Table of contents

The mixing of the axion with the eta ' is analysed from the point of view of the effective Lagrangian in the context of the 1/N expansion in QCD.

A momentum distribution function of recoil nucleons in hadron-nucleus collisions is determined consistently in the framework of the multichain model. The resulting distribution depends on the parameter a characterising the leading cluster momentum distribution. The results for the multiplicity ratio show good agreement with experimental data without introducing other effects such as the dibaryon resonances.

The yrast sequence of ¹⁶²Hf has been observed up to a level tentatively assigned as 38⁺ and reveals a continuing rotational character up to that spin. Sharp backbends at rotational frequencies of 0.27 and 0.42 MeV/h are attributed to i_13/2 neutron and h_11/2 proton alignments, respectively. A gradual increase in the aligned angular momentum of the yrast levels between these two sharp backbends is attributed to the rotational alignment of a pair of negative parity quasineutrons (mostly h_9/2 in character). The interpretation of this effect is supported by the failure of the negative parity bands, which already contain this aligned h_9/2 neutron, to gain alignment in the same rotational frequency range. While the alignment of the h_9/2 quasineutrons has been predicted in the cranked shell model to occur in the rare-earth region with a large interaction strength, this represents the first clear observation of such a band crossing.

Coupled channel calculations for elastic scattering of ¹⁷O on ¹²C and excitation of the first ¹/₂⁺ state of ¹⁷O were carried out in the molecular particle-core model. In this model the extra neutron of ¹⁷O occupies molecular single-particle states of the two-centre shell model. The enhancement of the radial coupling strength due to an avoided level crossing leads to structures in the differential cross sections in agreement with measured data. These structures are understood as the signals of a Landau-Zener-type nuclear transition, uniquely proving the existence of nuclear molecular orbitals and consequently of nuclear molecules.

Recent microscopic finite-range DWBA analyses of (p, alpha ) and ( alpha , p) reactions based on Saxon-Woods optical potentials underestimate the absolute values of the cross section by about two orders of magnitude. Therefore, the authors have re-analysed reaction data using double-folded alpha potentials. It was possible to obtain almost the correct magnitude of the cross sections. Simultaneously, the angular distributions of the elastic and reaction cross sections and the analysing powers could be reproduced.

At low energies ( approximately=20 MeV) the Johnson-Soper adiabatic model (1976) for deuteron break up is inaccurate when applied to transfer reaction calculations. The assumed degeneracy of the n-p centre-of-mass energy in all break-up configurations results in a lack of phase averaging and, in particular, an overestimation of the break-up component of the entrance channel wavefunction near the nuclear surface, of most importance to transfer reactions. This inadequacy of the break-up wavefunction is studied quantitatively and shown not to be significant for application of the adiabatic theory to the (d, p) reaction at 79 MeV incident deuteron energy.

The role of the spin dependence associated with Pauli-principle-induced break-up is studied quantitatively in the case of intermediate-energy deuteron elastic scattering from a ⁵⁸Ni target. The magnitude of the momentum-dependent T_p tensor interaction, calculated using realistic theoretical models of the nucleon-nucleon interaction, is shown to pass through a local maximum in the region of 400 MeV incident deuteron energy. Comparison of numerical calculations with the available experimental data at this energy shows that the Pauli mechanism is not responsible for the outstanding discrepancies between theory and experiment.

The cross section for a direct Coulomb break-up reaction may be related, within the restriction of first-order processes, to the cross section for the inverse photocapture reaction. This relationship has a possible application in determining astrophysical interesting photocapture cross sections from break-up data. By specific analysis for a particular break-up reaction, ⁷Li to alpha +t at 70 MeV, it is shown however that secondary effects considerably complicate the relationship, and therefore erode its value for extracting astrophysical data.

The authors have analysed the database of cosmic gamma rays above 100 MeV from the SAS-2 satellite to search for gamma -ray fluxes from extragalactic active galactic nuclei (AGN). The correlation function method devised by the COS-B group to search for galactic gamma -ray sources has been used. They have thus identified 15 gamma -ray AGN, including 11 quasars, 3 BL Lac objects and 1 radiogalaxy. They have further investigated the relationship between their gamma -ray fluxes in different energy regions (>100 MeV and 35-100 MeV) and the relationship between their gamma -ray fluxes and their red shifts, and they have also derived their gamma -ray spectral indices. The results give further support to the claim that the sources detected are genuine extragalactic objects.

Solutions of sheet field equations for classical strings have been obtained that represent the open helical string with a single-frequency mode.

The authors explicitly express the C and D event shape variables proposed by Parisi (1978) to describe the multijet structure of hadronic final states in e⁺e^- annihilation in terms of the 4-momentum fractions of the QCD interacting partons (whose hadronisation gives rise to the observed jets), and by so doing simply highlight their infrared-safe property for 3- and 4-jet final states, respectively.

Using the theory of weak interactions, several decay channels of muonium ( mu ⁺e^-)_JM (J=0, 1) and pionium ( pi ⁺e^-)₁2M/ with zero orbital angular momentum are calculated systematically. The decay probabilities, the angular distributions, the selection rules and the energy spectra are obtained.

The wavefunction of a system consisting of 2n fermions has been written as a linear combination of products of n pair functions in such a way that antisymmetrisation of the total wavefunction is ensured. Proper orthonormalisation conditions necessary for a finite system have been introduced. The self-consistent field integrodifferential coupled equations for the n pair functions have then been derived from a variational principle by minimising the total energy of the system with the auxiliary orthonormality conditions. These equations are the Hartree-Fock analogues for the n pair functions in a finite system of 2n fermions. The calculated energy is clearly an upper bound to the exact energy. The relation between the total energy and the eigenvalues of the pair equation is derived. The total energy is not the sum of the pair eigenvalues. The generalisation to three, four and higher clusters is indicated.

The dynamic equations are obtained for the moments of the Wigner function (density matrix) in the phase space of a nucleon. The rules of closure of the set of equations for these moments are formulated. The negative-parity vibrational states (octupole, magnetic quadrupole and isoscalar dipole toroidal) are analysed. The theory gives an adequate description for all these excitations using as an input the values of the nuclear radius (R=r₀A¹3/, r₀=1.2 fm) and the surface tension (T=17/4 pi r₀² MeV fm^-2).

The two parameters of the variable moment of inertia model and the three parameters of the variable anharmonic vibrator model have been studied as functions on the valence proton-neutron product N_pN_n in several regions of the periodic table. All the quantities studied show simple systematic behaviour as functions of N_pN_n, paving the way for a unified description of a wide range of nuclei in the variable moment of inertia framework in terms of few parameters, and allowing for reliable predictions of properties of nuclei far from stability to be made by interpolation rather than extrapolation. The few exceptions observed serve to clarify the underlying differences between the two models.

The alpha decay and the exotic radioactive emission of heavy ions are assumed to be superasymmetric fission processes. These processes are studied as transitions from Coulomb-excited states of the parent nucleus into excited states of the daughter nucleus. For their description the authors use the superasymmetric fission model and generalise it. Within this generalised model they calculate the cross section for the Coulomb-induced alpha decay in collisions of ²³⁸U on ²³⁸U below the Coulomb barrier and estimate the cross section for the Coulomb-induced cluster emission. The influence of prompt Coulomb-induced alpha decay is also discussed.

A partial diagonalisation of a coupled-channels system is obtained for arbitrary momentum-independent interactions and excitation energies when the angular momentum dependence of the centrifugal barrier is treated approximately. This is a consequence of the 'tidal symmetry' of the interaction, that is, the invariance of the interaction with respect to rotations of the interacting particles about their relative coordinate. Closed expressions are found for the transition amplitudes of the full coupled-channels system as a function of those of a reduced coupled-channels calculation involving only spin-zero states. In particular it is found for elastic scattering that the projection of the spin along the recoil direction is conserved. A consequence of this is that the tensor analysing powers obey the shape effect model relations in a strong-coupling non-adiabatic situation. The consequences for inelastic scattering are also discussed.

The ²⁴Mg(t, alpha )²³Na reaction has been studied at an incident energy of 33 MeV. A comparison of DWBA and CCBA calculations reveals the sensitivity of this reaction to details of the collectivity of both the target and residual nuclei. Spectroscopic strengths extracted from the DWBA analysis are in good agreement with the predictions of the Wildenthal 'USD' interaction. Inelastic effects are found to account for the significant population of high-spin states in the residual nucleus.

The prediction made by Linsley (1986,7) that the angular resolution of a scintillator array can be improved by placing lead sheets over its constituent detectors has been tested experimentally with the gamma-ray telescope at Haverah Park. The authors find that the addition of lead significantly improves the prompt timing and density response of detectors, and that the optimum thickness of lead, for detectors at sea level, is about 9 mm (1.6 radiation lengths). The solid-angle uncertainty in the direction of showers found using a small array shielded with this thickness is about 60% less than for directions found using an identical, unmodified, array. They also find that there is a 30% enhancement in the density signal observed for showers falling in the distance range 40<r<60 m: further work is required to derive the lateral distribution function which needs to be used with an array of lead-covered detectors.

It is argued that the spin (6) limit is inappropriate for representing the nuclei ^131,133Xe and ¹³⁵Ba. The importance of fitting transition rates as well as energy levels is stressed. This is a comment on a paper by Michailova (ibid., vol.13, p.L149, (1987)).