in bound gyrating orbits. (iv) and those with initial positions corresponding to the extrema ofV eff follow circular orbits which are stable for non-relativistic particles and unstable for relativistic ones. We have also considered the case of negative canonical angular momentum and found that no trapping in bound orbits occur for this case. In the case when particles are not confined to the equatorial plane we have found that the particles execute oscillatory motion between two mirror points if the magnetic field is sufficiently high, but would continuously fall towards the event horizon otherwise.

Short range orientational order in nematic liquid crystals

Abstract: The different approximations that have been used in applying Bethe’s cluster model to the nematic-isotropic phase transition are examined. It is shown that the introduction of a higher order term in the mean field potential of an outer molecule of the cluster improves the consistency of the theory considerably. In particular, the importance of satisfying Chang’s relation is emphasized. Calculations are presented of the long and short range order parameters, heat of transition and specific heat for different values ofz, the number of nearest neighbours around any given molecule, for both nonpolar and antiparallel near neighbour correlations. Even the new mean field potential appears to be inadequate forz=3.

Amplitude scintillations of ATS-6 radio beacon signals within the equatorial electrojet region (Ootacamund, dip 4° N)

Abstract: The recordings of the amplitudes of radio beacon signals on 40, 140 and 360 MHz from ATS-6 (at 34° E longitude) recorded at Ootacamund, India (11.43° N, 76.70°E, dip 4°N, elevation angle 41°) have revealed largest occurrence of scintillations for about 60% of cases around 2200 hr during the nighttime, and two secondary peaks (25% of cases) around 0900 hr and 1400 hr during the daytime. During the daytime, the scintillation decreases approximately as the inverse of the frequency for higher frequencies while for lower frequencies the law is valid till scintillation index at 40 MHz does not exceed 0.9. The temporal variation of daytime scintillation shows impulsive character, the duration of activity lasts for 1–2 hours at a time. During the nighttime, the scintillation decreases inversely with frequency for weak and moderate scintillation activity. The scintillation index at 360 MHz becomes independent of that at 140 MHz when the index at 140 MHz exceeds 0.85. For the set of frequencies 40–140 MHz, on some occasions scintillation index at 40 MHz is seen to be less than that at 140 MHz. The nighttime scintillations are in general stronger and remain so for extended length of time. The daytime scintillations are suggested to be due to blanketing or some other non-q type of sporadicE layer. The nighttime scintillations are most probably due to spreadF condition and the abnormal frequency variation of the scintillations may be due to multiple scattering layer during periods of intense spreadF.

A. C. behaviour and dielectric relaxation in indium oxide films

Abstract: Vacuum deposited blackish indium oxide films (In-O) as well as the oxidised films (In2O3) were studied for their a.c. behaviour at different temperatures and at various film thicknesses in the audio frequency region. e of In-O films was thickness dependent and also showed dielectric relaxation at lower frequencies due to the dipolar orientation arising from their non-stoichiometric nature. However at liquid nitrogen temperature region e was thickness independent similar to the oxidised films which neither showed any relaxation effect nor any thickness dependent e. The results have been discussed from the classical theory of dielectric polarisation.

X-ray determination of the mean amplitudes of vibration and debye temperatures of some rare earth monochalcogenides

Abstract: The x-ray diffraction intensities of Bragg reflections have been measured at room temperature for thulium selenide, samarium sulphide, samarium selenide and samarium telluride. On the basis of a common amplitude approximation, the Debye-Waller factor, the mean amplitude of vibration and the Debye temperature have been evaluated. The values of the Debye temperatures and mean amplitudes of vibration are 176±16°K, 0·185 ± 0·017 A (TmSe), 155 ± 7°K, 0·244 ± 0·012 A (SmS), 153 ± 14°K, 0·221 ± 0·020 A (SmSe) and 151 ± 20°K, 0·204 ± 0·027 A (SmTe).

Dirac equation in Kerr space-time

Abstract: The weak field-low velocity approximation of Dirac equation in Kerr space-time is investigated. The interaction terms admit of an interpretation in terms of a ‘dipole-dipole’ interaction in addition to coupling of spin with the angular momentum of the rotating source. The gravitational gyro-factor for spin is identified. The charged case (Kerr-Newman) is studied using minimal prescription for electromagnetic coupling in the locally inertial frame and to the leading order the standard electromagnetic gyro-factor is retrieved. A first order perturbation calculation of the shift of the Schwarzschild energy level yields the main interesting result of this work: the anomalous Zeeman splitting of the energy level of a Dirac particle in Kerr metric.

An ab-initio derivation of the pi-electron hamiltonian by a nonperturbative open-shell formalism

Abstract: A method for generating a pi-electron hamiltonian in an ab-initio manner using the non-perturbative open-shell many-body formalism recently developed by us is presented. The π-hamiltonian thus derived is energy-independent, and is also proved to be spin-independent. A recipe is given with the help of which Goldstone—like matrix-elements ofH π can be extracted up to three body terms. It has been demonstrated that the use of diagrammatics considerably simplifies the algebra and allows one to keep track of the various quantities involved. Up to a given order of approximation, an explicit form ofH π containing up to the three body terms has been given, and some of the important physical effects embedded in the hamiltonian are discussed. A comparative analysis of the various formalisms currently in use forms the concluding section of the paper.

Total (p,n) reaction cross section study on 51 V over the incident energy range 1.56 to 5.53 MeV

Abstract: 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 atE p ∼ 2·340 has been shape analysed to extract the proton width Γ p , the spreading withW and the spectroscopic factor.

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

Abstract: 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.

Effect of collisions on spectral lines in gases

Abstract: Collisions between an emitter and the surrounding buffer gas particles influence the emission lines. A collision may perturb the emitter in its excited and ground states, cause direct transitions between the levels, and at the same time, change the velocity of the emitter. In this paper, we present a model which deals for the first time with all these effects when they occursimultaneously in each collision. The model assumes the emitter to be subject to random collisions by the surrounding gas particles which are taken to constitute a heat bath in thermal equilibrium. The collisions are assumed to be binary, instantaneous, and to occur with a probabiity given by the Poisson distribution. These assumptions are shown to be equivalent to the widely used impact approximation in the collision broadening theory. We discuss several special cases of the general result for the line shape obtained in the paper.

A new criterion for the mixed crystal behaviour in the diatomic linear chain model

Abstract: Lucovsky, Brodsky, Burstein (LBB) have studied the behaviour of mixed crystals by setting up a criterion for the existence of local mode frequencies in real crystals starting from a diatomic linear chain model. This, while successfully predicting the one and two mode behaviour for some systems fails to predict the mixed mode behaviour. We propose a similar criterion for the existence of gap modes, by demanding that the gap mode predicted by the diatomic linear chain model should lie within the gap of the real three dimensional solid for its existence. It is shown that the gap modes for various systems calculated using this criterion are in reasonable agreement with the experimental values. The infrared behaviour of mixed crystals has to be determined by examining the existence of local as well as gap modes for the two end members of the system. This generalized new criterion successfully predicts the mixed mode behaviour of III–V mixed crystals besides predicting the one and two mode behaviour, observed in infrared absorption of mixed alkali halioes and III-V compounds.

Triboluminescence, a new tool to investigate fracture-initiation time of crystals under stress

Abstract: The present paper reports that triboluminescence (TBL) does not appear at the instant of impact of the load but a certain time lag is required for its appearance which depends on the value of the stress applied to the crystal. Since TBL appears in sugar crystals during the creation of new surfaces, the fracture-initiation time of the crystal has been taken to be the delay time in observing TBL pulse after the application of stress. The dependence of fracture-initiation time,t f σ , of crystals on the stress, σ, may be expressed ast f σ =t o exp (− ασ), wheret o and α are constants. The values of the lattice energy, and the change in lattice energy per unit stress, of sugar crystals have been calculated from TBL measurements and they have been found to be 21·2 kcal mole−1 and 0·41 × 10−8 kcal mole−1 dyne−1 cm2 respectively.

Radiation effects in calcite

Abstract: Calcite single crystals on neutron bombardment change their colour to pale red, red and deep red, as the duration of irradiation increases. The irradiated surface becomes rugged and on etching, a large number of micropits are observed. Using Vicker’s hardness indenter, it is observed that the hardress of the irradiated crystal increases with the increase in the total dosage. From Laue x-ray diffraction studies of irradiated crystals, distortion of lattice is observed. Absorption spectra in the visible region show an extra peak for irradiated samples which is absent for radiated ones. The radiation effects are annealed to a considerable extent on heat treatment. The implications are discussed.

Yields and energy spectra of light charged particles emitted in neutron induced fission of 235 U

Abstract: The yields and energy spectra of light charged particles emitted in the fission of235U have been measured in the neutron energy range of 100 keV to 1 MeV. The yield of long range alpha particles is found to increase around 200 keV neutron energy compared to thermal fission. A low energy component observed in the energy spectrum was assigned to the tritons emitted in fission. The yield of this triton component is seen to have a marked increase around 500 keV. These results indicate that LCP yield is influenced by the transition state level characteristics.

An electron fluid model for the lattice dynamics of metals

Abstract: An electron fluid model is proposed for the lattice dynamics of metals which satisfies the requirement of translational invariance and the lattice is in equilibrium without recourse to external forces. The model is applied to calculate the phonon dispersion of sodium in the symmetry directions.

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

Abstract: 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.

Phonon dispersion in cubic transition metals: A comparison between Fielek and extended Cheveau models

Abstract: Phonon dispersion in bcc transition metals has been studied by Fielek and extended Cheveau models. A comparative study of the results shows that the wavy nature of [ξ00]-L branch in the dispersion curves of some metals is due to the long range forces which are well accounted for by the extended Cheveau model but not by the Fielek model. The peculiar behaviour of transverse branches are due to the presence ofd-electrons. It is found that the oscillatingd-shell model of Fielek is fairly good for tungsten, α-iron and molybdenum, but does not give satisfactory description ofd-electron effects in the remaining bcc transition metals.

Timelike and null geodesics in the Nordström field

Abstract: The timelike and null geodesics are investigated in the Nordstrom geometry and it is found that incoming geodesics always encounter a turning point at a finite radial distance. The limits for escape, bound and stable orbits are obtained and they are closer to the source as compared to their counterparts in the Schwarzschild’s field.

Effective two-body forces in light and medium mass nuclei

Abstract: From the spectroscopic information provided by single particle stripping reactions, we have extracted average effective two-body interaction between nucleons in thep-, sd-, andfp- shells. Using the derived interaction parameters, we have calculated the energy centroids of the level spectra of residual nuclei obtained via single nucleon stripping reactions involving light and medium mass nuclei as targets.

Single particle SU(3) parentage coefficients

Abstract: The single particle SU(3) parentage coefficients are calculated for the case of leading SU(3) representation in the highest orbital symmetry partition, using the method suggested by Hecht. Tabulations are given for all possible cases of identical nucleons in η=3 and η=4 shells.

CNDO/2 calculations on the structure and properties of chlorine difluoride

Abstract: According to the open-shell CNDO/2 calculations on ClF2, performed by using the computer programme developed by Pople, Beveridge and Dobosh, the molecule is linear and stable, with equilibrium bond length 1.507A and binding energy −173.7 kcal/mole. The molecule has a tendency to dimerise and to disproportionate into ClF3 and ClF. The netd-orbital population in the monomer is 0.88. Bonding characteristics and other molecular properties are also discussed.

Spectral features of radiation from Nordstrøm and Kerr-Newman white holes.

Abstract: Unlike the Schwarzschild white hole, Nordstrom and Kerr-Newman white holes cannot explode right down from the space time singularityR=0. For example a charged white hole has to commence explosion (i.e., comes into existence) with a radiusR 0=R c (2−R c /R b )−1 whereR c is the ‘classical radius’ andR b is the final radius attained when the stationary state is reached. That means charged and rotating black holes also cannot hit the singularityR=0 and perish. Here the explosion is decelerated by the presence of charge and rotation and hence the radiation emitted would be not as energetic as in the Schwarzschild case where its energy is infinitely large for emission fromR=0.

Cuspidal edges for elastic wave surfaces for cubic crystals

Abstract: The paper deals with a detailed numerical study of the sections of the inverse and ray velocity surfaces for cubic crystals. The figures for the sections of the inverse and ray surfaces by the (001) and (110) planes have been plotted for over 65 crystals and from these, the nature of the cuspidal edges has been discussed. Typical graphs of the inverse and ray surfaces have been given. The parameters characterising the dimensions of the cusps have been tabulated. It is shown that the A-15 compounds exhibit very unusual and interesting wave surfaces at temperatures below superconducting critical temperatures.

Anomalous conversion in the decay of197mHg

Abstract: The total andK-shell conversion coefficients of the 165 keV transition in the decay of197mHg are determined from intensity balance considerations and a coincidence technique using a Ge(Li)—NaI (Tl) system respectively. The resultant values areaT=274.8±19.2 andaK=47±12, whil the corresponding theoretical values are 344 and 77 respectively, indicating anomalous conversion. The gamma ray transition probability however, shows a hindrance of only about 6 and cannot be correlated with the present anomalous conversion data. TheK/L ratio of the 130 keV transition, determined using a summing method with a Ge(Li) detector, yielded 0.090±0.012, while the corresponding theoretical value is 0.048, indicating anomalous conversion. The corresponding gamma transition probability shows a hindrance of about 3000, in correlation with anomalous conversion.

Residue-squaring diagonalisation method and the anharmonic oscillator

Abstract: A recently-formulated residue-squaring method for perturbation problems is subjected to an exacting test in its application to the problem of diagonalising the Hamiltonian of the nonlinear oscillator with quartic anharmonicity. Unlike other methods, this new iterative diagonalisation method enables several eigenvalues to be calculated simultaneously with little more labour than for a single eigenvalue. Values obtained for the four lowest even-parity levels of the anharmonic oscillator from just two or three iterations are shown to agree well with earlier accurate calculations. An approximate analytical formula for the energy levels is also presented.

Dispersion and spectral characteristics of crossfield instability in collisional magnetoplasma

Abstract: Results on dispersion and spectral characteristics of crossfield instability in a collisional magnetoplasma are presented for different values of externally applied radial electric field. The dispersion relation obtained experimentally differs significantly from predictions of linear theory for strong electric fields.K-spectra for density and potential fluctuations follow power law with indices (−3·7 ± 0·5) and (−5·6 ± 0·6) respectively. At large values of applied electric fields, the main crossfield modes appear to give rise to a number of other modes possibly through non-linear wave-wave interactions. Large amplitudem = 2 mode displays strong sidebands indicating particle trapping by the non-linear waves. Non-linear wave-wave and wave particle interactions, thus, appear to play at important role in the saturation of the cross field instability.

Measurement ofL-shell photoelectric cross sections in highZ elements at 37 and 74 keV

Abstract: L-shell photoelectric cross section measurements have been made at 36·818 and 74·409 keV for four elements in the range 81 ≤Z ≤ 92. The measurements at 74·409 keV are found to agree with theory, within experimental uncertainties, but the experimental values at 36·818 keV are found to be higher than the theoretical predictions. The possible reasons for the observed discrepancy are discussed.

Tensor product composition of algebras in Bose and Fermi canonical formalism

Abstract: We consider a graded algebra with two products (σ, α) over ane-factor commutation. One of the products (σ) ise-commutative, but, in general non-associative; and the other (α) is a graded Liee-product and a gradede-derivative with respect to the first (σ). Using the obvious mathematical condition, namely—the tensor product of two graded algebras with the samee-factors is another with the samee-factor, we determine the complete structure of a two-product (σ, α) graded algebra. When thee-factors are taken to be unity and the gradation structure is ignored, we recover the algebras of the physical variables of classical and quantum systems, considered by Grgin and Petersen. With the retention of the gradation structure and the possible choice of two e-factors we recover the algebras of the canonical formalism of boson and fermion systems for the above classical and quantum theories. We also recover in this case the algebra of anticommutative classical systems considered by Martin along with its quantum analogue.