• Volume 32, Issue 4

      April 1989,   pages  307-603

    • Foreword

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    • Shell model in nuclei—a historical overview

      Sudhir P Pandya Jitendra C Parikh

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      The nuclear shell model is (over)viewed with examples from its early phase to its current status.

    • Infinite nuclear matter model and a new mass formula for atomic nuclei

      L Satpathy

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      The ground-state energy of an atomic nucleus with asymmetryβ is considered to be equivalent to the energy of a perfect sphere made up of the infinite nuclear matter of the same asymmetry plus a residual energyη called the local energy,η represents the energy due to shell, deformation, diffuseness and exchange Coulomb effect etc. Using this picture and the generalized Hugenholtz- Van Hove theorem of many-body theory a new mass formula has been developed. Based on this, a mass table containing the mass excesses of 3481 nuclei in the range 18 ⩽A ⩽ 267 has been made. This mass formula is compared with other mass models.

    • Schwinger boson mapping of SO(8) fermion model

      R S Nikam

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      The Schwinger representation of the SO(8) fermion pair algebra in terms ofd and quasispin vector (u, s, v) bosons is used in deriving a microscopic boson coherent state having both particle-hole and pair excitations. The coherent state is the exact boson image of the HFB variational solution. We can study the shape phase transition and pairing behaviour of the nuclear ground states using the coherent states.

    • Single particle equivalent transfer monopole sum rules involving general multishell target states

      R K Bansal Ashwani Kumar

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      Monopole sum rules for single particle equivalent transfer reactions have been derived for target states where both neutrons and protons occupy the last active shell. The Pauli Principle lends some complexity to the sum rule, which is general in the sense that the inequivalent transfer situation and where the active shell has only neutron occupancy, become its special cases. Calculated values of various energy centroids have reasonable agreement with their measured counterparts.

    • Hartree-Fock theory of nuclear deformations and high spin states

      C R Praharaj

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      A brief review of the Hartree-Fock approximation to nuclear shapes and nuclear spectra is presented. Various aspects of rotation-alignment effect and high spin states are illustrated in the Hartree-Fock model.

    • Structure of nuclei in the regionA = 70

      R Sahu S P Pandya

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      The structure of the selenium nuclei in the regionA = 70 is studied using our deformed configuration mixing (DCM) shell model based on Hartree-Fock states. An effective interaction given by Kuo and modified by Bhatt is used. An attempt is made to understand the coexistence of shapes in selenium nuclei.

    • Field-theoretic methods in nuclear physics: A reappraisal

      S P Misra

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      We discuss here some non-perturbative techniques of field theory as applied to nuclear physics. We first describe Walecka’s mean field approach and its generalizations with a many-body description. We next apply to nuclear matter a new approach to strongly interacting systems, of which Walecka’s method becomes a specific approximation. The phenomenological implications of the same are also discussed.

    • Relativistic mean-field description of the ground-state nuclear properties

      Y K Gambhir P Ring

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      The results of the relativistic mean field (RMF) calculations are presented for the ground-state properties of characteristic deformed nuclei covering the entire periodic table. The representative cases in the 2s − 1d, rare-earth and actinide regions are explicitly considered. The pairing correlations are considered in the constant gap approximation. It is observed that the set of parameters appearing in the Lagrangian chosen to reproduce the ground-state properties of nuclear matter and spherical doubly magic nuclei, also turns out to give a very satisfactory description of light- and heavy-deformed nuclei.

    • Studies in nuclear macrophysics through fission and fission-like reactions

      S S Kapoor

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      Recent developments in the study of fission and fission-like reactions are briefly reviewed. After a brief introduction of some of the important features of the fission process, binary fission and fission-like processes in heavy ion-induced reactions are discussed. It is shown that studies of the fission fragment angular distributions which provide a way to determine relative contributions of compound nucleus fission and non-equilibrium fission-like events in heavy ion-induced fission have proved to be quite valuable in investigating the very shortK-equilibration times of the order of 10−20 s involved in the nuclear dynamics of the dinuclear complex on its way to compound nucleus formation following nucleus-nucleus collision.

    • Clustering phenomena in radioactive and stable nuclei and in heavy-ion collisions

      S S Malik S Singh R K Puri S Kumar Raj K Gupta

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      A theory for clustering formation in nuclei and in heavy-ion collisions has been worked out in terms of the quantum-mechanical fragmentation process. Treating the mass fragmentation and relative separation coordinates as weakly coupled, the spontaneous cluster-decay of radioactive nuclei has been considered as a two-step process of clustering formation and tunnelling of the confining nuclear interaction barrier. This model has also been applied to “stable” nuclei, lighter than lead. The effects of adding more and more neutrons to collidingN =Z,A = 4n nuclei are studied for theα-clustering transfer phenomenon.

    • Heavy-ion fusion in classical and semiclassical microscopic approaches

      S S Godre Y R Waghmare

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      Classical and semi-classical microscopic approaches leading to fusion of two heavy nuclei are studied. Calculations show that the results depend strongly on the nature of the nucleon-nucleon interaction. It is also observed that there is no angular momentum window unlike in TDHF calculations.

    • Spin-isospin responses in nuclei

      B K Jain

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      The spin-isospin response of nuclei to hadronic probes is discussed. It includes the spin-isospin flip Gamow-Teller excitations at nuclear level and the delta-isobar excitations at nucleonic level. In GT excitations, while the energy dependence of the spin-isospin nucleon-nucleon interaction is understood, the cause for discrepancy between this extracted interaction from the (p, n) data and that coming from the free nucleon-nucleont-matrix is not fully understood. Cause for the observed deficit in the GT-strength with respect to shell model value is also far from settled. For the production of Δ-isobars in nuclear reactions, (3He,t) reaction on proton and12C targets is considered. It is found that the DWBA is a legitimate candidate theory for the description of these reactions.

    • State densities and spectrum fluctuations: information propagation in complex nuclei

      J B French V K B Kota

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      At excitation energies in nuclei where the state density is unambiguously defined there is a sharp separation between the smoothed spectrum (which defines the density) and fluctuations about it which have recently been studied with a view to understanding some aspects of quantum chaos. We briefly review these two complementary subjects, paying special attention to: the role of the effective interaction in determining the density; the calculation of interacting-particle state and level densities, and of expectation values of interesting operators; the information about the effective nucleon-nucleon interaction which is carried both by the density and the fluctuations.

    • Density and fluctuations in shell model spectra

      R U Haq

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      A simple method, based on the Gram-Charlier expansion, to separate the smooth and fluctuating parts of the level density is developed. Applications to spectra arising from large shell model calculations are considered. Fluctuations in the energy levels are studied. It is found that with this proper decomposition, even long range correlations agree with the predictions of the random matrix theory.

    • Tests of time-reversal invariance in complex systems

      Akhilesh Pandey

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      The random-matrix theory for the effects of time-reversal non-invariance (TRNI) on energy level, strength and cross-section fluctuations in complex systems is reviewed. Applied to the compound-nuclear data this gives bounds on rms TRNI matrix elements. Using a fluctuation-free form of statistical spectroscopy bounds on α, the relative magnitude of the TRNI nucleon-nucleon interaction, is deduced. In all three cases we find α ≲ (2–3) × 10−3 at high (∼ 99%) statistical confidence. Suggestions are made about experiments which should improve the bounds.

    • Spectral distribution theory—Some recent applications

      Kamales Kar Sukhendusekhar Sarkar

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      The application of spectral distribution theory for binding energy, spectra and occupancies using universal-sd interaction in (sd) shell and for Gamow-Teller and M1 strength sums in both (fp) and (sd)-shell is described.

    • Inverse energy weighted sum-rules

      V K B Kota V Potbhare P Shenoy N Tressler

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      A new derivation of the inverse energy-weighted sum-rules is given by applying the spectral distribution methods to the Rayleigh-Schrodinger perturbation theory. The scalar space result is then extended to the configurations. This is applied to obtain corrections to the ground-state energy estimates when the effective interaction is approximated by a model Hamiltonian obtained by taking linear combinations of various parts of the pairing and the Q.Q operators.

    • Statistical theory of hot nuclei and high spin states

      M Rajasekaran N Arunachalam T R Rajasekaran V Devanathan

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      The hot rotating compound systems formed in heavy ion collisions are studied using the statistical theory with a view to determine the spin and temperature dependence of nuclear shapes. Shape transitions are observed for these systems at particular spin values. The neutron and proton separation energies for heavier high spin systems have been evaluated. Results are presented for70170Yb and78194Pt.

    • The changing scenario of the atomic nucleus—from nucleons and mesons to quarks and gluons

      Bikash Sinha

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      The general characteristics of the transition from hadronic matter of nucleons, three quark bags, mesons of quark antiquark pairs to quark gluon plasma is discussed. The phenomenological approach essentially guided by the MIT bag model and general thermodynamic criteria of first-order phase transition is elaborated. The more realistic calculations using the QCD lattice renormalization quark are touched upon. Possible signals of quark-gluon plasma are discussed. The central issue of deciphering plasma signals from the signals of hot hadronic matter is discussed in detail.

      The signals of the quark-gluon plasma, a subject of considerable interest in contemporary literature are focussed only on (i) dileptons (ii) photon photon pairs and (iii)J/Ψ suppression (with special emphasis on CERN experiments). The lingering shadow of “EMC” effect is also mentioned.

      Relics of the very early universe microseconds after the big bang in today’s universe (∼ 15 billion years later) are discussed. Finally, the outlook of this very exciting field is presented, a purely personal viewpoint, generalized eventually to poetic signals of the creation of the universe.

    • Electric and magnetic polarizabilities of nucleon and QCD quark models

      C S Warke

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      The experimental and theoretical work carried out on electric and magnetic polarizabilities of a nucleon is reviewed. The results of an exactly solvable one-dimensional chiral bag model predicts correct signs and order of magnitudes of polarizabilities supporting the approximations used in their realistic calculations.

    • Calculation of energy density in the colour tube model of relativistic heavy ion collisions

      B Banerjee

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      The energy density produced in a relativistic heavy ion collision is calculated within the framework of the colour tube model. The chromoelectric field generated in the collision produces quark-antiquark pairs. The motion of these particles is described by Boltzmann equation. The interaction between the quarks and antiquarks is approximated by introducing a relaxation time.

    • Instability of the plasma oscillations in finite temperature perturbative QCD

      J C Parikh P J Siemens J A Lopez

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      We use linear-response dielectric theory to show that the baryon-poor QCD plasma based on the perturbative vacuum is unstable, even at a high temperature. If deconfinement occurs in nuclear collisions or the early universe, it is not accompanied by the restoration of the perturbative vacuum.

    • Gribov ambiguity in gauge theories

      R Parthasarathy

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      Gribov ambiguity in gauge field theories is discussed and it is shown that such an ambiguity exists even for Abelian theories in covariant gauge at finite temperature. Both geometric and algebraic proofs are presented. In view of the importance of non-perturbative methods, some special gauges are given in which such ambiguities do not exist or are not relevant. The significance of these in the study of confinement in QCD is pointed out.

    • Quark-lepton compositeness: A three-body scenario

      A N Mitra

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      The Harari-Shupe model of quarks and leptons is viewed, not as a gauge theory, but as a quantum-mechanical three-body problem of the extreme relativistic type involving massless preons. Considerations based onS3-symmetry in the available degrees of freedom (spin, isospin, space and hypercolour) are employed in conjunction with a spin-dependence ansatz on the three-preon forces (Σaμv(1)σλμ(3)) for an understanding of the three basic issues of (i) spin-1/2, (ii) generation structure and (iii) steeply rising mass patterns of quark-lepton families. The Σa-dynamics is compatible with the interpretation of colour as a manifestation ofS3-symmetry, as envisaged in the original Harari-Shupe proposal, while the interpretation of the generation structure devolves on the role of a certain quantum numberN which takes on three different classes of values (3n, 3n ± 1;n = 0, 1, 2, ...) according to theS3-symmetry of thespatial wavefunction.

    • Faster-than-light particles: An alternative view

      V K Thankappan

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      The existence of a wave motion analogous to the electromagnetic waves, but with wave velocity less than that of light, is postulated. It is suggested that these waves, which transport energy and momentum with subluminal velocities, could replace the concept of faster-than-light particles. The possibility of the neutrinos as being the source of the waves is suggested for further investigation.

    • Application of nuclear techniques to study of trace elements

      A S Divatia

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      Trace elements in various materials using nuclear techniques are studied. Results obtained by using energy-dispersive X-ray fluorescence (EDXRF) and proton-induced X-ray emission (PIXE) are examined. The EDXRF method is used to quantitatively evaluate trace elements such as Mn, Fe, Cu, Zn, Br, Rb and Sr in tea leaves. Correlation of trace elements in tea leaves and in the corresponding soil is studied.

    • Future directions

      S P Pandya

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